WO2024213928A1

WO2024213928A1 - A system for twin cold finger mechanically driven gm cryocooler and drive mechanism

Info

Publication number: WO2024213928A1
Application number: PCT/IB2023/058987
Authority: WO
Inventors: Dr. Upendra BEHERA; Dr. Debashis PANDA
Original assignee: Individual
Current assignee: Individual
Priority date: 2023-04-08
Filing date: 2023-09-11
Publication date: 2024-10-17
Anticipated expiration: 2025-10-08

Abstract

The system comprises a common drive mechanism to power the displacers inside the cold head cylinders of pair of linear gears (racks), a circular gear with teeth in its half circle, a common rack holder and a pair of connecting rods attached with the rack holder in opposite directions. The connecting rods are attached to the displacer drive stems of both top and bottom cylinder. The rotary motion of the motor is converted into reciprocating motion with this arrangement, which eventually helps to drive the displacers in both cylinders. This claim is one of the novel ideas to existing drive mechanisms for GM cryocooler. Different configurations of rack holders have been proposed to hold the racks (linear gears) in opposite sides. Journal bearing and sealing arrangements have been made to overcome the leakage of grease from the driving arrangement to the cold heads. Additionally, warm end buffer volumes have been attached at the cold heads to enhance the Carnot efficiency of this system.

Description

A SYSTEM FOR TWIN COLD FINGER MECHANICALLY DRIVEN GM CRYOCOOLER AND DRIVE MECHANISM

FIELD OF THE INVENTION

The present disclosure relates to simple gear drive mechanisms, in more detail, a system for twin cold finger mechanically driven GM cryocooler and drive mechanism.

BACKGROUND OF THE INVENTION

A Gifford-McMahon (GM) cycle cryocooler invented by W.E. Gifford and H.O. McMahon of AD Little Inc. operates on the Gifford-McMahon thermodynamic cycle and generates certain refrigerating effect in cryogenic temperature range by expanding the working fluid through a mechanical displacer. These cryocoolers are mostly used in ground-based applications to meet the cryogenic cooling needs like cooling of superconducting magnets in MRI and HTS motors, HTS generators, HTS transfers, MagLev vehicles, generating a small quantity of liquid nitrogen, liquid hydrogen, liquid oxygen, liquid helium for laboratory scale applications, calibration of cryogenic temperature sensors from 300 K to 4 K, etc. The physical size of a GM cryocooler is more than that of a Stirling cryocooler; however, its operating frequency is less than that of a Stirling cryocooler. One of the major advantages of a GM cryocooler over its peers (i.e., Stirling cryocooler), is it works on the normal air-conditioning compressor with minor structural modification to enhance the rate of heat transfer (by adding an extra heat exchangers), and oil separation mechanisms (i.e., by adding oil-separators). Extra heat is produced during its operation as helium is used as a refrigerant. Since its invention, several structural modifications have been made and novel regenerative materials have been developed to enhance the cooling performance of the GM cryocooler. By using multi-staging approaches and using rare-earth magnetic materials inside the regenerators of the second stage, it’s now possible to achieve a temperature lower than 2.2 K with this cryocooler.

Twin cold finger types of pneumatic drive GM cryocooler has been developed by combining two identical cold heads of commercial Cryomech GM cryocooler to enhance the cooling capacity and make it suitable for the cooling of superconducting coils. Later, some patents used the similar concept to couple the displacers of two cold heads by common driving mechanism consisting of a scotch-yoke drive arrangement to convert the rotating motion of motor into reciprocating motion. In the view of the forgoing discussion, it is clearly portrayed that there is a need to have a system for twin cold finger mechanically driven GM cryocooler and drive mechanism.

SUMMARY OF THE INVENTION

The present disclosure seeks to provide a simple gear drive mechanism by which the rotary motion is converted into reciprocating motion, which eventually helps to drive the displacers inside the respective cold heads of two axially located GM cryocooler.

In an embodiment, a system for twin cold finger mechanically driven GM cryocooler and drive mechanism is disclosed. The system includes a helium compressor unit comprising of a reciprocating/scroll compressor, an oil separator, a helium heat exchanger, an adsorber, an oil heat exchanger and a low- pressure buffer volume.

The system further includes a comprehensive oil separation mechanism adopted in the compressor unit to prevent the entry of oil to the cold head of the cryocooler, which may be solidified because of cryogenic temperature limit and causes the failure of the cold head unit.

The system further includes a valve control unit selected from one or more of a mechanical rotary valve or solenoid valve, wherein the mechanical rotary valve consists of a rotor which is held tightly over a valve plate and rotated with the help of a motor at a speed such that when the rotor rotates over the plate, a flow passage is produced between the high and low-pressure lines of compressor with common gas flow path which allow the high and low-pressure gas to alternatively connect the cold head and generate the oscillating pressure wave, whereas the solenoid valves with appropriate electronic control units also generate the oscillating pressure pulse for the cold head of the cryocooler. The system further includes a cold head comprising of a compression chamber, a displacer housing, a slipper seal, a regenerator, an inlet and an outlet gas flow paths, an inlet and an outlet flow straighteners, a cold head cylinder, a cold heat exchanger/cooling-head, wherein in between the outer surface of the displacer and the inner surface of the cold head cylinder, slipper seals are provided which prevent the flow of working fluid from the compression chamber to the expansion chamber and the slipper seal is preferably an O-ring seal surrounded by C-shaped Teflon seal, which prevent flow of refrigerant from compression to expansion chamber and forces the working fluid to flow through the regenerator flow passage, wherein the displacer is connected with a displacer drive stem, which is further connected to a connecting rod such that through the connecting rod and displacer drive stem power is transmitted from the motor to the displacer, and it reciprocates inside the cold head cylinder, as a result of this, volume of both compression and expansion chamber changes in a cycle, wherein the displacer drive stem journal bearing prevents the leakage of refrigerant and guides the displacer movement.

The system further includes a drive mechanism comprising of a motor, a circular gear, a left linear gear, a right linear gear, a rack holder, a connecting rod, and a connecting rod journal bearing, wherein the connecting rod journal bearing holds the connecting rod in its position and the sealing arrangement prevents the leakage of lubricant, wherein the connecting rod is attached to the displacer drive stem and the rotating motion of the circular gear is converted to reciprocating motion with this arrangement, wherein due to the reciprocating motion of the displacer within the cold head cylinder, the volume of both the expansion chamber and compression chamber changes in a cycle, and a cooling effect is produced, wherein the gear drive mechanism is covered with an outer casing to avoid the entry of dust and make the appearance simple.

An objective of the present disclosure is to develop a gear drive mechanism by which the rotary motion is converted into reciprocating motion, which eventually helps to drive the displacers inside the respective cold heads of two axially located GM cryocooler. Another objective of the present disclosure is to double the cooling capacity.

Another objective of the present disclosure is to modify the single-stage configuration into a two- stage configuration by coupling the warm end of the second stage with the cold end of the first stage via a coupling rod to further reduce the refrigeration temperature. Another objective of the present disclosure is to reduce the compressor load by adopting warm end buffer volumes and this finally helps to enhance the Carnot efficiency of the system. Yet another objective of the present invention is to deliver an expeditious and cost-effective system for twin cold finger mechanically driven GM cryocooler and drive mechanism.

To further clarify advantages and features of the present disclosure, a more particular description of the invention will be rendered by reference to specific embodiments thereof, which is illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail with the accompanying drawings.

BRIEF DESCRIPTION OF FIGURES

These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

Figure 1 illustrates a single-stage single displacer mechanical drive GM cryocooler in accordance with an embodiment of the present disclosure;

Figure 2 illustrates a valve timing diagram for single cylinder single displacer GM cryocooler;

Figure 3 illustrates an expansion chamber P-V diagram;

Figure 4 illustrates comparison between ideal and real/numerical expansion chamber P-V diagram;

Figure 5 illustrates schematics of a single-stage twin cold finger mechanical drive GM cryocooler;

Figure 6 illustrates valve timing diagram for single cylinder twin cold finger GM cryocooler;

Figure 7 illustrates bottom expansion chamber P-V diagram;

Figure 8 illustrates top expansion chamber P-V diagram;

Figure 9 illustrates schematics of a two-stage single cold finger mechanical drive GM cryocooler;

Figure 10 illustrates solidworks drawing with cold cylinder housing; Figure 11 illustrates schematics of a two-stage twin cold finger mechanical drive GM cryocooler;

Figure 12 illustrates solidworks drawing of a two-stage twin cold finger mechanical drive GM cryocooler;

Figure 13 illustrates solidworks drawing of a two-stage twin cold finger mechanical drive GM cryocooler in isometric view;

Figure 14 illustrates different types of drive mechanisms for driving the displacers of both cold fingers based on the present investigation. (A) Rectangle-Rectangle shaped, (B) Rectangle-Elliptical shaped, (C) Rectangle-Octagonal shaped, (D) Octagonal-Octagonal shaped (E) Elliptical-Elliptical shaped;

Figure 15 illustrates schematics of a single-stage twin cold finger mechanical drive GM cryocooler with warm end buffer volumes;

Figure 16 illustrates valve timing diagram for single cylinder twin cold finger GM cryocooler with warm end buffer volumes;

Figure 17 illustrates schematics of a two-stage single cold finger mechanical drive GM cryocooler with warm end buffer volume;

Figure 18 illustrates valve timing diagram for two-stage GM cryocooler with warm end buffer volume;

Figure 19 illustrates schematics of a two-stage twin cold finger mechanical drive GM cryocooler with warm end buffer volumes;

Figure 20 illustrates schematics of a single-stage twin cold finger mechanical drive GM cryocooler with common warm end buffer volume; and

Figure 21 illustrates schematics of a two-stage twin cold finger mechanical drive GM cryocooler with common warm end buffer volume.

Further, skilled artisans will appreciate that elements in the drawings are illustrated for simplicity and may not have necessarily been drawn to scale. For example, the schematics illustrate the method in terms of the most prominent steps involved to help to improve understanding of aspects of the present disclosure. Furthermore, in terms of the construction of the device, one or more components of the device may have been represented in the drawings by conventional symbols, and the drawings may show only those specific details that are pertinent to understanding the embodiments of the present disclosure so as not to obscure the drawings with details that will be readily apparent to those of ordinary skill in the art having benefit of the description herein.

DETAILED DESCRIPTION:

For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated system, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the invention and are not intended to be restrictive thereof. Reference throughout this specification to “an aspect”, “another aspect” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. Thus, appearances of the phrase “in an embodiment”, “in another embodiment” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process or method that comprises a list of steps does not include only those steps but may include other steps not expressly listed or inherent to such process or method. Similarly, one or more devices or sub-systems or elements or structures or components proceeded by "comprises...a" does not, without more constraints, preclude the existence of other devices or other sub-systems or other elements or other structures or other components or additional devices or additional sub-systems or additional elements or additional structures or additional components. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The system, methods, and examples provided herein are illustrative only and not intended to be limiting. Embodiments of the present disclosure will be described below in detail with reference to the accompanying drawings. Referring to Figure 1, a single-stage single displacer mechanical drive GM cryocooler is illustrated in accordance with an embodiment of the present disclosure. The system 100 includes a helium compressor unit comprising of a reciprocating/scroll compressor (1), an oil separator (2), a helium heat exchanger (3), an adsorber (4), an oil heat exchanger (5) and a low-pressure buffer volume (6).

In an embodiment, a comprehensive oil separation mechanism is adopted in the compressor unit to prevent the entry of oil to the cold head of the cryocooler, which may be solidified because of cryogenic temperature and causes the failure of the cold head unit. In an embodiment, a valve control unit is selected, which is either a mechanical rotary valve or solenoid valve, wherein the mechanical rotary valve consists of a rotor which is held tightly over a valve plate and rotated with the help of a motor at a speed such that when the rotor rotates over the plate, a flow passage is produced between the high and low-pressure lines of compressor with common gas flow path (7) which allow the high and low-pressure gas to alternatively connect the cold head and generate the oscillating pressure wave, whereas the solenoid valves with appropriate electronic control units also generate the oscillating pressure pulse for the cold head of the cryocooler. In an embodiment, a cold head comprising of a compression chamber (8), displacer housing (10), a slipper seal (11), a regenerator (13), inlet (9) and outlet (15) gas flow paths, inlet (12) and outlet (14) flow straighteners, a cold head cylinder (17), cold heat exchanger/cooling-head (18), wherein in between the outer surface of the displacer and the inner surface of the cold head cylinder, slipper seals are provided which prevent the flow of working fluid from the compression chamber to the expansion chamber and the slipper seal is preferably an O-ring seal surrounded by C-shaped Teflon seal, which prevent flow of refrigerant from compression to expansion chamber and forces the working fluid to flow through the regenerator flow passage, wherein the displacer is connected with a displacer drive stem, which is further connected to a connecting rod such that through the connecting rod and displacer drive stem power is transmitted from the motor to the displacer, and it reciprocates inside the cold head cylinder, as a result of this, volume of both compression and expansion chamber changes in a cycle, wherein the displacer drive stem journal bearing prevents the leakage of refrigerant and guides the displacer movement.

In an embodiment, a drive mechanism comprising of a motor (25), circular gear (23), left linear gear (22), right linear gear (27), rack holder (24), connecting rod (21), and connecting rod journal bearing (26), wherein the connecting rod journal bearing holds the connecting rod in its position and the sealing arrangement prevents the leakage of lubricant, wherein the connecting rod is attached to the displacer drive stem and the rotating motion of the circular gear is converted to reciprocating motion with this arrangement, wherein due to the reciprocating motion of the displacer within the cold head cylinder, the volume of both the expansion chamber and compression chamber changes in a cycle, and a cooling effect is produced, wherein the gear drive mechanism is covered with an outer casing to avoid the entry of dust and make the appearance simple. In another embodiment, the valves VI and V2 are situated at the high- pressure line and low-pressure line of the compressor, respectively, when valve VI is open, the high- pressure refrigerant (helium gas) enters to the gas chambers of cold head from compressor, and when valve V2 is opened, the expanded refrigerant returns back from the gas chambers of cold head to the compressor, wherein the refrigerant flows from valve VI and V2 to the cold head by common gas flow path.

In another embodiment, a typical twin-cold finger mechanical drive GM cryocooler, which consists of a top cold head and a bottom cold head, wherein the structural configuration of each cold head is identical with the single-stage configuration, wherein the bottom cold head consists of a bottom common gas flow path (7b), bottom compression chamber (8b), bottom inlet gas flow path (9b), bottom displacer housing (10b), bottom slipper seal (11b), bottom inlet flow straightener (12b), bottom regenerator (13b), bottom outlet flow straightener (14b), bottom outlet gas flow path (15b), bottom expansion chamber (16b), bottom cylinder (17b) and bottom cooling head (18b), wherein the bottom displacer is connected with bottom connecting rod (21b) by means of bottom displacer drive stem (19b), and bottom displacer drive stem journal bearing (20b) holds it in its position and avoids the leakage of refrigerant, wherein the top cold head consists of a top common gas flow path (7t), top compression chamber (8t), top inlet gas flow path (9t), top displacer housing (lOt), top slipper seal (1 It), top inlet flow straightener (12t), top regenerator (13t), top outlet flow straightener (14t), top outlet gas flow path (15t), top expansion chamber (16t), top cylinder ( 17t) and top cooling head ( 18t).

In another embodiment, the top displacer is connected with top connecting rod (2 It) by means of top displacer drive stem (19t), and top displacer drive stem journal bearing (20t) holds it in its position and avoids the leakage of the refrigerant, wherein the bottom cold head is connected to the compressor system by valve VI and V2, wherein the top cold head is connected to the compressor system by valve V3 and V4, wherein VI and V3 are intake valves for bottom cold head and top cold head, V2 and V4 are exhaust valves for bottom cold head and top cold head, wherein when bottom cylinder is connected to the discharge side of compressor by VI, top cylinder should be connected to the suction side of compressor by V4, wherein when the bottom cylinder is connected to the suction side of compressor by V2, top cylinder is connected with the discharge side by V3 such that when one cylinder goes for charging process, the other cylinder goes for discharging process and vice-versa, in such a manner, the compressed working fluid passes from the compressed gas supply unit to the cold heads alternatively.

In another embodiment, a two-stage single cold-head GM cryocooler, in which the hot end of the second stage is coupled with the cold end of the first stage such that the working fluid gets cooled from room temperature to a certain temperature in the first stage and then to a further low temperature in the second-stage expansion chamber, wherein the first stage acts like a precooling of refrigerant before entering the second stage and the second-stage components are second-stage inlet gas flow path (31), second-stage inlet flow-straightener (34), second-stage regenerator (35), second-stage outlet flowstraightener (37), second-stage outlet gas flow path (38), and second-stage expansion chamber (39), wherein the gas gets expanded in the expansion chamber of the second-stage and cooling effect is provided by second-stage cooling head (40) and the second-stage slipper seal (32) prevents leakage of refrigerant and is placed between second-stage displacer housing (33) second-stage cylinder (36).

In another embodiment, the working fluid passes from the first stage expansion chamber to the second stage expansion chamber via second-stage inlet gas flow path, second-stage inlet flow-straightener, second-stage regenerator, second-stage outlet flow-straightener, second-stage outlet gas flow path, wherein after expansion of the refrigerant in the bottom second stage expansion chamber, a cooling effect is produced which is absorbed at bottom second-stage cooling head thereby the expanded refrigerant then returns from the second-stage expansion chamber to the first-stage expansion chamber by second-stage outlet gas flow path, second-stage outlet flow-straightener, second-stage regenerator, second-stage inlet flow-straightener, and second-stage inlet gas flow path, wherein the hot end of second stage displacer is connected to the bottom end of first stage displacer by a coupling rod (29) and the coupling rod is attached to the first stage displacer and second stage displacer by first-stage connecting pin (28), and second-stage connecting pin (30) respectively such that the motion is transferred from first stage to second stage, wherein the second stage displacer, as a result, undergoes reciprocating motion inside the second stage cylinder and the volume of second stage expansion chamber changes in a cycle. In another embodiment, the second-stage components of both top and bottom cold heads are attached to the cold end of their respective first stages via a coupling rod.

In another embodiment, the first stage bottom cold head consist of bottom common gas flow path (7b), bottom first-stage compression chamber (8b), bottom first-stage inlet gas flow path (9b), bottom first- stage displacer housing (10b), bottom first-stage slipper seal (11b), bottom first-stage inlet flowstraightener (12b), bottom first-stage regenerator (13b), bottom first-stage outlet flow-straightener (14b), bottom first-stage outlet gas flow path (15b), bottom first-stage expansion chamber (16b), bottom first- stage cylinder (17b), bottom first-stage cooling head (18b), bottom displacer drive stem (19b), bottom displacer drive stem journal bearing (20b), bottom connecting rod (21b), left linear gear (22), circular gear (23), rack holder (24), motor (25), bottom connecting rod journal bearing (26b), and right linear gear (27), wherein the displacer is driven inside the cylinder by the driving mechanism as discussed in the case of single displacer twin cold finger mechanically driven GM cryocooler.

In another embodiment, the warm end of second stage displacer of bottom cylinder is connected with the cold end of first stage displacer by a bottom coupling rod (29b) and the bottom coupling rod is attached to the first stage displacer and second stage displacer by bottom first stage connecting pin (28b) and bottom second-stage connecting pin (30b) such that the motion is transferred from first-stage to second stage, wherein the second stage displacer, as a result, undergoes reciprocating motion inside the second stage cylinder and the volume of second stage expansion chamber changes in a cycle, wherein a bottom second-stage slipper seal (32b) is placed between the outer diameter second-stage displacer housing (33b) and inner diameter of bottom second-stage cylinder (36b) to prevent the leakage of refrigerant, wherein the working fluid passes from the bottom first stage expansion chamber (16b) to the bottom second stage expansion chamber (39b) via bottom second-stage inlet gas flow path (31b), bottom second-stage inlet flow-straightener (34b), bottom second-stage regenerator (35b), bottom second-stage outlet flowstraightener (37b), and bottom second-stage outlet gas flow path (38b). In another embodiment, after expansion of the refrigerant in the bottom second stage expansion chamber, a cooling effect is produced which is absorbed at bottom second-stage cooling head (40b) and the expanded refrigerant returns from the bottom second-stage expansion chamber to the compressor via bottom second-stage outlet gas flow path, bottom second-stage outlet flow-straightener, bottom second- stage regenerator, bottom second-stage inlet flow-straightener, bottom second-stage inlet gas flow path, bottom first-stage expansion chamber, bottom first-stage outlet flow-straightener, bottom first stage regenerator, bottom first-stage inlet flow-straightener, bottom first-stage inlet gas flow path, bottom first- stage compression chamber, bottom common gas flow path, valve V2, low-pressure buffer, and compressor suction line.

A simple refrigerator consists of a compressor, a condenser, an expansion device, and an evaporator. The cryogenic refrigeration/ liquefaction cycles are capable to achieve a cryogenic temperature (typically < 120 K) for many industrial applications. The cryogenic refrigeration cycles are categorized into a regenerative cryocooler and a recuperative cryocooler based on the mode of heat exchange. In the recuperative cryogenic refrigerator, a major portion of heat transfer happens by a recuperative heat exchanger and gas gets expanded either by a JT valve or by a turboexpander/piston-cylinder expander. The expansion process in a JT valve is isenthalpic, which is isentropic in a turboexpander. Some cryogenic liquefaction cycles contain both turboexpanders and JT valves.

In regenerative cryocoolers, the maximum percentage of heat exchange happens through the fixed bed regenerator, although recuperative heat exchangers are placed for minor heat interaction with the surroundings. The recuperator used in recuperative cryocoolers is a two-fluid heat exchanger, in which hot fluid flows through one flow path, and cold fluid flow through other flow paths. Therefore, the flow of working fluid is steady in recuperative cryocoolers and is oscillating in regenerative cryocoolers. Stirling cryocooler, GM cryocooler, VM cryocooler, and pulse tube cryocoolers are some of the basic configurations of regenerative cryocoolers, whereas the JT cryocooler, Brayton cryocooler, Claude cycle, Kapitza cycle, and Collin cycle based cryocoolers are some basic configurations of recuperative cryocoolers.

The single-stage GM cryocooler consists of a helium compressor, a valve control unit (either a mechanical rotary valve or solenoid valve), and a cold head as shown in Figure 1. A comprehensive oil separation mechanism is adopted in the compressor unit to prevent the entry of oil to the cold head of the cryocooler, which may be solidified because of cryogenic temperature and causes the failure of the cold head unit. The helium compressor unit consists of a reciprocating/scroll compressor (1), an oil separator (2), a helium heat exchanger (3), an adsorber (4), an oil heat exchanger (5) and a low-pressure buffer volume (6) as shown in Figure 1. The valve control unit may be a mechanical rotary valve or electronically controlled solenoid valves. The mechanical rotary valve consists of a rotor which is held tightly over a valve plate and rotated with the help of a motor at a speed. When the rotor rotates over the plate, a flow passage is produced between the high and low-pressure lines of compressor with common gas flow path (7) which allow the high and low-pressure gas to alternatively connect the cold head and generate the oscillating pressure wave. Solenoid valves with appropriate electronic control units also generate the oscillating pressure pulse for the cold head of the cryocooler. Here, valves VI and V2 are situated at the high-pressure line and low-pressure line of the compressor respectively. When valve VI is open, the high- pressure refrigerant (helium gas) enters to the gas chambers of cold head from compressor, and when valve V2 is opened, the expanded refrigerant returns back from the gas chambers of cold head to the compressor. Refrigerant flows from valve VI and V2 to the cold head by common gas flow path (7). A typical cold head consists of a compression chamber (8), displacer housing (10), a slipper seal (11), a regenerator (13), inlet (9) and outlet (15) gas flow paths, inlet (12) and outlet (14) flow straighteners, a cold head cylinder (17), cold heat exchanger/cooling-head (18). Normally, the displacer is made hollow shaped and is filled with regenerator screen meshes to make the overall configuration sophisticated, simple, and reliable. Holes are made on the displacer at its top and bottom end through which gas flows into the regenerator and out of the regenerator. Typically, axial holes (9) are made at the inlet to the displacer flow passage and radial holes (15) are made at the outlet passage as shown in Figure 1. In between the outer surface of the displacer and the inner surface of the cold head cylinder, slipper seals (11) are provided which prevent the flow of working fluid from the compression chamber to the expansion chamber. The slipper seal in present invention is an O-ring seal surrounded by C-shaped Teflon seal, which prevent flow of refrigerant from compression to expansion chamber. This forces the working fluid to flow through the regenerator flow passage. The gap between the top of the displacer and the cold head cylinder housing is called the compression chamber (8). The space between the bottom of the displacer and the bottom cylinder is the expansion chamber (16). The displacer is connected with a displacer drive stem (19), which is further connected to a connecting rod (21). Through the connecting rod and displacer drive stem power is transmitted from the motor to the displacer, and it reciprocates inside the cold head cylinder, as a result of this, volume of both compression and expansion chamber changes in a cycle. The displacer drive stem journal bearing (20) prevents the leakage of refrigerant and guides the displacer movement. A motor (25), circular gear (23), left linear gear (22), right linear gear (27), rack holder (24), connecting rod (21), and connecting rod journal bearing (26) constitute the drive mechanism of the cryocooler. The connecting rod journal bearing holds the connecting rod in its position and the sealing arrangement prevents the leakage of lubricant. The connecting rod is attached to the displacer drive stem. The rotating motion of the circular gear is converted to reciprocating motion with this arrangement. Due to the reciprocating motion of the displacer within the cold head cylinder, the volume of both the expansion chamber and compression chamber changes in a cycle, and a cooling effect is produced. The gear drive mechanism is covered with an outer casing to avoid the entry of dust and make the appearance simple. Individual components of Figure 1 are written below.

[1: Compressor, 2: Oil separator, 3: Heat exchanger (Helium), 4: Adsorber, 5: Heat exchanger (Oil), 6: Low-pressure buffer volume, 7: Common gas flow path, 8: Compression chamber, 9: Inlet gas flow path, 10: Displacer housing, 11: Slipper seal, 12: Inlet flow-straightener, 13: Regenerator, 14: Outlet flow-straightener, 15: Outlet gas flow path, 16: Expansion chamber, 17: Cylinder, 18: Cooling head, 19: Displacer drive stem, 20: Displacer drive stem journal bearing, 21: Connecting rod, 22: Left linear gear, 23: Circular gear, 24: Rack housing, 25: Motor, 26: Connecting rod journal bearing, 27: Right linear gear, VI: Intake valve, V2: Exhaust valve]

Figure 2 illustrates a valve timing diagram for single cylinder single displacer GM cryocooler. Figure 2 illustrates the valve-timing arrangement of the cryocooler illustrated in Figure 1. The opening durations of individual valves are represented by bold lines in a cycle. During the charging period, VI opens, and during the discharging period, V2 opens. There are certain durations in which both VI and V2 are closed and this is called a waiting period. The cooling mechanism of a single-stage GM cryocooler is explained as follows:

At the start of the cycle, the displacer is near the bottom dead center, thus the volume of the expansion chamber is minimum, and the volume of the compression chamber is maximum. The intake valve VI opens, thus high-pressure working gas (helium) enters into the compression chamber from the discharge port of the compressor via the discharge line. The gas passes through an oil separator, helium heat exchanger, and adsorber to remove oil and impurities from helium. The displacer starts moving from the bottom dead center (BDC) to the top dead center (TDC) due to the movement of the displacer by a drive mechanism, thus compressed gas starts flowing from the compression chamber to the expansion chamber via the inlet gas flow path, inlet flow-straightener, regenerator, outlet flow-straightener, and outlet outflow path. During this, the gas rejects heat to the regenerator matrix and gets cooled. The inlet valve closed and gas flowed between the compressor and cold head ends. The exhaust valve V2 opens, gas is then expanded inside the expansion chamber, and the displacer starts moving downward, as a result of this volume of the expansion chamber decreases and the volume of the compression chamber increases. The gas while flowing through the regenerator absorbs heat from the regenerator matrix and flows to the compressor. Expanded refrigerant enters the compressor from the expansion chamber via the outlet gas flow path, outlet flow-straightener, regenerator, inlet flow-straightener, inlet gas flow path, compression chamber, common gas flow path, and suction line of compressor. One cycle ends.

Figure 3 illustrates an expansion chamber P-V diagram. The P-V diagram is drawn in Figure 3, and it completes in four steps as discussed below.

Process ‘l’-‘2’: VI opens, and high-pressure gas enters the expansion chamber. Thus, the pressure value increases.

Process ‘2’-‘3’: VI opens and the displacer starts moving from the bottom dead center to the top dead center. Thus, the volume of the expansion chamber increases.

Process ‘3 ’-‘4’: V2 opens and low-pressure expanded refrigerant flows from the expansion chamber to the compressor.

Process ‘4’-‘l’: Valve V2 opens and the displacer starts moving from the top dead center to the bottom dead center. Thus, the volume of the expansion chamber decreases and one cycle gets over. The gross refrigeration capacity/ expansion work is calculated as the enclosed area under the P-V diagram. Figure 4 illustrates comparison between ideal and real expansion chamber P-V diagram. Figure 4 compares the ideal P-V diagram with a real/numerical P-V diagram, which considers the displacement path of the displacer. The ideal P-V diagram is a rectangular shape, whereas, the real P-V diagram is oval shape. The real P-V diagram is drawn based on the mathematical model discussed in Panda et al. [Vacuum, 199, 110938],

Figure 5 illustrates schematics of a single-stage twin cold finger mechanical drive GM cryocooler using this novel gear drive mechanism. Figure 5 shows a typical twin-cold finger mechanical drive GM cryocooler, which consists of a top cold head and a bottom cold head. The structural configuration of each cold head (i.e. either bottom cold head or top cold head) is identical with the single-stage configuration. The bottom cold head consists of a bottom common gas flow path (7b), bottom compression chamber (8b), bottom inlet gas flow path (9b), bottom displacer housing (10b), bottom slipper seal (11b), bottom inlet flow straightener (12b), bottom regenerator (13b), bottom outlet flow straightener (14b), bottom outlet gas flow path (15b), bottom expansion chamber (16b), bottom cylinder (17b) and bottom cooling head (18b). The bottom displacer is connected with bottom connecting rod (21b) by means of bottom displacer drive stem (19b), and bottom displacer drive stem journal bearing (20b) holds it in its position and avoids the leakage of refrigerant. Likewise, the top cold head consists of a top common gas flow path (7t), top compression chamber (8t), top inlet gas flow path (9t), top displacer housing (lOt), top slipper seal (l it), top inlet flow straightener (12t), top regenerator ( 13t), top outlet flow straightener (14t), top outlet gas flow path (15t), top expansion chamber ( 16t), top cylinder ( 17t) and top cooling head ( 18t). The top displacer is connected with top connecting rod (2 It) by means of top displacer drive stem ( 19t), and top displacer drive stem journal bearing (20t) holds it in its position and avoids the leakage of the refrigerant. Both top (2 It) and bottom (21b) connecting rods are made on the common rack holder in opposite directions as shown in Figure 5. The common drive mechanism is similar in structural configuration as discussed earlier in Figure 1. The bottom cold head is connected to the compressor system by valve VI and V2. The top cold head is connected to the compressor system by valve V3 and V4. In another way, VI and V3 are intake valves for bottom cold head and top cold head, V2 and V4 are exhaust valves for bottom cold head and top cold head. Individual components of Figure 5 are written below.

[1: Compressor, 2: Oil separator, 3: Heat exchanger (Helium), 4: Adsorber, 5: Heat exchanger (Oil), 6: Low-pressure buffer volume, 7b: Bottom common gas flow path, 7t: Top common gas flow path, 8b: Bottom compression chamber, 8t: Top compression chamber, 9b: Bottom inlet gas flow path, 9t: Top inlet gas flow path, 10b: Bottom displacer housing, lOt: Top displacer housing, 11b: Bottom slipper seal, l it: Top slipper seal, 12b: Bottom inlet flow-straightener, 12t: Top inlet flow-straightener, 13b: Bottom regenerator, 13t: Top regenerator, 14b: Bottom outlet flow-straightener, 14t: Top outlet flow-straightener, 15b: Bottom outlet gas flow path, 15t: Top outlet gas flow path, 16b: Bottom expansion chamber, 16t: Top expansion chamber, 17b: Bottom cylinder, 17t: Top cylinder, 18b: Bottom cooling head, 18t: Top cooling head, 19b: Bottom displacer drive stem, 19t: Top displacer drive stem, 20b: Bottom displacer drive stem journal bearing, 20t: Top displacer drive stem journal bearing, 21b: Bottom connecting rod, 2 It: Top connecting rod, 22: left linear gear, 23: circular gear, 24: Rack housing, 25: Motor, 26b: Bottom connecting rod journal bearing, 26t: Top connecting rod journal bearing, 27: Right linear gear, VI: Bottom intake valve, V2: Bottom exhaust valve, V3: Top intake valve, V4: Top exhaust valve]

Figure 6 illustrates valve timing diagram for single cylinder twin cold finger GM cryocooler. The valve-timing chart is drawn in Figure 6 and is identical with U.S. Patent No. 10184693, and only difference lie in the drive mechanism of displacer. It is seen that the gas charging procedures for both cylinders happen in mutually opposite phases. When bottom cylinder is connected to the discharge side of compressor by VI, top cylinder should be connected to the suction side of compressor by V4. Similarly, when bottom cylinder is connected to the suction side of compressor by V2, top cylinder will be connected with the discharge side by V3. Thus, when one cylinder goes for charging process, the other cylinder goes for discharging process and vice-versa. In this manner, the compressed working fluid passes from the compressed gas supply unit to the cold heads alternatively.

Figure 7 illustrates bottom expansion chamber P-V diagram. Depending upon this, the expansion chamber diagram of the twin cold-finger mechanical drive GM cryocooler is drawn in Figures 7 and 8 for bottom and top cold heads respectively. Both P-V diagrams are identical to Figure 3. The gas flow processes in Figure 8 are opposite due to the opposite phase of operation. In absence of any waiting period, the ideal PV-diagram looks like a purely rectangular shape, however, slanted cut-offs are visible in the topright and bottom-left comers in the presence of a certain waiting time. The presence of a certain optimal waiting time reduces the cooling capacity but improves the specific cooling capacity. More detailed theoretical descriptions on valve-timing are available in Panda et al. [Vacuum, 199, 110938],

The working mechanism of the twin cold-finger GM cryocooler, valve-timing operation, and the thermodynamic cycle is explained as follows:

When the bottom cold head is connected with the high-pressure line of the compressor, high- pressure compressed working fluid enters the bottom compression chamber (8b) of the cold head. During this period, the bottom displacer (10b) is either at the bottom dead center or in the vicinity of the bottom dead center. Thus, the pressure value increases from PL to PH as shown by lines ‘lb’-‘2b’ in the P-V diagram in Figure 7. Due to the motion of the bottom displacer in the vertically upward direction, the volume of the bottom expansion chamber increases, and the volume of the bottom compression chamber decreases as shown by lines ‘2b’-‘3b’ in the P-V diagram. During this movement, working fluid at high- pressure flows to the bottom expansion chamber (16b) from the bottom compression chamber (8b) by bottom inlet gas flow path (9b), bottom inlet flow-straightener (12b), bottom regenerator (13b), bottom outlet flow-straightener (14b), bottom outlet gas flow path (15b). Port 9b and port 15b are made on the axial and radial directions of the displacer respectively. Thus, inflow to the bottom regenerator from the bottom compression chamber is in the axial direction, and outflow from the bottom regenerator to the bottom expansion chamber is in the radial direction. After the closing of VI, bottom intake period finished, and the bottom waiting period starts, during which gas flow is prevented between the bottom cold head and the helium compressor. After the bottom waiting period, the bottom exhaust valve V2 opens, therefore the bottom cold head is coupled with the low-pressure line of the compressor, the gas expands, its pressure drops, and starts moving out from the bottom expansion chamber. Thus, pressure drops from PH to PL, shown by line ‘3b’-‘4b’ as illustrated in Figure 7. Subsequently, the displacer moves from bottom TDC to bottom BDC in a vertically downward direction (plotted by path ‘4b’-‘ lb’ in Figure 7) and this completes one working cycle for the bottom cylinder. In between the closing of the bottom exhaust valve V2 and bottom intake valve VI, gas flow is prevented in between the bottom cylinder and helium compressor, however gas may pass from bottom compression chamber to bottom expansion chamber because of displacer movement. The opening closing angles of both top and bottom cylinder may or may not be identical.

Figure 8 illustrates top expansion chamber P-V diagram. The working mechanism, the thermodynamic cycle of the top cold finger happens in the exact opposite direction as both the top displacer and bottom displacer are connected with the top and bottom connecting rods of the common drive mechanism. At the beginning of the cycle, the top displacer is situated at the top TDC (or, in the vicinity of top TDC), the volume of the top compression chamber is minimum and volume of the top expansion chamber is maximum, and the exhaust valve is opened. This is because, at the beginning of the cycle, bottom displacer is near bottom BDC. This is represented by point ‘ It’ in the P-V diagram in Figure 8. During this time, the top exhaust valve opens, thus pressure drops and this is plotted by ‘ lt’-‘2t’ in the P-V diagram in Figure 8. When the bottom displacer moves up, the top displacer travels from the top TDC to the top BDC, as a result, the volume of the top expansion chamber decreases and the volume of the top compression chamber increases. This is shown by ‘2t’ to ‘3t’ in Figure 8. During this top exhaust period, working gas flows from the top expansion chamber ( 16t) to the helium compressor via top outlet gas flow path (15t), top outlet flow-straightener (14t), top regenerator (13t), top inlet flow-straightener (12t), top inlet gas flow path (9t), top compression chamber (8t), top common gas flow path (7t) and valve V4. Once, the displacer reaches the top BDC, the bottom displacer reaches the top TDC. During this period, valve V3 opens, and high-pressure compressed gas starts flowing from the helium compressor to the top compression chamber by V3 and top common gas flow path (7t). Pressure starts increasing and this is shown by line ‘3t’-‘4t’ in the P-V diagram in Figure 8. As the volume of the top compression chamber is maximum, maximum gas gets accumulated at this position. Once, the displacer starts movement from the top BDC to the top TDC volume of the top expansion chamber increases and volume of the top compression chamber decreases (shown by ‘3t’-‘4t’), and high-pressure gas is allowed to flow from the top compression chamber (8t) to the top expansion chamber (16t) by top inlet gas flow path (9t), top inlet flow-straightener (12t), top regenerator (13t), top outlet flow-straightener (14t), bottom outlet gas flow path ( 15t), and once cycle gets completed for the top cold head. The cryocooler continuously goes through several such processes to generate the cooling effect, which is later absorbed by external objects through the top cold stage and bottom cold stage. Thus, the gross cooling capacity of bottom cold head is the enclosed area under lb-2b-3b-4b and lb, and gross cooling capacity of top cold head is the enclosed area under lt-2t-3t-4t and It in absence of any waiting periods. In presence of certain waiting period, area is lb*-2b-2b*-3b*-4b-4b*-lb* for bottom cold head, and It*-2t-2t*-3t*-4t-lt* for top cold head. Figure 9 illustrates schematics of a two-stage single cold finger mechanical drive GM cryocooler. Figure 9 illustrates the schematics of a two-stage single cold-head GM cryocooler, in which the hot end of the second stage is coupled with the cold end of the first stage. Thus, the working fluid gets cooled from room temperature to a certain temperature in the first stage and then to a further low temperature in the second-stage expansion chamber. In other words, the first stage acts like a precooling of refrigerant before entering the second stage. The components of the first-stage are similar to that of Figure 1. The second- stage components are second-stage inlet gas flow path (31), second-stage inlet flow-straightener (34), second-stage regenerator (35), second-stage outlet flow-straightener (37), second-stage outlet gas flow path (38), and second-stage expansion chamber (39). The gas gets expanded in the expansion chamber of the second-stage and cooling effect is provided by second-stage cooling head (40). The second-stage slipper seal (32) prevents leakage of refrigerant and is placed between second-stage displacer housing (33) second-stage cylinder (36). The working fluid passes from the first stage expansion chamber to the second stage expansion chamber via second-stage inlet gas flow path, second-stage inlet flow-straightener, second-stage regenerator, second-stage outlet flow-straightener, second-stage outlet gas flow path. After expansion of the refrigerant in the bottom second stage expansion chamber, a cooling effect is produced which is absorbed at bottom second-stage cooling head (40). Expanded refrigerant then returns from the second-stage expansion chamber to the first-stage expansion chamber by second-stage outlet gas flow path, second-stage outlet flow-straightener, second-stage regenerator, second-stage inlet flow-straightener, and second-stage inlet gas flow path. The gas flows from first stage expansion chamber to the compressor in the similar way as discussed in Figure 1. The hot end of second stage displacer is connected to the bottom end of first stage displacer by a coupling rod (29). The coupling rod is attached to the first stage displacer and second stage displacer by first-stage connecting pin (28), and second-stage connecting pin (30) respectively. Thus, the motion can be transferred from first stage to second stage. The second stage displacer, as a result, undergoes reciprocating motion inside the second stage cylinder and the volume of second stage expansion chamber changes in a cycle. The valve opening-closing intervals are similar to that of single-stage GM cryocooler shown in Figure 2. Individual components of Figure 9 are written below.

[1: Compressor, 2: Oil separator, 3: Heat exchanger (Helium), 4: Adsorber, 5: Heat exchanger (Oil), 6: Low-pressure buffer volume, 7: Common gas flow path, 8: First-stage compression chamber, 9: First-stage inlet gas flow path, 10: First-stage displacer housing, 11: First-stage slipper seal, 12: First-stage inlet flow-straightener, 13: First-stage regenerator, 14: First-stage outlet flow-straightener, 15: First-stage outlet gas flow path, 16: First-stage expansion chamber, 17: First-stage cylinder, 18: First-stage cooling head, 19: Displacer drive stem, 20: Displacer drive stem journal bearing, 21: Connecting rod, 22: Left linear gear, 23: Circular gear, 24: Rack housing, 25: Motor, 26: Connecting rod journal bearing, 27: Right linear gear, 28: First-stage connecting pin, 29: Coupling rod, 30: Second-stage connecting pin, 31: Second- stage inlet gas flow path, 32: Second-stage slipper seal, 33: Second-stage displacer housing, 34: Second- stage inlet flow-straightener, 35: Second-stage regenerator, 36: Second-stage cylinder, 37: Second-stage outlet flow-straightener, 38: Second-stage outlet gas flow path, 39: Second-stage expansion chamber, 40: Second-stage cooling head,Vl: Intake valve, V2: Exhaust valve]

Figure 10 illustrates solidworks drawing with cold cylinder housing. Figure 10 illustrates a typical solid model for a two-stage mechanical drive GM cryocooler and its cut-section view.

Figure 11 illustrates schematics of a two-stage twin cold finger mechanical drive GM cryocooler. Figure 11 illustrates the schematics of a two-stage twin cold finger mechanical drive GM cryocooler. The second-stage components of both top and bottom cold heads are attached to the cold end of their respective first stages via a coupling rod. Since, the first stage components are similar with Figure 5; these are not discussed once again. The second stage components and their connection modes are discussed in detail here. The first stage bottom cold head consist of bottom common gas flow path (7b), bottom first-stage compression chamber (8b), bottom first-stage inlet gas flow path (9b), bottom first-stage displacer housing (10b), bottom first-stage slipper seal (11b), bottom first-stage inlet flow-straightener (12b), bottom first- stage regenerator (13b), bottom first-stage outlet flow-straightener (14b), bottom first-stage outlet gas flow path (15b), bottom first-stage expansion chamber (16b), bottom first-stage cylinder (17b), bottom first- stage cooling head (18b), bottom displacer drive stem (19b), bottom displacer drive stem journal bearing (20b), bottom connecting rod (21b), left linear gear (22), circular gear (23), rack holder (24), motor (25), bottom connecting rod journal bearing (26b), and right linear gear (27). The displacer is driven inside the cylinder by the driving mechanism as discussed in the case of single displacer twin cold finger mechanically driven GM cryocooler. The warm end of second stage displacer of bottom cylinder is connected with the cold end of first stage displacer by a bottom coupling rod (29b). The bottom coupling rod is atached to the first stage displacer and second stage displacer by botom first stage connecting pin (28b) and botom second-stage connecting pin (30b). Thus, the motion can be transferred from first-stage to second stage. The second stage displacer, as a result, undergoes reciprocating motion inside the second stage cylinder and the volume of second stage expansion chamber changes in a cycle. A botom second- stage slipper seal (32b) is placed between the outer diameter second-stage displacer housing (33b) and inner diameter of botom second-stage cylinder (36b) to prevent the leakage of refrigerant. The working fluid passes from the botom first stage expansion chamber (16b) to the botom second stage expansion chamber (39b) via botom second-stage inlet gas flow path (31b), botom second-stage inlet flowstraightener (34b), botom second-stage regenerator (35b), botom second-stage outlet flow-straightener (37b), and botom second-stage outlet gas flow path (38b). After expansion of the refrigerant in the botom second stage expansion chamber, a cooling effect is produced which is absorbed at botom second-stage cooling head (40b). After expansion, the expanded refrigerant returns from the botom second-stage expansion chamber to the compressor via botom second-stage outlet gas flow path, botom second-stage outlet flow-straightener, botom second-stage regenerator, botom second-stage inlet flow-straightener, botom second-stage inlet gas flow path, botom first-stage expansion chamber, botom first-stage outlet flow path, botom first-stage outlet flow-straightener, botom first stage regenerator, botom first-stage inlet flow-straightener, botom first-stage inlet gas flow path, botom first-stage compression chamber, botom common gas flow path, valve V2, low-pressure buffer, and compressor suction line.

The top cold head of the twin-cold finger mechanical drive GM cryocooler also contains similar components that of botom cylinder, only the difference lie in its orientation; i.e. it is kept in mutually opposite directions. Thus, when the working fluid undergoes a charging process in the expansion chambers of botom cylinder, it will undergo discharging process in the top cylinder. This can be adjusted by adjusting the opening closing intervals of valve VI, V2, V3 and V4 as explained in Figure 6 for single- stage twin cold finger mechanical drive GM cryocooler. The gas flows from the compressor to the top second-stage expansion chamber by discharge pipe, valve V3, top common gas flow path, top first-stage compression chamber, top first-stage inlet flow path, top first-stage inlet flow-straightener, top first-stage regenerator, top first-stage outlet flow-straightener, top first-stage outlet flow path, top first-stage expansion chamber, top second-stage inlet flow path, top second-stage inlet flow-straightener, top second- stage regenerator, top second-stage outlet flow-straightener, and top-second-stage outlet flow path. After expansion, the expanded gas at a lower pressure returns from the second-stage expansion chamber to the compressor via the top-second-stage outlet flow path, top second-stage outlet flow-straightener, top second-stage regenerator, top second-stage inlet flow-straightener, top second-stage inlet flow path, top first-stage expansion chamber, top first-stage outlet flow path, top first-stage outlet flow-straightener, top first-stage regenerator, top first-stage inlet flow-straightener, top first-stage inlet flow path, top first-stage compression chamber, top common gas flow path, valve V4, low-pressure buffer and compressor suction line. In this way the gas parcel finishes one cycle. The refrigeration effect is produced in the first-stages and second-stages of both botom and top cold heads and these are absorbed by the respective thermal stages. Cooling power at botom first and second-stage expansion chamber will be absorbed at botom first and second-stage thermal stages, and top first and second-stage expansion chamber will be absorbed at top first and second-stage thermal stages. Individual components of Figure 11 are writen below.

[1: Compressor, 2: Oil separator, 3: Heat exchanger (Helium), 4: Adsorber, 5: Heat exchanger (Oil), 6: Low-pressure buffer volume, 7b: Botom common gas flow path, 7t: Top common gas flow path, 8b: Botom first-stage compression chamber, 8t: Top first-stage compression chamber, 9b: Botom first- stage inlet gas flow path, 9t: Top first-stage inlet gas flow path, 10b: Botom first-stage displacer housing, lOt: Top first-stage displacer housing, 11b: Botom first-stage slipper seal, l it: Top first-stage slipper seal, 12b: Botom first-stage inlet flow-straightener, 12t: Top first-stage inlet flow-straightener, 13b: Botom first-stage regenerator, 13t: Top first-stage regenerator, 14b: Botom first-stage outlet flow-straightener, 14t: Top first-stage outlet flow-straightener, 15b: Botom first-stage outlet gas flow path, 15t: Top first- stage outlet gas flow path, 16b: Botom first-stage expansion chamber, 16t: Top first-stage expansion chamber, 17b: Botom first-stage cylinder, 17t: Top first-stage cylinder, 18b: Botom first-stage cooling head, 18t: Top first-stage cooling head, 19b: Botom displacer drive stem, 19t: Top displacer drive stem, 20b: Botom displacer drive stem journal bearing, 20t: Top displacer drive stem journal bearing, 21b: Botom connecting rod, 2 It: Top connecting rod, 22: Left linear gear, 23: Circular gear, 24: Rack housing, 25: Motor, 26b: Botom connecting rod journal bearing, 26t: Top connecting rod journal bearing, 27: Right linear gear, 28b: Botom first-stage connecting pin, 28t: Top first-stage connecting pin, 29b: Botom coupling rod, 29t: Top coupling rod, 30b: Botom second-stage connecting pin, 30t: Top second-stage connecting pin, 31b: Botom second-stage inlet gas flow path, 3 It: Top second-stage inlet gas flow path, 32b: Botom second-stage slipper seal, 32t: Top second-stage slipper seal, 33b: Botom second-stage displacer housing, 33t: Top second-stage displacer housing, 34b: Botom second-stage inlet flowstraightener, 34t: Top second-stage inlet flow-straightener, 35b: Botom second-stage regenerator, 35t: Top second-stage regenerator, 36b: Botom second-stage cylinder, 36t: Top second-stage cylinder, 37b: Botom second-stage outlet flow-straightener, 37t: Top second-stage outlet flow-straightener, 38b: Botom second-stage outlet gas flow path, 38t: Top second-stage outlet gas flow path, 39b: Botom second-stage expansion chamber, 39t: Top second-stage expansion chamber, 40b: Botom second-stage cooling head, 40t:Top second-stage cooling head,Vl: Botom intake valve, V2: Botom exhaust valve, V3: Top intake valve, V4: Top exhaust valve]

Figure 12 illustrates solidworks drawing of a two-stage twin cold finger mechanical drive GM cryocooler. Figure 12 & 13 shows the model of a twin cold finger two-stage mechanical drive GM cryocooler with outer components. Another view of the twin cold finger two-stage mechanical drive GM cryocooler is presented in Figure 12 with a cross-sectional view to demonstrate its internal parts.

Figure 13 illustrates solidworks drawing of a two-stage twin cold finger mechanical drive GM cryocooler. Figure 14 illustrates different types of drive mechanisms for driving the displacers of both cold fingers based on the present investigation. (A) Rectangle-Rectangle shaped, (B) Rectangle-Elliptical shaped, (C) Rectangle-Octagonal shaped, (D) Octagonal-Octagonal shaped (E) Elliptical-Elliptical shaped. Figure 14 shows the schematics of the common gear drive mechanism for different shapes of rack holder. As shown in the figure, the common gear drive mechanism consists of a pair of linear gears (left linear gear 22 and right linear gear 27) placed in parallel with each other and a circular gear (23) with half plain surface and half teach surface. The circular gear is attached to a rotating device (i.e., a motor 25, not shown) through a shaft. A key groove is made on the gear to avoid the slip of the gear during its rotation. During the rotation of the motor, the circular gear rotates at a certain speed. The linear gears are oriented parallel to each other, and their teeth are in contact with the teeth of the circular gear. Both linear gears are atached with a common rack holder (24) with the help of a certain detachable joint, so they can be easily assembled and disassembled. The rack holder on both the top and botom end is further atached with connecting rod. In order to make proper alignment, it is attached in vertical direction. It means botom connecting rod (21b) is in vertically downward direction and top connecting rod (2 It) is in vertically upward direction. The top connecting rod is connected to the top displacer drive stem, and botom connecting rod is connected to the botom displacer drive stem. Thus, the circular motion of the linear gear is transmited to the displacers, and it causes both top and botom displacers to reciprocate within the cold head cylinders of both cold fingers. When the circular gear rotates, its teeth will be in contact with the linear gear, which causes the linear movement of the linear gear. During this period, the other linear gear is in contact with the smooth surface of the gear, but its motion happens due to the motion of the common rack holder because of the motion of the other linear gear. When the gear rotates, the teeth surface remains connected with the right linear gear, thus, its motion starts along the vertically upward direction, and during this period, the left gear comes in contact with the smooth surface of the circular gear. However, its upward movement happens because of the motion of the rack holder. In another word, the movement of the rack holder and associated connecting rod along a vertically downward direction happens during the communication of circular gear teeth, and left linear gear teeth, and vertically upward direction occurs during the communication of circular gear teeth and right linear gear teeth. During the vertical downward motion, the botom displacer moves from the botom TDC to the botom BDC, and the top displacer moves from the top BDC to the top TDC. During the vertically upward motion, the botom displacer travels from the botom BDC to the botom TDC and the top displacer travels from the top TDC to the botom TDC. Thus, the stroke length of both the top displacer and botom displacer within the top cylinder and botom cylinder should match the number of teeth of the left and right linear gears. In this manner, in each rotation, both left linear gear and right linear gear, rack holder, connecting rods, and displacers move together to generate the reciprocating motion inside the cold head cylinders of both cold fingers. The reciprocating motion expands the working fluid and generates cooling effects in both cylinders.

Figure 14 also illustrates the different structural shapes of the rack holder for holding the linear gears on both sides. Among them, the elliptical-elliptical shape and octagonal-octagonal shapes are simple in construction. Each shape is capable of generating the desired motion, but the criteria for choosing a particular shape depends on the weight, material requirement, fabrication challenges, etc. The pressure angles and modules of both linear and circular gears should be chosen wisely in order to transfer the motion appropriately to generate the reciprocating motion of the connecting rods. The rack holder, linear gears, circular gear are together placed inside an outer casing. Journal bearings and O-ring seals are provided to prevent the leakage of lubricating grease from the common gear drive mechanism for the bottom and top cold heads.

Figure 15 illustrates schematics of a single-stage twin cold finger mechanical drive GM cryocooler with warm end buffer volumes. Figure 15 illustrates the schematic of a single-stage twin cold finger type mechanical drive GM cryocooler with warm end buffer volume, which is an extension of single-stage cold head depicted in US Patent No. 8783045. The components are similar to that shown in Figure 5 except bottom (41b) and top (4 It) warm end buffer volumes for bottom and top cold head respectively. The bottom warm end buffer volume is connected with bottom common gas flow path (7b) via valve V5, and the top warm end buffer volume is connected with the top connecting flow path (7t) via valve V6. The warm end buffer volumes contain some gas at a pressure closer to the charging pressure of the system and supply it to the respective cold heads and drain from the cold head during the closing period of VI, V2, V3 and V4. Therefore, certain work load reduces on the compressor and this decreases the compressor power. This on the other hand, enhances the percentage Carnot efficiency of the overall configuration. Individual components of Figure 15 are written below.

[1: Compressor, 2: Oil separator, 3: Heat exchanger (Helium), 4: Adsorber, 5: Heat exchanger (Oil), 6: Low-pressure buffer volume, 7b: Bottom common gas flow path, 7t: Top common gas flow path, 8b: Bottom compression chamber, 8t: Top compression chamber, 9b: Bottom inlet gas flow path, 9t: Top inlet gas flow path, 10b: Bottom displacer housing, lOt: Top displacer housing, 11b: Bottom slipper seal, l it: Top slipper seal, 12b: Bottom inlet flow-straightener, 12t: Top inlet flow-straightener, 13b: Bottom regenerator, 13t: Top regenerator, 14b: Bottom outlet flow-straightener, 14t: Top outlet flow-straightener, 15b: Bottom outlet gas flow path, 15t: Top outlet gas flow path, 16b: Bottom expansion chamber, 16t: Top expansion chamber, 17b: Bottom cylinder, 17t: Top cylinder, 18b: Bottom cooling head, 18t: Top cooling head, 19b: Bottom displacer drive stem, 19t: Top displacer drive stem, 20b: Bottom displacer drive stem journal bearing, 20t: Top displacer drive stem journal bearing, 21b: Bottom connecting rod, 2 It: Top connecting rod, 22: Left linear gear, 23: Circular gear, 24: Rack housing, 25: Motor, 26b: Bottom connecting rod journal bearing, 26t: Top connecting rod journal bearing, 27: Right linear gear, 41b: Bottom warm buffer volume, 4 It: Top warm buffer volume, VI: Bottom intake valve, V2: Bottom exhaust valve, V3: Top intake valve, V4: Top exhaust valve, V5: Bottom warm buffer volume connecting valve, V6: Top warm buffer connecting valve]

Figure 16 illustrates valve timing diagram for single cylinder twin cold finger GM cryocooler with warm end buffer volumes. The valve timing charts of the single-stage twin-displacer type GM cryocooler containing warm end buffer volumes are shown in Figure 16. The working mechanism is explained as follows:

During the start, the bottom displacer is near the bottom BDC or in the vicinity of bottom BDC, during this time volume of bottom expansion chamber is minimum and volume of bottom compression chamber is maximum. As both bottom and top displacers are connected with a common displacer drive mechanism, the top displacer is located near the top TDC, as a result volume of the top expansion chamber is maximum and volume of top compression chamber is minimum. Now, valve V5 opens, thus due to high- pressure compressed working fluid starts flowing from bottom warm end buffer volume (41b) to the gas chambers of bottom cold head and pressure increases in the bottom compression chamber and bottom expansion chamber. Once the pressure value reached the pressure of the bottom warm end buffer volume, V5 closed and VI opens, thus high-pressure gas from the compressor starts flowing to the gas chambers of bottom cold head. As the displacer moves up, volume of bottom expansion chamber increases, and volume of bottom compression chamber decreases. Once the displacer reaches the bottom TDC, the volume of bottom expansion chamber is maximum and the volume of bottom compression chamber is minimum. Then, the valve VI closes; it may close before 180° too with some waiting time. Then valve V5 opens, and due to high-pressure in the bottom cold head cylinder, gas starts flowing from the gas chambers of bottom cold head cylinder to the bottom warm end buffer volume. Thus, pressure value decreases in bottom cylinder compression and expansion chambers, but its value increases in bottom warm end buffer volume. Once, the pressure in gas chambers of bottom cold head cylinder approaches the pressure of bottom warm end buffer volume, V5 closes and V2 opens, thus remaining gas drains from the gas chambers of bottom cold head cylinder to the compressor. Now, the displacer starts moving from bottom TDC to bottom BDC and bottom cold head cylinder is coupled with suction port of compressor. Thus, pressure value is minimum inside the cold head. One cycle gets finished for bottom cold head. The top cold head cylinder also operates on a similar cycle but in an opposite phase angle. At the start of the cycle, the top displacer is located at the top TDC as the bottom displacer is at bottom BDC. Thus, pressure is maximum inside the top expansion and compression chambers. Valve V6 opens, thus high-pressure gas starts flowing into the top warm end buffer volume and pressure value decreases in the gas chambers of top cold head cylinder, and the pressure value increases in the top warm end buffer volume. Once the pressure value of top cylinder gas chamber approaches pressure of top warm end buffer volume, V6 closes, and V4 opens, thus gas starts flowing from the gas chambers of top cold head cylinder towards the compressor. Subsequently, displacer starts moving from top TDC to top BDC, as at this time bottom displacer moves from bottom BDC to bottom TDC. When the discharging period ends, valve V4 closed, pressure is minimum inside the gas chambers of top cold head cylinder. Then, pre-charging starts to the gas chambers of top cold head by opening V6, and gas starts flowing from hot warm end buffer volume (4 It) towards the gas chambers of top cold head cylinder and hence pressure drops in top warm end buffer volume but its values increase in top compression and expansion chambers. Once pressure in top gas expansion chamber approaches the system charging pressure, V6 closes and V3 opens. Thus, high-pressure gas starts flowing from the compressor to the gas chambers of top cylinder and hence the pressure value increases. The displacer starts moving from top BDC to top TDC and hence the volume of top expansion chamber increases, and high- pressure gas is supplied by V3. Thus, once the cycle gets finished for top expansion chamber.

Figure 17 illustrates schematics of a single-stage single cold finger mechanical drive GM cryocooler with warm end buffer volume. Figure 17 illustrates the schematics of a two-stage twin cold finger mechanically driven GM cryocooler. Here, the structural configurations are similar to that of Figure 9, only warm end buffer volumes have been attached for both cold head as discussed in the gas supplying circuits in Figure 15 for single-stage twin cold finger mechanically driven GM cryocooler. Thus, gas can be supplied to the gas chambers of both top and bottom cylinders for some duration without putting load on compressor and this will enhance the efficiency of the system. Individual components of Figure 17 are written below.

[1: Compressor, 2: Oil separator, 3: Heat exchanger (Helium), 4: Adsorber, 5: Heat exchanger (Oil), 6: Low-pressure buffer volume, 7: Common gas flow path, 8: First-stage compression chamber, 9: First-stage inlet gas flow path, 10: First-stage displacer housing, 11: First-stage slipper seal, 12: First-stage inlet flow-straightener, 13: First-stage regenerator, 14: First-stage outlet flow-straightener, 15: First-stage outlet gas flow path, 16: First-stage expansion chamber, 17: First-stage cylinder, 18: First-stage cooling head, 19: Displacer drive stem, 20: Displacer drive stem journal bearing, 21: Connecting rod, 22: Left linear gear, 23: Circular gear, 24: Rack housing, 25: Motor, 26: Connecting rod journal bearing, 27: Right linear gear, 28: First-stage connecting pin, 29: Coupling rod, 30: Second-stage connecting pin, 31: Second- stage inlet gas flow path, 32: Second-stage slipper seal, 33: Second-stage displacer housing, 34: Second- stage inlet flow-straightener, 35: Second-stage regenerator, 36: Second-stage cylinder, 37: Second-stage outlet flow-straightener, 38: Second-stage outlet gas flow path, 39: Second-stage expansion chamber, 40: Second-stage cooling head, 41: Warm end buffer volume, VI: Intake valve, V2: Exhaust valve, V5: Warm buffer volume connecting valve]

Figure 18 illustrates valve timing diagram for two-stage GM cryocooler with warm end buffer volume. The opening closing intervals of valves VI, V2 and V5 is similar to that of discussed above (bold lines shows the opening durations of valve) and hence does not explain once again.

Figure 19 illustrates schematics of a two-stage twin cold finger mechanical drive GM cryocooler with warm end buffer volume. The valve opening and closing intervals are shown in Figure 16. Individual components of Figure 19 are written below.

[1: Compressor, 2: Oil separator, 3: Heat exchanger (Helium), 4: Adsorber, 5: Heat exchanger (Oil), 6: Low-pressure buffer volume, 7b: Bottom common gas flow path, 7t: Top common gas flow path, 8b: Bottom first-stage compression chamber, 8t: Top first-stage compression chamber, 9b: Bottom first- stage inlet gas flow path, 9t: Top first-stage inlet gas flow path, 10b: Bottom first-stage displacer housing, lOt: Top first-stage displacer housing, 11b: Bottom first-stage slipper seal, l it: Top first-stage slipper seal, 12b: Bottom first-stage inlet flow-straightener, 12t: Top first-stage inlet flow-straightener, 13b: Bottom first-stage regenerator, 13t: Top first-stage regenerator, 14b: Bottom first-stage outlet flow-straightener, 14t: Top first-stage outlet flow-straightener, 15b: Bottom first-stage outlet gas flow path, 15t: Top first- stage outlet gas flow path, 16b: Bottom first-stage expansion chamber, 16t: Top first-stage expansion chamber, 17b: Botom first-stage cylinder, 17t: Top first-stage cylinder, 18b: Botom first-stage cooling head, 18t: Top first-stage cooling head, 19b: Botom displacer drive stem, 19t: Top displacer drive stem, 20b: Botom displacer drive stem journal bearing, 20t: Top displacer drive stem journal bearing, 21b: Botom connecting rod, 2 It: Top connecting rod, 22: Left linear gear, 23: Circular gear, 24: Rack housing, 25: Motor, 26b: Botom connecting rod journal bearing, 26t: Top connecting rod journal bearing, 27: Right linear gear, 28b: Botom first-stage connecting pin, 28t: Top first-stage connecting pin, 29b: Botom coupling rod, 29t: Top coupling rod, 30b: Botom second-stage connecting pin, 30t: Top second-stage connecting pin, 31b: Botom second-stage inlet gas flow path, 3 It: Top second-stage inlet gas flow path, 32b: Botom second-stage slipper seal, 32t: Top second-stage slipper seal, 33b: Botom second-stage displacer housing, 33t: Top second-stage displacer housing, 34b: Botom second-stage inlet flowstraightener, 34t: Top second-stage inlet flow-straightener, 35b: Botom second-stage regenerator, 35t: Top second-stage regenerator, 36b: Botom second-stage cylinder, 36t: Top second-stage cylinder, 37b: Botom second-stage outlet flow-straightener, 37t: Top second-stage outlet flow-straightener, 38b: Botom second-stage outlet gas flow path, 38t: Top second-stage outlet gas flow path, 39b: Botom second-stage expansion chamber, 39t: Top second-stage expansion chamber, 40b: Botom second-stage cooling head, 40t:Top second-stage cooling head, 41b: Botom warm buffer volume, 4 It: Top warm buffer volume, VI: Botom intake valve, V2: Botom exhaust valve, V3: Top intake valve, V4: Top exhaust valve, V5: Botom warm buffer volume connecting valve, V6: Top warm buffer volume connecting valve]

Figure 20 shows a twin cold finger single-stage GM cryocooler with common buffer volumes at the warm end. The explanation and valve-timing operations are similar to that of Figure 16; only the second-stage arrangement reduces the refrigeration temperature. Figure 20 illustrates schematics of a single-stage twin cold finger mechanical drive GM cryocooler with common warm end buffer volume. Figure 20 illustrates the schematics of a single-stage twin cold finger mechanically driven GM cryocooler with common warm end buffer. This configuration differs from the configuration illustrated in Figure 15, as it contains a common buffer volume and two valves V5 and V6 for both botom and top cold head cylinders. In another word, both top and botom warm end buffer volumes are replaced by a common buffer volume with comparatively larger volume for stabilizing the pressure and this is connected with top cold finger by valve V5 and botom cold finger by V6. The opening and closing sequences of valves VI to V6 are similar as represented in Figure 16. Individual components of Figure 20 are writen below.

[1: Compressor, 2: Oil separator, 3: Heat exchanger (Helium), 4: Adsorber, 5: Heat exchanger (Oil), 6: Low-pressure buffer volume, 7b: Botom common gas flow path, 7t: Top common gas flow path, 8b: Botom compression chamber, 8t: Top compression chamber, 9b: Botom inlet gas flow path, 9t: Top inlet gas flow path, 10b: Botom displacer housing, lOt: Top displacer housing, 11b: Botom slipper seal, l it: Top slipper seal, 12b: Botom inlet flow-straightener, 12t: Top inlet flow-straightener, 13b: Botom regenerator, 13t: Top regenerator, 14b: Botom outlet flow-straightener, 14t: Top outlet flow-straightener, 15b: Botom outlet gas flow path, 15t: Top outlet gas flow path, 16b: Botom expansion chamber, 16t: Top expansion chamber, 17b: Botom cylinder, 17t: Top cylinder, 18b: Botom cooling head, 18t: Top cooling head, 19b: Botom displacer drive stem, 19t: Top displacer drive stem, 20b: Botom displacer drive stem journal bearing, 20t: Top displacer drive stem journal bearing, 21b: Botom connecting rod, 2 It: Top connecting rod, 22: Left linear gear, 23: Circular gear, 24: Rack housing, 25: Motor, 26b: Botom connecting rod journal bearing, 26t: Top connecting rod journal bearing, 27: Right linear gear, 41: Common warm buffer volume, VI: Botom intake valve, V2: Botom exhaust valve, V3: Top intake valve, V4: Top exhaust valve, V5: Botom warm buffer volume connecting valve, V6: Top warm buffer connecting valve]

Figure 21 illustrates schematics of a two-stage twin cold finger mechanical drive GM cryocooler with common warm end buffer volume. Figure 21 shows the schematics of two-stage twin cold finger mechanically driven GM cryocooler with a common warm end buffer volume. The two-stage configuration has been made by connecting the second stage components at the cold end of the first-stage to further reduce the refrigeration temperature. This configuration is also similar to that of Figure 19; only the two buffer volumes (top and botom warm end buffer volumes) are replaced by a single buffer volume of comparatively larger size for pressure stabilization. The common warm-end buffer volume is connected to both top and botom cold fingers by valves V5 and V6. The opening closing sequences of valves are similar to that of represented in Figure 16. Individual components of Figure 21 are writen below. [1: Compressor, 2: Oil separator, 3: Heat exchanger (Helium), 4: Adsorber, 5: Heat exchanger (Oil), 6: Low-pressure buffer volume, 7b: Bottom common gas flow path, 7t: Top common gas flow path, 8b: Bottom first-stage compression chamber, 8t: Top first-stage compression chamber, 9b: Bottom first- stage inlet gas flow path, 9t: Top first-stage inlet gas flow path, 10b: Bottom first-stage displacer housing, lOt: Top first-stage displacer housing, 1 lb: Bottom first-stage slipper seal, 1 It: Top first-stage slipper seal, 12b: Bottom first-stage inlet flow-straightener, 12t: Top first-stage inlet flow-straightener, 13b: Bottom first-stage regenerator, 13t: Top first-stage regenerator, 14b: Bottom first-stage outlet flow-straightener, 14t: Top first-stage outlet flow-straightener, 15b: Bottom first-stage outlet gas flow path, 15t: Top first- stage outlet gas flow path, 16b: Bottom first-stage expansion chamber, 16t: Top first-stage expansion chamber, 17b: Bottom first-stage cylinder, 17t: Top first-stage cylinder, 18b: Bottom first-stage cooling head, 18t: Top first-stage cooling head, 19b: Bottom displacer drive stem, 19t: Top displacer drive stem, 20b: Bottom displacer drive stem journal bearing, 20t: Top displacer drive stem journal bearing, 21b: Bottom connecting rod, 2 It: Top connecting rod, 22: Left linear gear, 23: Circular gear, 24: Rack housing, 25: Motor, 26b: Bottom connecting rod journal bearing, 26t: Top connecting rod journal bearing, 27: Right linear gear, 28b: Bottom first-stage connecting pin, 28t: Top first-stage connecting pin, 29b: Bottom coupling rod, 29t: Top coupling rod, 30b: Bottom second-stage connecting pin, 30t: Top second-stage connecting pin, 31b: Bottom second-stage inlet gas flow path, 3 It: Top second-stage inlet gas flow path, 32b: Bottom second-stage slipper seal, 32t: Top second-stage slipper seal, 33b: Bottom second-stage displacer housing, 33t: Top second-stage displacer housing, 34b: Bottom second-stage inlet flowstraightener, 34t: Top second-stage inlet flow-straightener, 35b: Bottom second-stage regenerator, 35t: Top second-stage regenerator, 36b: Bottom second-stage cylinder, 36t: Top second-stage cylinder, 37b: Bottom second-stage outlet flow-straightener, 37t: Top second-stage outlet flow-straightener, 38b: Bottom second-stage outlet gas flow path, 38t: Top second-stage outlet gas flow path, 39b: Bottom second-stage expansion chamber, 39t: Top second-stage expansion chamber, 40b: Bottom second-stage cooling head, 40t:Top second-stage cooling head, 41: Common warm buffer volume, VI: Bottom intake valve, V2: Bottom exhaust valve, V3: Top intake valve, V4: Top exhaust valve, V5: Bottom warm buffer volume connecting valve, V6: Top warm buffer volume connecting valve]

A single-stage twin-cold finger mechanically driven GM cryocooler that uses a common drive mechanism to power the displacers inside the cold head cylinders of pair of linear gears (racks), a circular gear with teeth in its half circle, a common rack holder and a pair of connecting rods attached with the rack holder in opposite directions. The connecting rods are attached to the displacer drive stems of both top and bottom cylinder. The rotary motion of the motor is converted into reciprocating motion with this arrangement, which eventually helps to drive the displacers in both cylinders. This claim is one of the novel ideas to existing drive mechanisms for GM cryocooler.

Different configurations of rack holders have been proposed to hold the racks (linear gears) in opposite sides. Journal bearing and sealing arrangements have been made to overcome the leakage of grease from the driving arrangement to the cold heads.

The single-stage twin cold-finger mechanically driven GM cryocooler is further converted into a two-stage twin cold finger mechanically driven GM cryocooler by attaching the second stage components (second-stage displacer housing, regenerator, flow-straighteners, cylinders, coupling rod and its connecting pins) at the cold end of the first-stage. As a result, the refrigeration temperature can be further reduced.

In another arrangement, pair of warm end buffer volumes is placed at the warm end of both cold fingers of the twin cold finger mechanically driven GM cryocooler. The buffer volumes are charged to a pressure closer to the charging pressure of the system and are connected to the bottom and top common gas flow paths. Therefore, fluidic communication happens between the gas chambers of both top and bottom cold heads with the respective warm end buffer volumes. This will reduce the load on compressor and increase the percentage Carnot efficiency of the system. This arrangement is made for both single-stage and two-stage twin cold finger mechanically driven GM cryocoolers. This claim is also one of the novel ideas to existing drive mechanisms for GM cryocooler.

In another arrangement, a single-stage and two-stage twin cold finger mechanically driven GM cryocooler is developed with a common warm end buffer volume with pair of connecting valves. The gas from common buffer volume will communicate with the gas chambers of both top and bottom cylinder during certain durations of the waiting period of compressor valves. Therefore, the load on the compressor system will be reduced and the percentage Carnot efficiency of the system will increase further. Additionally, by keeping a single warm end buffer volume for both cold fingers, the system can be easily miniaturized, and will increase its reliability. Also, the valve-timing arrangements are provided for each configuration, expansion chamber P-V diagrams are drawn to explain the cooling mechanism in expansion chambers of both top and bottom cold head cylinders.

The present invention is related to single-stage and two-stage twin cold finger GM cryocooler that is capable of producing certain refrigeration capacity up to 2.2 K. The cryocoolers are used in a variety of industrial applications as listed below:

• Liquefaction of cryogenic fluids such as helium, hydrogen, neon, nitrogen, argon, oxygen, etc.

• Helium recondensation in Magnetic Resonance Imaging (MRI) Machines.

• Cooling of both low-temperature and high-temperature superconducting magnets to retain their superconducting states.

• Cooling of high-temperature superconductivity (HTS) cable, HTS motors, and HTS generators. Cooling of low-temperature superconductivity magnets.

• Cooling of HTS magnet in MagLev trains.

• Calibration of cryogenic temperature sensors.

• Testing of material properties in cryogenic temperature limits.

The objectives of this invention include:

• A single-stage twin cold finger type GM cryocooler containing two cold heads situated axially (one at the bottom and other at the top) and the displacers of both cold heads are connected to a common gear driving mechanism. The common gear drive mechanism consists of a motor, a circular gear having teeth in half of its circle, a pair of linear gear (called as racks), a rack holder and a pair of connecting rods. Different shapes of mechanical supporting structures are identified as rack holders as a part of this invention.

• A single-stage twin cold finger type GM cryocooler containing two cold heads situated axially and first-stage displacers of both cold head are connected with a common gear drive mechanism. Additionally, the second-stage displacer of the bottom cold head is connected with the first-stage displacer by means of a coupling rod (aluminum). In this manner, the single-stage twin cold finger mechanically drive GM cryocooler is converted into a two-stage twin cold finger mechanically drive GM cryocooler. Thus, the refrigeration temperature can be further reduced (~ 4.2 K or lower temperature can be achieved in the thermal stages of both second-stage cold head).

• Both single-stage and two-stage twin cold finger GM cryocooler configurations are further attached with warm end buffer volumes and connecting valves in another invention in addition to the invention presented earlier. This arrangement (i.e. the warm end buffer and valve) charges and discharges gas to the gas chambers of both cold heads during some lengths of waiting periods of the compressor valve section. As a result of this arrangement, the percentage Carnot efficiency of the system increases.

• Thermodynamic cycles are drawn at both bottom expansion chamber and top expansion chamber to explain the movement of gas parcels to generate a productive cooling effect.

The drawings and the forgoing description give examples of embodiments. Those skilled in the art will appreciate that one or more of the described elements may well be combined into a single functional element. Alternatively, certain elements may be split into multiple functional elements. Elements from one embodiment may be added to another embodiment. Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments.

Claims

1. A system for twin cold finger mechanically driven Gifford-McMahon cryocooler and drive mechanism, the system comprises: a helium compressor unit comprising of a reciprocating/scroll compressor, an oil separator, a helium heat exchanger, an adsorber, an oil heat exchanger and a low-pressure buffer volume; a comprehensive oil separation mechanism adopted in the compressor unit to prevent the entry of oil to the cold head of the cryocooler, which may be solidified because of cryogenic temperature and causes the failure of the cold head unit; a valve control unit selected from one or more of a mechanical rotary valve or solenoid valve, wherein the mechanical rotary valve consists of a rotor which is held tightly over a valve plate and rotated with the help of a motor at a speed such that when the rotor rotates over the plate, a flow passage is produced between the high and low-pressure lines of compressor with common gas flow path which allow the high and low-pressure gas to alternatively connect the cold head and generate the oscillating pressure wave, whereas the solenoid valves with appropriate electronic control units also generate the oscillating pressure pulse for the cold head of the cryocooler; a cold head comprising of a compression chamber, a displacer housing, a slipper seal, a regenerator, an inlet and an outlet gas flow paths, an inlet and an outlet flow straighteners, a cold head cylinder, a cold heat exchanger/cooling-head, wherein in between the outer surface of the displacer and the inner surface of the cold head cylinder, slipper seals are provided which prevent the flow of working fluid from the compression chamber to the expansion chamber and the slipper seal is preferably an O-ring seal surrounded by C-shaped Teflon seal, which prevent flow of refrigerant from compression to expansion chamber and forces the working fluid to flow through the regenerator flow passage, wherein the displacer is connected with a displacer drive stem, which is further connected to a connecting rod such that through the connecting rod and displacer drive stem power is transmitted from the motor to the displacer, and it reciprocates inside the cold head cylinder, as a result of this, volume of both compression and expansion chamber changes in a cycle, wherein the displacer drive stem journal bearing prevents the leakage of refrigerant and guides the displacer movement; and a drive mechanism comprising of a motor, a circular gear, a left linear gear, a right linear gear, a rack holder, a connecting rod, and a connecting rod journal bearing, wherein the connecting rod journal bearing holds the connecting rod in its position and the sealing arrangement prevents the leakage of lubricant, wherein the connecting rod is attached to the displacer drive stem and the rotating motion of the circular gear is converted to reciprocating motion with this arrangement, wherein due to the reciprocating motion of the displacer within the cold head cylinder, the volume of both the expansion chamber and compression chamber changes in a cycle, and a cooling effect is produced, wherein the gear drive mechanism is covered with an outer casing to avoid the entry of dust and make the appearance simple.

2. The system as claimed in claim 1, wherein the valves VI and V2 are situated at the high-pressure line and low-pressure line of the compressor, respectively, when valve VI is open, the high-pressure refrigerant (helium gas) enters to the gas chambers of cold head from compressor, and when valve V2 is opened, the expanded refrigerant returns back from the gas chambers of cold head to the compressor, wherein the refrigerant flows from valve VI and V2 to the cold head by common gas flow path.

3. The system as claimed in claim 1, wherein a typical twin-cold finger mechanical drive GM cryocooler, which consists of a top cold head and a bottom cold head, wherein the structural configuration of each cold head is identical with the single-stage configuration, wherein the bottom cold head consists of a bottom common gas flow path, bottom compression chamber, bottom inlet gas flow path, bottom displacer housing, bottom slipper seal, bottom inlet flow straightener, bottom regenerator, bottom outlet flow straightener, bottom outlet gas flow path, bottom expansion chamber, bottom cylinder and bottom cooling head, wherein the bottom displacer is connected with bottom connecting rod by means of bottom displacer drive stem, and bottom displacer drive stem journal bearing holds it in its position and avoids the leakage of refrigerant, wherein the top cold head consists of a top common gas flow path, top compression chamber, top inlet gas flow path, top displacer housing, top slipper seal, top inlet flow straightener, top regenerator, top outlet flow straightener, top outlet gas flow path, top expansion chamber, top cylinder and top cooling head.

4. The system as claimed in claim 3, wherein the top displacer is connected with top connecting rod by means of top displacer drive stem, and top displacer drive stem journal bearing holds it in its position and avoids the leakage of the refrigerant, wherein the bottom cold head is connected to the compressor system by valve VI and V2, wherein the top cold head is connected to the compressor system by valve V3 and V4, wherein VI and V3 are intake valves for bottom cold head and top cold head, V2 and V4 are exhaust valves for bottom cold head and top cold head, wherein when bottom cylinder is connected to the discharge side of compressor by VI, top cylinder should be connected to the suction side of compressor by V4, wherein when the bottom cylinder is connected to the suction side of compressor by V2, top cylinder is connected with the discharge side by V3 such that when one cylinder goes for charging process, the other cylinder goes for discharging process and vice-versa, in such a manner, the compressed working fluid passes from the compressed gas supply unit to the cold heads alternatively.

5. The system as claimed in claim 1, wherein a two-stage single cold-head GM cryocooler, in which the hot end of the second stage is coupled with the cold end of the first stage such that the working fluid gets cooled from room temperature to a certain temperature in the first stage and then to a further low temperature in the second-stage expansion chamber, wherein the first stage acts like a precooling of refrigerant before entering the second stage and the second-stage components are second-stage inlet gas flow path, second-stage inlet flow-straightener, second-stage regenerator, second-stage outlet flowstraightener, second-stage outlet gas flow path, and second-stage expansion chamber, wherein the gas gets expanded in the expansion chamber of the second-stage and cooling effect is provided by second-stage cooling head and the second-stage slipper seal prevents leakage of refrigerant and is placed between second-stage displacer housing second-stage cylinder .

6. The system as claimed in claim 5, wherein the working fluid passes from the first stage expansion chamber to the second stage expansion chamber via second-stage inlet gas flow path, second- stage inlet flow-straightener, second-stage regenerator, second-stage outlet flow-straightener, second-stage outlet gas flow path, wherein after expansion of the refrigerant in the bottom second stage expansion chamber, a cooling effect is produced which is absorbed at bottom second-stage cooling head thereby the expanded refrigerant then returns from the second-stage expansion chamber to the first-stage expansion chamber by second-stage outlet gas flow path, second-stage outlet flow-straightener, second-stage regenerator, second-stage inlet flow-straightener, and second-stage inlet gas flow path, wherein the hot end of second stage displacer is connected to the bottom end of first stage displacer by a coupling rod and the coupling rod is attached to the first stage displacer and second stage displacer by first-stage connecting pin, and second-stage connecting pin respectively such that the motion is transferred from first stage to second stage, wherein the second stage displacer, as a result, undergoes reciprocating motion inside the second stage cylinder and the volume of second stage expansion chamber changes in a cycle.

7. The system as claimed in claim 1, wherein the second-stage components of both top and bottom cold heads are attached to the cold end of their respective first stages via a coupling rod.

8. The system as claimed in claims 1 and 7, wherein the first stage bottom cold head consist of bottom common gas flow path, bottom first-stage compression chamber, bottom first-stage inlet gas flow path, bottom first-stage displacer housing, bottom first-stage slipper seal, bottom first-stage inlet flowstraightener, bottom first-stage regenerator, bottom first-stage outlet flow-straightener, bottom first-stage outlet gas flow path, bottom first-stage expansion chamber, bottom first-stage cylinder, bottom first-stage cooling head, bottom displacer drive stem, bottom displacer drive stem journal bearing, bottom connecting rod, left linear gear, circular gear, rack holder, motor, bottom connecting rod journal bearing, and right linear gear, wherein the displacer is driven inside the cylinder by the driving mechanism as discussed in the case of single displacer twin cold finger mechanically driven GM cryocooler.

9. The system as claimed in claims 7 and 8, wherein the warm end of second stage displacer of bottom cylinder is connected with the cold end of first stage displacer by a bottom coupling rod and the bottom coupling rod is attached to the first stage displacer and second stage displacer by bottom first stage connecting pin and bottom second-stage connecting pin such that the motion is transferred from first-stage to second-stage, wherein the second stage displacer, as a result, undergoes reciprocating motion inside the second stage cylinder and the volume of second stage expansion chamber changes in a cycle, wherein a bottom second-stage slipper seal is placed between the outer diameter second-stage displacer housing and inner diameter of bottom second-stage cylinder to prevent the leakage of refrigerant, wherein the working fluid passes from the bottom first stage expansion chamber to the bottom second stage expansion chamber via bottom second-stage inlet gas flow path, bottom second-stage inlet flow-straightener, bottom second- stage regenerator, bottom second-stage outlet flow-straightener, and bottom second-stage outlet gas flow path.

10. The system as claimed in claim 9, wherein after expansion of the refrigerant in the bottom second stage expansion chamber, a cooling effect is produced which is absorbed at bottom second-stage cooling head and the expanded refrigerant returns from the bottom second-stage expansion chamber to the compressor via bottom second-stage outlet gas flow path, bottom second-stage outlet flow-straightener, bottom second-stage regenerator, bottom second-stage inlet flow-straightener, bottom second-stage inlet gas flow path, bottom first-stage expansion chamber, bottom first-stage outlet flow-straightener, bottom first stage regenerator, bottom first-stage inlet flow-straightener, bottom first-stage inlet gas flow path, bottom first-stage compression chamber, bottom common gas flow path, valve V2, low-pressure buffer, and compressor suction line.