US3361340A - High vacuum pumping system - Google Patents

Description

Jan. 2, 1968 w'. M. BRUBAKER HIGH VACUUM PUMPING SYSTM Filed oct. 18, 1965 5 Sheets-Sheet 1 SMN N Q. QQ uw w @S @S Sx:

Jan. 2, 1968' w. M. BRUBAKER 3,361,340

HIGH VACUUM PUMPING SYSTEM Filed Oct. 18, 1965 5 Sheets-Sheet 2 Wm/r /4/4) I NVEN TOR. h//zfm/ /l ez/Ziff@ Jan. 2, 1968 w. M. BRUBAKER 3,361,340

HIGH VACUUM PUMPING SYSTEM Filed oct. 18, 1965 5 sheets-sheet s /'Z INVENTOR.

' Jan. 2, 1968 w. M. BRUBAKER HIGH VACUUM PUMPING SYSTEM 5 Sheets-Sheet 4 Filed oct. 18, 1965 Jam` 2, 1968 w. M.- BRUBAKER 3,361,340

HIGH VACUUM PUMPING SYSTEM Filed oct. 18, 1965 5 sheets-sheet 5 United States Patent O 3,361,340 HIGH VACUUM PUMPING SYSTEM Wilson M. Brubaker, Arcadia, Calif., assigner to Consolidated Vacuum Corporation Filed Oct. 18, 1965, Ser. No. 497,030 Claims. (Cl. 23m-69) ABSTRACT OF THE DISCLOSURE Prior art ion pumps have power sources which are selected to provide input power for the ion pump rated at a value based upon a continuous pumping operation for a given pump capacity. The rated power for such prior art pumps corresponds to a maximum edective power dissipation level which safeguards the pump electrodes from high temperature damage within the pump which otherwise might result from power dissipation therein. In accordance with the invention disclosed herein by one representative embodiment, a power supply having a rating in excess of that for the given ion pump rating, is employed in a continuous ON manner at high pressure to quickly achieve rapid pump starting time until a predetermined upper upper temperature limit for the pump iS sensed and automatically an intermittent ON/ OFF powersupplying cycle is established. The intermittent cycle is automatically controlled Iby sensing the pump temperature and progressively varying the ON/ OFF power-supplying cycle for the pump in accordance with the temperature established within the pump, until at a low pressure the temperature within the pump is sensed to lbe at a safe level and continuous power is again supplied.

This invention relates in general to high vacuum pumps and in particular relates to a new and improved method and apparatus for increasing the `gas evacuation eiciency of an ion pump.

Recent developments have refined high vacuum methods and apparatus in the field of ion high vacuum pumps in which gas molecules in a vessel to be evacuated are either expelled or are transferred into a solid phase. Numerous techniques are available in the art today and some of the most recent ultrahigh vacuum break-throughs are described in considerable detail in Ultrahigh Vacuum And Its Application, Roberts & Vanderslice, Prentice- Hall, New Jersey, 1963. Reference may be made to the early chapters of the foregoing text if a detailed analysis of ion pump principles is desired. Basically, however, the term pumping is any process by which the number `of molecules in the gas phase of the Vessel to Ibe evacuated is caused to decrease. This delinition also includes other widely used terms such as cleanup or throughput which are used to denote reduction in pressure caused by electrical and chemical effects in a sealed pumping system.

In general, ion pumps include an electric iield which exists between the pumps electrodes. This electric field produces a space charge limited discharge which is frequently called a Penning Discharge. In this discharge some of the neutral molecules become ionized, others become excited to form metastable molecules. Under the influence of the electric field the ions are driven into the cathode electrode. Their paths are influenced to a limited amount by magnetic fields present in the space charge area.

The ions strike the cathode with large amounts of kinetic energy. Thus, they penetrate the surface a small distance, and may become buried therein. The ionic bombardment rof the cathode surfaces results in sputtering of the cathode material. In general, the cathode is made of a reactive metal. The freshly sputtered material is dispersed over the other surfaces of the pump structure, and

3,351,340 imatented Jan. 2, 1968 ICC reacts with the molecules of many gases. This process is called gettering.

The foregoing phenomena is of particular interest during starting periods of the pumping cycle or when a heavy load, as represented by a sudden increase in gas is applied to a pump which is cool and has previously been pumping at a low pressure or high vacuum. It is also this pressure range to which, generally speaking, this invention is directed, although it is by no means limited to these pressure values.

When a pump is working at a low pressure, and a sudden gas load is applied, a pressure-runaway condition may occur. The power supply for a prior art pump had to be designed to provide a relatively low value of maximum power under these conditions.

Design limits for prior art pumps have been set in View of the foregoing factors. For example, in prior art pumps the applied power is chosen in view of a continuous pumping operation such that the maximum effective power dissipation level for the pump is not exceeded. If the maximum level were exceeded by prior art pumping techniques, excessive heatin" of the various parts of the pump would occur. Such excessive heating would result in melted electrodes and damage to other internal parts of the pump. In attempts to safeguard against the foregoing sensitivities such as pressure-runaway and excessive heating, elaborate and complex power supplies have been developed and/or constant personal attention by trained personnel to various factors such as Voltage, current, pressure and temperature is provided during pumping operations.

l have discovered that the foregoing complex circuitry and personal attention can be avoided by the principles of this invention in which the maximum power dissipation level of the pump is not exceeded on the average, but is greatly exceeded for short time intervals during an intermittent application of a supply voltage to the pump.

In accordance with my invention a power supply for an ion pump is chosen which is rated considerably higher than the standard design criteria lof the ion pump to which it is connected. This power supply when delivering power to the ion pump does so at higher voltage and current for only a short ON period whereby the pumping action may approach that of a runaway condition but is interrupted short of that condition by an OFF period. This OFF period removes power from the pump and provides for cooling of the pump electrodes. These intermittent ON and OFF power intervals are automatically regulated to develop an average power that is equivalent to the same continuous power dissipation rating for a prior art continuous-mode operation for a chosen pump. This power cycle is regulated such that power is ON continuously at the start of the pumping cycle until the point of maximum effective power dissipation level is reached and the power is turned OFF. Thereafter the alternating power application sequence begins, with the OFF time progressively increasing with respect to the ON intervals until maximum power is applied, at which point the OFF and ON times are about of equal duration. The OFF time thereafter progressively decreases with respect to the ON time until at high vacuum the power is continually ON, and the pump maintains its high vacuum indefinitely.

This intermittent operation, experiments have proven, greatly increases the efficiency of a pumping system and provides a decreased starting time for the pumping cycle of this invention which is considerably improved over prior art pumps.

In one possible embodiment, the apparatus for controlling this intermittent variable energy operation is a self-timing temperature sensitive circuit connected between the power source and the ion pump.

rent characteristic for an ion pump;

FIG. 2 depicts an ideal power-current and voltagercurrent curve showing the maximum effective power dissipation level for an ionV pump;

FIG. 3 depicts ideal power-pressure curves for a prior art ion pump and the ion pump in accordance with the principles of this invention;

FIG. 3A depicts the power-pressure characteristic of FIG. 2 and a curve representing the intermittent operation in accordance with principles of this invention;

FIG. 4 is a block diagram of a circuit providing an automatic intermittent operation similar to the curve shown in FIG. 3A;

FIG. 5 is a circuit diagram illustrating in detail one automatic power input interrupting circuit in accordance with the principles of this invention; and

FIG. 6 is a temperature-time characteristic for an ion pump and a power interrupting circuit of FIG. 5, in accordance with the principles of this invention.

Turning nowV to FIG. 1, a typical power-current and voltage-current characteristic for an ion pump is shown. The v-oltage current curve l0 represents a maximum voltage which may be 6 kilovolts when there is zero current for the system. Further, as depicted by curve 8, at this instant of maximum voltage and zero current there isV a corresponding zero power input for the pump system. In practice, as power 10 is applied to the pump the discharge selects its own voltage and current values. The characteristics of the power supply are such that at very low values of current the voltage impressed across the electrodes of the pump is relatively high. As a higher and higher vacuum is achieved, i.e., the number of molecules in the gas phase in the system decreases, the power decreases in the manner shown in curve 8. At low presfsures the current is proportional to the pressure and the applied voltage is nearly constant. Thus the power input is essentially proportional to the pressure as will -be discussed in detail hereinafter with reference to FIGS. 2 and 3.

Line 6, FIG. 1, which may be for example 300 watts,

represents as Vassumed maximum effective power dissipation level designed for an ion pump. This power dissipation` level is equivalent to a temperature level lbecause power is dissipated in the pump system as heat.

As viewed in light of the prior art, the characteristics for an ideal power curve and an ideal voltage and current curve are depicted in FIG. 2. In FIG. 2 the power characteristic is shown as 12 and the voltage characteristic is shown as 14. The dashed power characteristic 8 is that of FIG. l. As can be observed from FIG.2, the power increases at a rapid rate to a point corresponding to the maximum effective power dissipation level for the pump. Ideally the power input would parallel this maximum dis- Vsipation level throughout the higher pressure ranges, eg.,

pressures of 103 to 10-5 torr. At the lower pressure ranges,e.g., 105 to 10-10 torr, however, the power would -drop to a sustaining amount providing for a continual high vacuum.

I have discovered that if the rated power dissipation level for the pump is indeed observed onthe average, but is achieved by an intermittent power application by way of a power supply far in excess of that designed for the pump, improved operation results. This improved operation results in a decrease in pump starting time.

In accordance with the principles of my invention, the maximum effective pump power dissipation level 6, in FIG. 3 is exceeded by the employment of a power supply such as curve 20 which is larger than the pumps standard designed power supply of curve S. At the extreme right in FIG. 3 a pressure in the order of l0-1 torr is assumed to be present in the ion pump. This starting pressure is achieved by a pre-pumping procedure as is well known in the prior art. At this point in the pumping cycle, which is the starting time, T0, FIG. 3A, the power is ON continuously. As the power Vis appliedto the pump the larger amount of power 20 supplied during the time interval T0 through T1 in FIG. 3A reaches the maximum etective power dissipation level 6 for the ion pump. This maximum power level is reached considerably sooner than it would be for the power supply shown in curve 8. At time T1 the continuous power application is interrupted and is thereafter intermittently applied during the time interval T1 through T3, FIG. 3A.

As was stated earlier, it is the temperature resulting from power dissipation within the pump that is a critical factor. If the power input is too great over an extended time interval, then the temperature rise exceeds the safe temperature level for the pump and melts the pump electrodes and other pump parts.

The safe temperature levels for the pump are observed by the principles of this invention which auto-7' matically interrupts the power to the pump in a properly 'timed manner. This properly timed intermittent power down operation. At this pressure portion of the'pump-V Aing cycle, the ONv and OFF intervals follow the pattern shown by curve 16 in FIG. 3A, which represents Vthe ratio of OFF time to ON time. Y

The ON and OFF times of intermittent power curv 15, FIG. 3A, are of dierent duration relative to one another. During interval P1 through P2 the power` curve 20 is approaching maximum value. Throughout this interval the OFF periods progressively increase with respect to the ON periods so that the average maximum dissipation level is not exceeded. Once that maximum level for power curve 2Q, P2 of FIG. 3A, is passed the pressure in the pump becomes lower and lower. The power OFF periods progressively decrease with respect to power ON periods until the point of maximum effective power dissipation level is reached.

At higher vacuum when the power curve 20 is less than the maximum effective power dissipation level.6, the power is ON continuously. A power sustaining point is developed at approximately 10-6 torr. At this point Vlittle gas remains in the system, and although voltage and current is continually applied to the pump electrodes the consumed power is very low.

As will be described hereinafter, I haveprovided in my invention that the intermittent power application be controlled automatically. However, it should be understood that an automaticV operation is not necessarily required. If an automatic operation is not provided, however, constant personal attention by an operating technician will be required.

In the normal operation of a pump at low pressures, a small amount of gas is often admitted into the pump. If the admission of gas raises lthe pressure above 10's torr, the pump power is increased as shown by the'cur've 20 in FIG. 3. Such an increase in power may cause a further increase in pressure, perhaps because of release of gas from the pump electrodes, again causing an increase in pump power. This condition is often referred to as pressure runaway. In FIG. 3 one portion of the power curve 20 is shown as the pressure runaway danger area.

If the admission of gas into the system or pressure runaway causes the pressure to exceed about 10-5 torr, the pump power will exceed the maximum eiective power dissipation level. According to my invention, the automatic power interrupting circuit is then employed to prevent damage to the pump parts. p

Prior to a description of the automatic power interrupting circuit of this invention a brief summary of the advantages over the prior art'attendant to this operation is in order. The intermittent supply of power to an ion pump higher than its design criteria, in accordance with the principles of this invention, results in a faster starting of the gas evacuation process by the utilization of an initially high applied voltage. This higher voltage is possible because the average power input to the pumping system is kept within the maximum effective power dissipation level for pump. At this high voltage the sputtering action of the discharge is greatly enhanced and the elective pumping action is also enhanced.

In addition, I have discovered experimentally that the intermittent operation increases not only the starting rate but may actually increase the ultimate evacuation pressure capabilities of the system. In numerous experiments it has been observed that the pump pressure and the current often tend to level off or hang up at one pressure for a considerable length of time. This hang up, however, may be overcome it the power applied to the pump is interrupted for a small fraction of a minute. The complete theory of operation of this hang up phenomenon and the way it is overcome is not fully understood, but the cycling operation which is described hereinafter represents an automatic intermittent operating technique which overcomes this hang up.

The automatic cycling circuit of this invention is shown in block form in one illustrative embodiment in FIG. 4. In FIG. 4 an alternating current supply source 40 is shown connected through an ON-OFF switch 39 to an ion pump power supply 41 by a pair of power lines 43. Power supply 4l is rated much higher than the standard design supply for an ion pump shown in block form as 42. The power supply 41 for ion pump 42 may advantageously be any known prior art supply in which the initial operating voltage is greater than the initial operating voltage for a power supply intended for pump 42 at the same starting pump pressures.

A anged vessel 45 to be evacuated is connected to ion pump 42 in any well known manner. The temperature of the pump is monitored by a thermal sensing device 59. This thermal sensing device opens the A-C power circuit for ion pump 42 by disconnecting the A-C source 40, when the temperature of pump 42 reaches its maximum upper temperature limit as previously described in connection with FIG. 3 and FIG. 3A.

A more detailed description of one possible thermal sensing device 5t) is shown in FIG. 5. The source 40, supply 41, and pump 42 are not repeated in FIG. 5. A portion on one side wall of ion pump 42 is shown connected to a thermal bridge 49. A thermal switch 48 in dashed lines is contained in a thermal block that forms a part of thermal bridge 49 connected between the power lines 43 and the wall of ion pump 42. This thermal switch 48 may advantageously be any temperature-sensitive switch which has two definite predetermined temperature settings at which it opens and closes contacts 38. Numerous suitable switches are known in the art and one typical approach is shown utilizing a round, fiat thermal disk 39 of temperature-sensitive material which is cornmercially available. The disk 39 should ideally have a low thermal capacity. n

The dat thermal disk 39 is convex on its upper side, as shown, at a rst predetermined temperature. It becomes concave with respect to the upper side, as shown by dashed lines, when it is heated to a second predetermined temperature. A moveable Contact, of Contact pair 33, is mechanically operable in response to the concave and convex movement of the ilat disk 39. For example, in the circuit of FIG. 5, thermal switch 48 normally includes contact 33 held in a closed condition unless a suihcient rise in temperature causes disk 39 lto assume a concave shape. A concave shape, dashed lines, allows the spring loaded contacts 38 to open.

Heater coil 47 wrapped around the thermal disk 39 heats the disk a controlled predetermined amount so that it is normally at a higher temperature than the pump wall when the pump is running at its cool high-vacuum temperature. This difference in temperatures between the thermal disk 39 and the pump wall is shown by the respective curves 58 and S9 at time To in FIG. 6.

Admission of gas to the pumpincreases its load and its power requirements in the manner shown between points 25 and 26 in FIGS. 3 and 3A. Thus, if it is assumed for purposes of explanation that the valve 46, FIG. 4, is opened to admit a very small amount of gas to the evacuated chamber, then the pressure inside the pump increases. This pressure increase is a heavy load condition and the pump draws more power from the supply as it works to evacuate the gas that has been introduced. Such power input to the pump, under a heavy load, is dissipated in the pump as heat. The temperature of pump 42 increases as shown by the temperature curve 6i? for pump 42 between times T1 and T2.

As previously stated, the power supply for this pump is significantly greater than the power supply normally available for a pump of this capacity and thus during time T1 through T2 the pump is working at a much higher power 2t?, FIG. 3, than the standard design power 8, FIG. 3, for a pump of this capacity rated at a prior art continuous pumping mode. Accordingly, the temperature of pump 42 increases rapidly, and in a manner similar to that shown by curve 61, FiG. 6, the temperature of the thermal disk 39 also rises rapidly. This temperature rise occurs because thermal bridge 49 closely follows the temperature rise of pump 42. At time T2, FIG. 6, the thermal disk 39 heats to a predetermined Contact opening point A. At point A contacts 38 open and line voltage is removed from heater coil 47 and from power supply 41. Reference to FIGS. 3 and 3A show this contact opening point A at the maximum elective power dissipation level 6 for pump 42.

Thermal bridge 49 is chosen to exhibit a low thermal capacity with respect to the thermal capacity of pump 42. Thus bridge 49 is capable of quickly dissipating its heat into the lower temperature of the pump wall and the surroundings. Pump 42, on the other hand, has a high thermal capacity and loses its heat slowly. Pump 42 has only the surroundings of the reorn as a sink for its heat and thus its temperature falls more slowly than that of thermal bridge 49. The difference in the rate of heat loss in thermal bridge 49, thermal disk 39, and pump 42 are shown in FIG. 6 by curves 63 and 62 respectively between times T2 and T3.

At time T3 thermal disk 39 drops to its first predetermined contact temperature and `contacts 38 close. This closure connects the line voltage of source 40 to the ion pump power supply 4l and to heater coil 47. Power 20, FIG. 3, results in an increased pumping action and an increase in temperature within pump 42. Accordingly, the pump temperature which has fallen during the OFF interval between T2 and T3 once again heats as shown by curve 64. Thermal bridge 49 and thermal disk 39 follow the rise in the pump temperature as shown by the temperature curve 65.

At time T4, FIG. 6, the temperature curve for the thermal disk 39 reaches the contact opening temperature, but does so at a diiferent slope or rate of temperature change than that of curve 61. The reason for this difference in temperature rate is that the pump has not cooled completely to the former ambient temperature 58. Instead, the pump has cooled only a small percentage of the temperature rise between time T1 through T2. In this case the asymptotic temperature to which the thermal disk 39 is rising, as shown by the dashed portion of curve 66, is to a temperature amount shown by the doubleheaded -arrows 67. It should be noted at this point in the operation that the ON time interval between T3 and T4 is considerably longer than the OFF time T2 through T3.

This intermittent power cycle will continue in :a similar manner during the portion of the curves not shown in FIG. 6. The temperature for pump 42 continues to rise above the ambient temperature 5S, and accordingly :the asymptotic temperature to which the thermal disk 39 climbs is higher and higher. For example the horizontal temperature line '7G at T5, and the solid and dashed temperature curve 7l, 72 represent the temperatures for disk 39 at a point such as B in the power curve 2) of FIG. 3.

At time T6 temperature curve 7 i for the thermal bridge 49 intercepts the switch opening temperature T3 `and f power is interrupted for the ion pump 42. it should be noted that in this case the ON time between T5 and T6 is approximately of equal duration to the GFF time TS t T7. As this intermittent power operation continues theV Ving temperature path. This decreased temperature curve is not shown in detail in FIG. 6 but is similar to the reverse of that just fully described.

The oreg ing described technique and apparatus of this invention thus achieves, during an entire pumping cycle, an increase in the pumping rate and a corresponding reduction in the number of gas molecules present in a vessel by applying power at a considerably higher value than the maximum power dissipation level of the pump. This technique and apparatus maintains the average maximum eective pump power dissipation level within the designed characteristics for thepump but does so by interrupting the power `application and obtaining the advantages listed hereinbefore which are achieved by the highest power input during times that the pump is actually on. Further, this intermittent ON and OFF power cycle for reasons discussed hereinbefore7 achieves a faster starting time and obtains a higher value of gas evacuation than has heretofore been possible by prior art pumping methods and apparatus.

It is to be understood that the foregoing features and principles of this invention are merely descriptive, and that many departures and variations thereof are possibile Iby those skilled in the art, without departing from the spirit and scope of this invention.

What is claimed is:

1. A method of increasing pumping ehciency for an ion pump comprising the steps of:

(a) applying a source of supply power to the ion pump greater in value than that rated for the pump based upon a continuous pumping mode of operation,

(b) sensing the temperature in the pump which results from power dissipation within the pump, and

(c) intermittently interrupting the supply of power to the pump each time its temperature approaches a maximum upper temperature limit for the pump.

2. The method of claim 1 comprising the additional steps of:

(a) intermittently reapplying the power supply source to the pump each time its temperature drops a. predetermined amount below the power interruption temperature, and

(b) continuous applying the supply of power to the pump at a high pumped pressure having low power dissipation resulting in the pump temperature being maintained below the maximum upper temperature limit for the pump.

3. The method of claim 2 comprising the additional step of:

(a) regulating the power applying and power interrupting intervals over one complete pumping cycle to supply only the average maximum effective power dissipation level `for the pump.

4. The method of increasing pumping eiciency for an ion pump comprising the steps of:

(a) connecting a source yof supply power to the pump greater than that of the design criteria for the ion Pump,v

3 (b) regulating power applying :and power interrupting intervals over one complete pumping cycle to supply only the average rated maximum eliective power dissipation level for the pump,

5 (c) sensing a predetermined critical sensing temperature of said pump established by power dissipation therein in excess of the normal power dissipation rated for fthe pump, and

(d) regulating in response t0 the sensed temperature the duration of power applying intervals relative to the power interrupting intervals at a ratio that assures the pump temperature is maintained below the critical temperature for the pump.

5. A high vacuum pumping system comprising:`

power dissipation level,

6. A high vacuum pumping system in accordance with claim 5 wherein said intermittent :power applying means comprises:

(a) a switch for opening and closing the power supply circuit to the pump,

(b) said temperature sensing means being connected in a thermal circuit to said pump for developing an exteriorly-available temperature proportional to the dissipation of power in the pump, and

(c) means coupled to said temperature sensing means and said switch for repetitively opening and closing said switch between first and second predetermined temperatures.

7. A high vacuum pumping system in -accordance with claim 6 wherein said last mentioned means comprises:

(a) a thermal sensitive device positioned to hold said switch in a closed condition during a starting period and a high vacuum period where said pump temperature is less than said iirst predetermined temperature, said thermal sensitive device further being operative during pumping periods between said starting and high vacuum periods when said pump temperature exceeds said rst temperature for varying the repetitive switch closure times and switch opening times in a predetermined.progressively increasing and decreasing ratio.

8. A high vacuum pumping system comprising: Y

(a) an ion pump having a maximum effective input dissipation level,

(b) Y.a source capable of continuously supplying input power in excess of that level, and

' (c) means intermittently operative for applying power from said source to said pump during a pumping cycle to maintain an average power input within the power dissipation level for the pump, said intermittent power applying means comprising:

Y (l) a switch for opening and closing the power Y supply circuit to the pump,

(2) temperature sensing means connected in aY (a) an ion pump having a maximum eective input c (a) a thermal sensitive device positioned to hold said switch in a closed condition during `a stanting period and 4a high vacuum period where said pump temperature is less than said rst predetermined temperature, said thermal senstiVe device further being operative during pumping periods between said starting and high vacuum periods when said pump temperature exceeds said rst temperature for varying the repetitive switch closure times and switch opening times in a predetermined progressively increasing and decreasing ratio.

10. A high vacuum pumping system having an ion .pump

rated with a given power dissipation level based upon a continuous pumping operation and equipped with internal pumping elements which are susceptible to thermal darnage at temperatures above a predetermined temperature level, said system comprising:

(a) a power supply for said pump having a power rating in excess of the given rating for the pump, (b) selectively operative switching means for supplying or removing power from said power supply to said pump,

(c) temperature sensing means connected in a thermal circuit to said pump and connected to said switching means for selectively supplying power continuously at high pressure and low pressure periods wherein power dissipation in said pump is below said predetermined temperature level and for intermittently supplying and removing power at medium pressures so that the pump temperature does not exceed said predetermined temperatures.

References Cited UNITED STATES PATENTS 3/ 1962 Milleron 230-69 3,118,103 l/1964 Mandoli et al 230-69 X 3,159,332 12/1964 Rutherford 230--69 ROBERT M. WALKER, Prima/y Examiner.

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