HK1142991B - High frequency helical amplifier and oscillator - Google Patents

Description

High frequency helical amplifier and oscillator

Priority

This application claims the benefit of U.S. provisional application No.60/902,537, filed on day 2, 21, 2007. Statement regarding jointly sponsored research

Funding for the project is provided under U.S. government contracts No. fa9550-07-C-0076, FA9550-06-C-0081, W911NF-06-C-0086 and W911NF-06-C-0026, and the U.S. government may have certain rights in the invention.

Background

The invention relates to millimeter and sub-millimeter wavelength generation, amplification and processing technology. And more particularly to an electronic device such as a traveling wave tube for millimeter and sub-millimeter wavelength amplifiers and oscillators, the invention will be described with particular reference thereto. However, the invention also finds application in other devices operating at millimeter and sub-millimeter wavelengths, and in other devices employing slow wave circuits.

Traveling Wave Tubes (TWTs) are electronic devices that typically include slow wave circuitry, which is generally defined by a hollow, vacuum-tight barrel with optional additional millimeter and sub-millimeter wavelength circuitry disposed within the barrel. An electron source and appropriate steering magnets or electric fields are arranged around the slow wave circuit to transmit a beam of electrons through a generally hollow beam channel. The electrons interact with a slow wave circuit and the energy of the electron beam is converted into microwaves which are guided by the slow wave circuit. Such traveling wave tubes provide millimeter and sub-millimeter wavelength generation and amplification.

Approximately 30 years ago, a helicon Backward Wave Oscillator (BWO) was the signal source of choice for a microwave swept frequency oscillator (BWO). However, this application has now been replaced by solid state devices. Helical slow wave circuits are still used as high power millimeter wave Traveling Wave Tube (TWT) amplifiers, producing approximately 200 watts CW at 45GHz, but the fundamental problem associated with conventional manufacturing, thermal management and electron beam transmission is the barrier to higher frequency applications. For decades, the conventional practice of spiral manufacturing has included winding round wire or rectangular tape around a cylindrical mandrel. As the desired frequency of operation increases, the mandrel diameter must decrease, causing the stress between the inner and outer radii of the helix to increase as the wire thickness becomes a significant fraction of the mandrel radius. The heat generated on the spiral, whether by electron beam interception or resistive losses from the RF current, must be conducted away through a dielectric support rod, which is a secondary thermal conductor and often makes no specific thermal contact with the spiral. As the frequency increases, the inner diameter of the helix decreases, providing reduced space for conventional electron beam transmission and thus reducing the achievable output power.

The present invention contemplates a new and improved vacuum electronic device that addresses the above-referenced difficulties and others.

Disclosure of Invention

In one aspect of the invention, a slow wave circuit of an electronic device is provided. The slow wave circuit comprises a helical conducting structure, wherein electron beams flow around the outside of the helical conducting structure and are formed as an array of sub-beams arranged in a circular pattern around the helical conducting structure; a generally hollow diamond barrel containing the helical conductive structure, wherein the hollow barrel is cylindrical in shape; and a pair of diamond dielectric support structures bonded to the spiral conducting structure and the hollow barrel.

In another aspect of the invention, a slow wave circuit for an electronic device having a cathode and a collector is provided. The slow wave circuit includes: a spiral conducting structure between the cathode and the collector, wherein electron beams flow around the outside of the spiral conducting structure and are formed as an array of beamlets arranged in a circular pattern around the spiral conducting structure; a generally hollow diamond barrel containing the helical conductive structure, wherein the barrel is square in shape; and a pair of continuous diamond dielectric support structures bonded to the spiral conducting structure and the hollow barrel.

In a further aspect of the invention there is provided a slow wave circuit for a helical travelling wave tube, wherein output power from the tube is deployed directly into free space of a helical antenna, the helical antenna being an extension of the slow wave circuit.

Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, that the detailed description and the specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.

Drawings

The invention resides in the construction, arrangement, and combination of the various parts of the apparatus, and steps of the method, so that the objects which will be more fully hereinafter set forth, pointed out in detail in the claims, and illustrated in the accompanying drawings in which:

FIGS. 1A and 1B illustrate a diamond supported micro-helical slow wave circuit according to aspects of the present invention;

FIG. 2 is a dispersion diagram for the operation of a helix;

fig. 3 is a diagram showing the distortion of an incomplete hollow electron beam at the cathode (left) and after strong magnetic field propagation (right);

FIG. 4 shows the stable propagation of a circular array of beamlets in a strong magnetic field;

FIGS. 5A and 5B show a front view (5A) and a cross-sectional view (5B) of an exemplary magnetic circuit design;

FIG. 6 illustrates the axial magnetic field generated by the circuit shown in FIG. 5;

FIG. 7 shows a segment of a dispersion map for operation as a 650GHz BWO;

FIG. 8 illustrates BWO with a grooved bucket for suppressing unwanted patterns;

FIG. 9 is a cross-sectional view of a probe in a waveguide coupler;

FIG. 10 is a graph showing return loss for a probe in a waveguide configuration;

FIG. 11 is a graph showing the tail magnetic field near the collector;

FIG. 12 shows the geometry of the collector in cross-section (left) and side view (right);

FIG. 13 is a side view of an electron track in a BWO collector;

FIG. 14 is a layout of the BWO body portion (half) and an end view of an assembled BWO structure;

FIG. 15 is a computer simulation of the electron gun with the side removed;

FIG. 16 is a diagram of an assembled TWT with a diamond enclosure as a transparent box;

FIG. 17 is a diagram showing a resonant loss structure placed on a TWT diamond support sheet;

FIG. 18 is a cross section of the output of the helical antenna;

FIGS. 19A-C illustrate one method of making a diamond supported spiral; and

fig. 20 is a diagram illustrating the actual twist of an ideal helical geometry that may be introduced by manufacturing techniques.

Detailed Description

Disclosed herein is a micro-helix slow wave structure in which a helix is fabricated by selectively plating metal onto lithographically patterned circular trenches fabricated by reactive ion etching of a silicon wafer. The spiral is supported by a diamond dielectric support rod. Diamond is the best possible thermal conductor and can be bonded to the helix. The electron beam is transmitted not through the center of the helix but around the outside of the helix. While all of these may not be practical in C-Band, such structures may be fabricated for operation in the millimeter and sub-millimeter wavelength ranges. Since this concept applies to both TWT and BWO, we will describe this concept.

Referring now to the drawings, wherein the showings are for purposes of illustrating exemplary embodiments only and not for purposes of limiting the claimed subject matter, FIGS. 1A and 1B provide views of a micro-helical slow wave circuit. As shown in fig. 1A, in a round diamond drum 12, one turn of the helix 10 may be supported by diamond studs 14 attached to each half turn. The diamond stud 14 is typically formed by Chemical Vapor Deposition (CVD).

Diamond synthesis by CVD has become a well established technique. It is known to synthesize diamond coatings on various objects, as well as free-standing objects. Free-standing objects are typically fabricated by depositing diamond on a planar substrate or a substrate having relatively simple holes formed therein. For example, U.S. patent No.6,132,278, the disclosure of which is incorporated herein by reference, discloses a generally pyramidal or conical diamond microchip emitter that forms a solid with plasma enhanced CVD by growing diamond to fill a hole formed in a silicon substrate, and U.S. patent No.7,037,370, the disclosure of which discloses an alternative method of fabricating a free-standing, internally supported, three-dimensional object having an outer surface comprising a plurality of intersecting facets, at least a subset of which have a diamond layer.

The inner surface 16 of the barrel 12 is metallized. Fig. 1B shows a plurality of turns of a spiral wire 20 supported by a continuous sheet 24 of CVD diamond in a square diamond drum 22. As in the previous case, the barrel may be fabricated from CVD diamond such that the inner surface 26 of the barrel 22 is selectively metallized. The introduction of the non-conventional square tub 22 enhances the microfabrication process and effectiveness in suppressing unwanted modes. The dimensions of these structures may vary depending on several factors, such as the frequency of operation and whether the device is an amplifier or an oscillator, and are determined using well-known computational techniques previously introduced by the inventors. See "Accurate Cold-Test Model of pharmaceutical TWT Slow-Wave Circuits", C.L.Kory and J.A.Dayton, Jr., IEEE Trans.ED, Vol.45, No.4, pp.966-971 (4 months 1998); "Effect of Manual circuits Variations on TWT Cold-TestCharacteristics," C.L.Kory and J.A.Dayton, Jr., IEEE trans.ED, Vol.45, No.4, pp.972-976 (4 months 1998); "Computational Investigation of Experimental interaction Impedance based on simulation transformation for reliable transforming-Wave tube Structure," C.L.Kory and J.A.Datyton, Jr., IEEE Transactions on electronic devices, Vol.45, No.9, p.2063, 9 months 1998; "First Pass TWT Design Success," R.T.Benton, C.K.Chong, W.L.Menninger, C.B.Throington, X.ZHai, D.S.Komm and J.A.Dayton, Jr., IEEE trans.ED, Vol.48, No.1, pp.176-178 (1 month 2001).

In the normal mode of operation, the electron beam is directed along an axis passing through the center of the helix. This is one of the reasons that has hitherto prevented operation of the helix at very high frequencies, as the helix inner diameter becomes too small to allow large currents to pass. One of the innovations herein is to allow current to pass through a relatively larger space outside the spiral. The electromagnetic fields here are very different. The helical dispersion relationship for the case of 95GHz TWT as shown in fig. 2 indicates the presence of three modes. All of the spiral structures described herein have a pattern similar to that of figure 2. The configuration shown in fig. 1 is ideal for the actual circuit being fabricated. Although the actual fabricated structures may differ slightly in detail, they are useful for accurately simulating the performance of micro-spiral devices. The computational techniques used to create fig. 2 are readily applicable and simulate the exact details of the fabricated structure.

The slope of the line drawn from the origin 30 of fig. 2 is proportional to the electron velocity. The slope of the mode line is proportional to the group velocity of the wave. The intersection of the electron velocity line and the mode line indicates a potential operating point where the velocity of the oscillometric and electron are close to synchronous. Two electron velocity lines are depicted in fig. 2. The upper line 32 intersects mode 1 at 95GHz, mode 2 at 270GHz, and mode 3 at 480 GHz. The slope at the operating point for mode 1 is positive, indicating a positive group velocity, and thus a traveling wave amplifier (TWT). However, at the operating point for modes 2 and 3, the slope is negative, indicating a potentially unwanted mode that can lead to unwanted back wave oscillations. The intersection with mode 1 is the first operating point and thus the dominant mode. It is often desirable to inhibit operation in modes other than the primary mode.

The slower electron velocity line 34 indicates that for operation at lower voltages, the primary operating point will be at the intersection with mode 2 at 170GHz, at which frequency the device will oscillate (operating as BWO versus TWT). The phase velocity line also crosses mode 1 at 250GHz and mode 3 at 270 GHz. Both of these operating points are potential sources of oscillation that would interfere with the primary mode if they were not suppressed.

Depending on the selected size and operating voltage, these spiral devices can be configured as amplifiers (TWTs) or oscillators (BWOs). Several methods for suppression of unwanted modes of operation will be described. The output power is coupled from the BWO circuit into a waveguide that is an integral part of the bucket. The horn antenna at the output waveguide end may radiate directly from the BWO for quasi-optical operation or the waveguide may terminate in a flange for operation with a closed system. Input power to the TWT may be achieved using quasi-optical coupling or by a waveguide that is an integral part of the barrel. The output power from the TWT may be directly radiated from a helical antenna fabricated as an integral part of the helical slow wave circuit or may be coupled to a waveguide as an integral part of the barrel. The electron beam for both TWT and BWO may consist of a circular array of beamlets that are held in place by a balance of forces that result in their mutual electrostatic repulsion and their interaction with the axial magnetic focusing field. By capturing the used electron beam in the new depressed collector with the tail of the focusing magnetic field, the efficiency of both BWO and TWT can be significantly enhanced.

Annular multi-beam array

The electron beam around the helix typically consists of several sub-beams arranged in a circular array. The number of beamlets and the current in each beamlet depends on the outer diameter of the helix and the current requirements of the device. The beamlets may originate from a field emission array that has been lithographically patterned, from a grid thermionic cathode, or from an array of small thermionic cathodes. The electron beam is immersed in a focusing axis magnetic field. A continuous hollow beam can be intercepted on the diamond support structure. However, as can be seen from fig. 3 (right), the discontinuous hollow beam becomes unstable. A circular array of beamlets is one solution for generating a steady stream of electrons. Electrostatic forces between equally spaced beamlets tend to push the beamlets away from each other and from the helix they surround. They are held in place by the axial magnetic field. In conventional spiral devices, electrostatic forces in the beam push electrons toward the spiral, resulting in an undesirable intercepted current.

An example of this multibeam propagation is shown in fig. 4, which shows the stationary propagation of an annular array of beamlets at progressively increasing distances from the cathode in a strong magnetic field. After a few millimeters of travel, the entire array is rotated a few degrees around the axis, which can be offset by deploying the beam at an offset angle. The individual beamlets also rotate about their own axes. Again, this example is for 650ghz bwo. Each sub-beam contains 0.75mA for a total beam current of 4.5 mA. For other applications at other frequencies, the number of beamlets and the current of each beamlet are designed as desired.

The calculations shown in fig. 4 are based on an array of beamlets deployed from a field emission cathode immersed in a 0.85 tesla axis magnetic field. The magnetic circuit 40 shown in fig. 5A and 5B demonstrates the possibility of generating the desired magnetic field plotted in fig. 6. The ordinate in fig. 6 is tesla and the abscissa is mm. The magnetic circuit 40 generally includes a center magnet 42, a pair of end magnets 44, and a pair of pole pieces 46. In this example, the permanent magnets 42, 44 are NdFeB 55 and the pole pieces 46 are permendur. Further, the outer diameter of the magnets 42, 44 is 70 mm and the inner diameter is 6 mm. The length is 30 mm for the center magnet 42 and 12 mm for the side magnets 44. The pole piece 46 is 60 mm in diameter and 4 mm long.

Sub-millimeter BWO

Fig. 2 shows the operation of a micro-helical slow-wave circuit as a BWO with a dominant oscillation mode and two competing higher order modes. A section of a dispersion map modified from fig. 2 for BWO operation at 650GHz is shown in fig. 7. For convenience purposes, the primary oscillation mode is designated as mode 1 in fig. 7. For example, a dispersion map of the graph is generated from computer simulations using the exact circuit dimensions. In this case, the configuration simulated in fig. 7 is for a BWO with a barrel and diamond stud support. The electron velocity line is plotted for a 12kV electron beam. Three approaches were found to suppress two undesirable higher order modes with relatively little effect on the dominant mode: the inner wall of the tub can be coated with a high resistivity material. The barrel may be made square as shown in fig. 1B.

Fig. 8 shows a single turn of the helix 50 supported in a grooved diamond drum 52 by diamond studs 54 attached at each half turn. As in the former case, the barrel may be fabricated from CVD diamond, with the inner surface 56 of the barrel 52 being selectively metallized. Including slot 58 to interrupt higher order modes. The helix as shown in fig. 1A and 8 is supported by diamond studs, which is the most efficient configuration. However, replacing the diamond studs with the continuous diamond sheet shown in fig. 1B may provide a more robust structure with an acceptably low loss of efficiency in some cases. The final design can be obtained by optimizing the computer simulation.

For example, the dimensions of a typical BWO circuit operating at 6kV using a square barrel and supported by a continuous diamond sheet are shown in table 1 below. The predicted power output from this design depends on the current and current density in the electron beam and the proximity of the beam to the circuit. The selection of these factors involves engineering tradeoffs. Increasing the current and current density places more stress on the electron source and the magnetic focusing system, while bringing the electron beam closer to the helix increases the probability of beam interception. For the BWO described in table 1 and shown in fig. 4 operating at 650GHz with an electron beam of 4.5mA, the computer predicted an output power indicating 70 mW. If the current can be increased to 10mA, the output power is 270 mW. By operating at higher voltages, the power can be further increased.

Table 1: circuit size (micron) for spiral BWO with square barrel

Pitch of thread, p	44.76
		Thickness of the support bar th	10
External diameter of the helix, diamo	62.5
		Internal diameter of the helix, diami	42.5
Width of helical band, tapew	26
		Barrel width, barreld	200
Thickness of the helix, rth	10

Helix to waveguide coupler

A spiral-to-waveguide coupler is necessary to provide an output path for BWO generated power. One form of this coupler is shown in figure 9. The same scheme can be used at the input to the TWT and can be used as an optional output coupler to the TWT. The end of the helix 60 is expanded to create a probe 62, the probe 62 being able to pass through the broad wall of a rectangular waveguide 64 built into the tube body. Also shown are a continuous diamond support sheet 66 and a matching short 68. The return loss of such a coupler designed for a 650GHz BWO is shown in fig. 10.

BWO collector design

The helical slow wave circuit extracts only a small fraction of the power in the electron beam. After passing through the slow wave circuit, the electron beam speed is reduced and the electron beam is captured at a relatively low energy in the depressed collector. Fig. 11 shows the tail of the magnetic field first seen in fig. 6. The magnetic field coupled with the transverse electrostatic field formed by the collector electrodes 68, 69 shown in fig. 12 lowers the electrons in the spent beam to about 5% of their energy and captures them on a support structure thermally isolated from the slow wave circuit. One collector geometry that meets our needs is a split cylinder with the upper half set at the cathode voltage and the lower half set at the collector voltage, typically biased 300V higher than the cathode voltage. For operation at 650ghz bwo, fig. 13 shows simulated electron trajectories in the collector.

BWO body layout

A BWO body housing a slow wave circuit and an electron gun may be formed by depositing diamond onto a ridge array of a silicon die, patterned by deep reactive ion etching. When the silicon is removed, the remaining diamond is in the form of a half-cell array. FIG. 14 shows a detailed sketch of an example BWO housing 70. The left side of the figure shows the location of the cathode mount 72 and the first anode 74 separated by a length 76 of insulating diamond. The cross-sectional shaded area indicates the location of the second anode 78. Details of the anode slots in the electron gun are shown on the left, and the output coupler 80 and tub 82 of the slow wave circuit are shown on the right. Also shown are a feedhorn 84 and an output waveguide 86. The bucket 82 has a depth of 100 microns and the remaining elements have a depth of 190 microns, which is typically required for 650GHz BWO. A cross-sectional view featuring the diamond enclosure 88, the barrel aperture 90, the helix 92, and the feedhorn aperture 94 is also shown. Portions of the barrel 82, waveguide 86, feedhorn 84, anode slots 74, 78, and cathode mount 72 are selectively metallized.

Fig. 15 shows a more detailed depiction of the electron gun, with the sides removed. Reference numerals 96 and 97 refer to the top and bottom portions of the diamond cassette 98, respectively, wherein the diamond cassette 98 houses the BWO and provides electronic isolation in the gun and barrel of the slow wave circuit. The slow wave circuit shown in fig. 14 is 6mm long. When a longer slow wave circuit is required, the layout can be extended in the length direction. The output waveguide, which is formed as an integral part of the housing, is flared at the end to create a feedhorn. After the anode and array of the helical slow wave circuit are inserted into the lower half of the body array, the upper half is added and the whole structure is combined. The individual BWOs are removed from the bonded array by laser cutting. A view of the output end of the assembled BWO is also shown in fig. 14. The slow wave circuit is positioned on the axis of the magnetic field. The RF output is off-axis and is directed through a window in the collector to the end of the evacuated envelope. For the 650GHz BWO case, bucket 82 is 100 microns deep, while the remaining area of the layout is 190 microns deep. Of course, when the two halves are assembled, these dimensions are doubled so that the depth of the slow wave circuit barrel 82 is 200 microns and the waveguide and electron gun dimensions are 380 microns.

Micro-helical TWT

Many of the already described for BWO apply to TWT. However, there are some differences. Since the TWT is an amplifier it must have an input coupler and since the output is at the end of the tube rather than in the middle, the output power can be radiated directly from the slow wave circuit without having to pass through a waveguide. Due to the very high frequency, coupling into the TWT quasi-optical input can be via the antenna as well as the waveguide. FIG. 16 is a view of the TWT 100 showing the diamond envelope as a transparent box surrounding the TWT 100. TWT 100 includes waveguide 102, probe 104, field emission cathode 106, first anode 108, second anode 110, and helix 112. The sketch of BWO looks very similar, except that there is no input waveguide.

As noted with respect to fig. 2, there are two undesirable modes of back wave in addition to the desired amplification mode for TWT. The method for suppressing undesired higher order modes in BWO is not applicable to TWT. If higher order modes are a problem, the higher order modes must be eliminated by inserting a resonant loss pattern 120 onto the diamond support structure 122 as shown in fig. 17. See "Resonant Loss for HelixTraceling Wave Tubes", C.E. Hobrecht, International Electron Devices Meeting, 1978.

The output from the TWT is directly radiated from the slow wave circuit by the helical antenna, which is fabricated as an integral part of the helical slow wave circuit. This eliminates one of the major failure points in the high power millimeter wave tube, the connection from the slow wave circuit to the output waveguide. In the computer simulation shown in fig. 18, half of the structure is cut away to show the details of the helical antenna 130. Also shown are a continuous diamond support sheet 132 and a helical slow wave circuit 134. The antenna generates linearly polarized waves. By using the antenna as a feed for the cone shaped horn, the antenna directivity can be enhanced. The antenna is directed towards a window in the evacuated envelope.

Helical slow wave circuit fabrication

All TWTs and BWOs described herein are based on micro-helical slow-wave circuits whereby the helix is fabricated using microfabrication techniques such as lithography, reactive ion etching, deep reactive ion etching and selective metallization. Given some beliefs, the outer diameter of the helix is only 62.5 microns for 650ghz bwo. The spiral is supported by a CVD diamond sheet or CVD diamond stud.

One method of fabricating a helical slow wave circuit is shown in fig. 19A-C. In fig. 19A, a half-spiral 140 of metal has been deposited in a cylindrical trench 142 etched into a diamond coated silicon wafer 144. Diamond sheets 146 are also shown on either end of the trench 142. In fig. 19B, two silicon supported spiral halves 140 are aligned and joined to form a spiral 148. In fig. 19C, the silicon 144 has been removed to complete the production of the diamond supported helix 148.

Silicon wafers are coated with a diamond film and then lithographically etched to produce an array of openings for the electron gun and the spiral. Circular grooves were etched into the diamond coated silicon wafer to form the desired shape of the spiral outer diameter. The circular trenches are lithographically patterned and selectively metallized to produce an array of half-spirals. These are bonded together and when the silicon is removed, an array of diamond supported spirals remains.

The barrels of the helix can also be manufactured using micro-fabrication techniques. The mold is created by etching an array of ridges into a silicon wafer. Diamond is grown on the wafer and silicon is removed. The result is an array of diamond half-boxes as the body of the tube. The tube body includes barrels of helical slow wave circuits, dielectric insulation for the electron gun, and input and output waveguides, as desired. Alignment of the parts is ensured because the parts are manufactured in the same operation and they become one solid piece of diamond. For lower frequency millimeter wave devices, more conventional processing techniques may be used to satisfactorily manufacture the body. An array of helices is placed on the bottom half-box, the top box is added and the entire assembly is bonded together.

The diagram shown in fig. 19 is an ideal state of the spiral structure. The sketch in fig. 20 shows a more realistic resulting structure, showing the actual twist of an ideal helical geometry that may be introduced by the manufacturing technique. The diamond support rods 150 overlie the bonding pads of the wire spiral 152. The bonding material typically includes solder balls 154. Depending on the shape of the trenches etched into the silicon, the actual outer surface of the spiral 156 may not be perfectly round as a result. The alignment of the helix 156 and the electron beam will be controlled by the detents 158 in the diamond support sheet 150, the detents 158 being aligned with the walls 160 of the barrel to direct the slow wave circuit into the center of the barrel. Note also that the inside of the barrel is metallized.

To complete the bond between the spiral and the diamond and between the two circuit halves, metal tabs must be present on each side of the structure and the bonding material itself can further distort the structure. The extent of these shifts from the ideal case depends on the manufacturing technology and also on the operating frequency. However, none of these will render the above analysis ineffective. The computer simulation techniques employed herein can provide the actual size and shape of the helix, and the actual size and shape of the helix can be adjusted to achieve the desired performance.

In conventional vacuum electronic devices, a skilled technician manufactures one device at a time from several hundred component parts. These devices will be manufactured on a wafer scale that is suitable for mass production. Two wafers would be required to make the spiral array and two wafers would also be required to make the body array. The four wafers are bonded together, the silicon is removed, and the devices are separated in a final step by laser dicing. Again using 650ghz bwo as an example, about 50 devices can be fabricated from 4 100mm diameter silicon wafers, greatly reducing the per unit cost of the device.

Typical helical slow wave circuits are limited to frequencies below 60GHz and typically operate at frequencies much lower than 60 GHz. The spiral circuit described herein can be designed to operate as a BWO or TWT in the range from 60GHz to several THz.

The helix is not manufactured in a conventional manner by winding a metal wire or tape onto a mandrel. These spirals are produced using microfabrication techniques that may include reactive ion etching, lithography, selective metallization, and mold bonding.

For high frequency conventional spirals, the thickness of the wire or ribbon becomes a significant fraction of the mandrel radius, which generates significant stress on the outside of the spiral and leads to twisting and structural failure. There is no such effect in these spirals.

The helix will take the approximate circular shape of a conventional helix. The actual details of the spiral shape will be computationally modeled to arrive at the final design.

For enhanced efficiency, the pitch can be lithographically controlled to produce a tapered circuit that keeps the electromagnetic waves and electron beams synchronized.

A conventional spiral is typically held in a barrel under high compression force by three dielectric rods. The helix is not under large compressive stress; the helix is bonded at 180 degree intervals to a Chemical Vapor Deposition (CVD) diamond support, which may be a continuous sheet or stud attached to each half turn of the helix.

Dielectric rods used in conventional spiral circuit fabrication have relatively poor thermal conductivity. The CVD diamond support used here has the highest known thermal conductivity.

The thermal conductivity between a conventional spiral and a dielectric rod is a highly non-linear function of the compressive force between the two. This force is a function of temperature, and therefore the thermal capacity of the tube decreases when the barrel is heated in high power operation. Here the CVD diamond support is bonded to the spiral. The thermal conductivity across the bond is not a function of temperature.

In conventional helical vacuum electronics, the electron beam passes through the center of the helix. At high frequencies, the diameter of the helix is reduced to a value at which significant current cannot pass through the helix. In these devices, the electron beam is directed around a relatively larger space outside the helix.

Conventional hollow electron beams are susceptible to instability. As used herein, an electron beam is comprised of a plurality of beamlets arranged in a stable circular array.

The multibeam array may be formed from a grid thermionic cathode, a plurality of thermionic cathodes, or from a patterned field emission array.

In conventional helical vacuum electronics, space charge forces push electrons towards the helix causing beam interception, which can reduce efficiency and cause failure. In these devices, space charge forces between the beamlets will push each other away and thus away from the helix.

In conventional helical vacuum electronics, the barrel surrounding the helix is round. In this apparatus, the barrel may be square in some applications for ease of manufacture and to eliminate unwanted modes of operation.

In conventional vacuum electronics, the electron gun and slow wave circuit are fabricated separately and then soldered together. The accuracy of the alignment between these two parts (which is very important to the performance of the device) is compromised by the tolerances of the welding operation. In these devices, the barrel of slow waves and the walls of the electron gun are manufactured as a unit and are thus precisely aligned.

The electron gun wall will be grooved to receive the anode insert and provide electrical connection to the anode when selectively metallized.

The anode may be fabricated from a metal foil that has been formed using electrical discharge machining, or may be fabricated from highly conductive silicon that has been formed by lithography and deep reactive ion etching or other microfabrication processes.

In conventional spiral vacuum electronics, the barrel is fabricated from metal. In this apparatus, barrels may be made from CVD diamond that has been selectively metallized.

In conventional vacuum electronics, the electron gun, slow wave circuit and input/output coupler are fabricated as separate components and soldered together. In this apparatus, they are fabricated as a single unit in a CVD diamond enclosure to achieve precise alignment.

Conventional vacuum electronics are assembled from hundreds of components at a time by a skilled technician. The device will be manufactured in wafer scale mass production, which will produce up to 50 devices from a single operation using 4 100mm silicon wafers, resulting in significant cost savings for each unit.

In conventional TWTs, output power is coupled from a slow wave circuit to a waveguide or transmission line. The solution can also be adapted to the device. However, the TWT will be designed to radiate RF output power directly from the slow wave circuit through a helical antenna that is fabricated as an integral part of the helical slow wave circuit.

For conventional TWTs, input power is introduced into the device through a waveguide or coaxial line. In this device, due to the very high frequencies, input power can be introduced through the antenna or the quasi-optical coupler.

The output of the helical antenna may be fed into a small feedhorn to enhance the directivity of the antenna.

The waveguides are formed as integral elements of the device barrel as input or output transmission lines for TWTs or as output transmission lines for BWOs.

A probe fabricated as an extension of the helical slow wave circuit is coupled to the input or output waveguide through an opening in the broad wall of the waveguide.

A short circuit is fabricated into the waveguide to match the probe and waveguide.

For BWO, unwanted higher order modes are suppressed by coating the inside of the tub with a low conductive material, by periodically grooving the tub, or by manufacturing the tub in a square rather than circular configuration.

For TWTs, unwanted higher order modes are suppressed by adding resonance losses to the diamond support sheet.

The spent beam from the BWO is captured at low energy in a two-stage collector that captures electrons between orthogonal electromagnetic fields. The spent beam from the TWT is captured in a multistage depressed collector.

The output power from the BWO is radiated from the BWO enclosure by a horn antenna fabricated at the end of the output waveguide.

The above description merely provides a disclosure of particular embodiments of the invention and is not intended to limit the invention. Therefore, the present invention is not limited to only the above-described embodiments. On the contrary, it is recognized that one of ordinary skill in the art could conceive alternative embodiments that fall within the scope of the invention.

Claims (18)

1. A slow wave circuit of an electronic device, the slow wave circuit comprising:

a helical conducting structure, wherein electron beams flow around the exterior of the helical conducting structure and are formed as an array of beamlets arranged in a circular pattern around the helical conducting structure;

a hollow barrel containing the helical conductive structure, wherein the hollow barrel is cylindrical in shape; and

a pair of dielectric support structures bonded to the helical conductive structure and the hollow barrel.

2. The slow wave circuit of claim 1, wherein the electronic device comprises a Traveling Wave Tube (TWT).

3. The slow wave circuit of claim 2, wherein a spent beam from the TWT is captured in a multistage depressed collector.

4. The slow wave circuit of claim 1, wherein the electronic device comprises a Backward Wave Oscillator (BWO).

5. The slow wave circuit of claim 4, wherein a spent beam from the BWO at low energy is captured in a two-stage collector that captures electrons between orthogonal electromagnetic fields.

6. The slow wave circuit of claim 1, wherein the hollow barrel comprises four equally spaced slots symmetrically placed around the pair of dielectric support structures.

7. The slow wave circuit of claim 1, wherein the dielectric support structure is comprised of diamond.

8. The slow wave circuit of claim 1, wherein the hollow barrel is comprised of diamond.

9. The slow wave circuit of claim 1, wherein the circuit operates at a frequency greater than 60 GHz.

10. A slow wave circuit of an electronic device having a cathode and a collector, the slow wave circuit comprising:

a spiral conducting structure between the cathode and the collector, wherein electron beams flow around the outside of the spiral conducting structure and are formed as an array of beamlets arranged in a circular pattern around the spiral conducting structure;

a hollow barrel containing the helical conductive structure, wherein the barrel is square in shape; and

a pair of continuous dielectric support structures bonded to the helical conductive structure and the hollow barrel.

11. The slow wave circuit of claim 10, wherein the electronic device comprises a Traveling Wave Tube (TWT).

12. The slow wave circuit of claim 11, wherein a spent beam from the TWT is captured in a multistage depressed collector.

13. The slow wave circuit of claim 10, wherein the electronic device comprises a Backward Wave Oscillator (BWO).

14. The slow wave circuit of claim 13, wherein the spent beam from the BWO is captured at low energy in a two-stage collector that captures electrons between orthogonal electromagnetic fields.

15. The slow wave circuit of claim 10, wherein the continuous dielectric support structure is comprised of diamond.

16. The slow wave circuit of claim 10, wherein the hollow barrel is comprised of diamond.

17. The slow wave circuit of claim 10, wherein the circuit operates at a frequency greater than 60 GHz.

18. The slow wave circuit of claim 2 or 11, wherein output power from the tube is deployed directly into free space of a helical antenna, the helical antenna being an extension of the slow wave circuit.