US3065374A - Low noise electron discharge device - Google Patents

Description

Nov. 20, 1962 Filed 001,. 12, 1959 FIG. l

R. G. ROCKWELL LOW NOISE ELECTRON DISCHARGE DEVICE FIG.

2 Sheets-Sheet l ROBERT G. ROCKWELL BYu/( WV ATTORNEY NOV 20, 1962 R. G. ROCKWELL 3,065,374-

LOW NOISE ELECTRON DISCHARGE DEVICE Filed Oct. l2, 1959 2 Sheets-Sheet', 2

Low Noise Gun Dritt Space Input Drift Output a b c d Cavity Tube Cavity Plane i I l I l I I I Ij :I CGIIIOde I l I Electron] II I V Focus VI V2 V3 Beam V0 Vo V Collector I I I I I I I I I Characteristic w W COHSTunt l Impevalance VIV Sonsmm I I Amplified 'se I I I I I I I I FIG. 6

l Current e Sum of Thermal Noise I Noise Current Squared q2 I I u ent Wave IFrom Ante I ited by Ia we'ekf Current Wave Z -IIA ai Excited by Vq F IG. 4 II 22 35 B2 FIG. 5

VIIll 79 INVENTOR.

ROBERT G. RocKwE'Ll.

mx/(Hm ATTORNEY United States Patent 3,665,374 LW NGISE ELECRON DISCHARGE DEVICE Robert G. Rockwell, Los Altos Hills, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Oct. 12, 1959, Ser. No. 845,875 18 Claims. (Cl. S15- 5.34)

The present invention relates to electron discharge devices and more particularly to klystron tubes having an extremely low noise figure.

The use of electron discharge devices for amplifying relatively weak microwave signals is limited by the inherent internal noise of the device. in the past, klystron tubes have been considered as having inherent high noise figures of the order of 25410 db whereas noise figures as low as 6-11 db have Ibeen achieved with low noise traveling wave tubes and noise figures on the order of 3 db with backward wave amplifiers. A theory has been advanced without verification that low noise guns might be used with klystrons to reduce their noise figures as has been done with traveling wave tubes in the past.

The major difficulty in providing a klystron amplifier with a low noise electron gun such as has been utilized in traveling wave tubes is the higher beam current which is required for adequate klystron gain, and up until now it has been assumed that high beam current and low noise performance were incompatible. Some of the reasons for this were that l) a larger impedance transformation is required which increases the lens effect `by demanding a higher perveance gun; (2) a larger potential variation exists across the beam due to the space charge; and (3) the higher current density required necessitates a higher cathode temperature and hence a higher noise ligure. Furthermore, in order to develop a low noise klystron, new assembly techniques are required because of the need for an entirely non-magnetic tube, which is not necessary for most other klystrcns. In constructing a low noise klystron amplifier, it has been realized that of primary importance are the alignment of the low noise gun electrodes with the drift tube, alignment of the drift tube over its entire length, alignment of the beam with the magnetic eld, elimination of the collector secondary electrons from the beam and the cleanliness of the tube. By constructing a klystron tube in accordance with the present invention a noise figure of 6.7 db was obtained.

A by-product of the higher current in the high perveance gun of a low noise klystron is a very wide dynamic range.

The object of the present invention is to provide a novel low noise klystrony tube useful as an amplifier of relatively weak microwave signals.

One feature of the present invention is the provision of a novel cathode -mounting means including a cathode, a first mounting member, a plurality of stilts supporting the cathode on the first mounting member, a second mounting member, and a plurality of stilts supporting the first mounting member on the second mounting member, the cathode and the second mounting member being disposed in the same direction from the first mounting member whereby the position of the cathode with respect to the second mounting member is maintained substantially fixed over a wide temperature range.

Another feature of the present invention is the provision of a novel electron gun mounting assembly including a hollow gun mounting block, means for mounting an electron gun on the gun mounting block and axially aligned therewith, and a hollow gun mounting sleeve adapted for mounting on an electron discharge device, a surface of the gun mounting block and a surface of the gun mounting sleeve being mating conical surfaces where- 'icc lby the gun mounting sleeve can lbe mounted on the electron discharge device and aligned with the axis thereof and then the gun mounting block with an electron gun mountedV thereon can `be mounted on the gun mounting sleeve via said mating conical surfaces thereby to mount the electron gun axially of the electron discharge device.

Another feature of the present invention is the provision of a cathode mounting assembly including a cathode supported on a mounting member, a plurality of electrodes for mounting in front of the cathode, a plurality of insulator spacers, one to be positioned on each s'ide of the electrodes and the mounting member for keeping the electrodes and the mounting member in insulated, spaced-apart relation, a rst retaining member positioned against that spacer on the back of the mounting mem-ber, a second retaining member positioned against that spacer on the front of the forwardmost electrode, a plurality of mounting screws adapted -for connecting the first and the second retaining members and with the mounting member, the electrodes, and the spacers positioned under compression therebetween, and a plurality of hollow expansion members, one to be positioned between the head of each of the mounting screws and the adjacent retaining member whereby each of the expansion members expands when heated and compensates for the expansion in the respective mounting screw thereby keeping the mounting member, the electrodes, and the spacers under compression.

Another feature of the present invention is the provision of a novel drift tube assembly in an electron discharge device -comprising two axially aligned drift tubes with an annular header supporting each of the drift tubes within the body of the electron discharge device and axially aligned with the electron beam whereby the two drift tubes provide a single uniform `drift space accurately aligned with the axis of the electron beam.

Another feature of the present invention is the provision of the novel drift turbe assembly of the aforementioned feature including two hollow cylinders axially mounted adjacent one another, each closed at its outward end by one of said annular headers thereby providing an enlarged opening between the adjacent ends of said drift tubes whereby the reduced plasma radian frequency is increased and thus the required length of the drift space is reduced.

Still another feature of the present invention is the provision of a novel magnetic field distorting member adapted to surround the means Ifor collecting the electron beam in an electron ldischarge device, said field distorting member provided with a taper on the end thereof which surrounds the inner end of the means for collecting the electron beam whereby the magnetic field within' the means for collecting the electron beam is distorted, thereby preventing secondary electrons from returning to the cavity resonator means within the electron discharge device from the means for collecting the electron beam.

Other obiects and advantages of the present invention will become apparent `from the specification taken in connection with the accompanying drawings wherein,

FIG. 1 is a longitudinal cross sectional view of the low noise klystron embodiment of the present invention with the magnetic field distorting means .retracted from its operating position,

FIG. 2 is an enlarged view of the low noise gun assembly shown in FG. l,

FiG. 3 is a perspective View of the cathode mounting structure shown in FiG. 2,

FIG. 4 is an enlarged cross section view of the cathode shown in FIGS. 1 3.

FIG. 5 is an enlarged view of the collector assembly shown in FIG. l and delineated by line 5--5 with the magnetic field distorting means in operating position surrounding the collector, and

FIG. 6 is a graph showing a schematic of the low noise klystron embodiment of the present invention with a curve plotted for the characteristic impedance and curves plotted `for the noise current squared over the different regions of the low noise klystron amplifier.

Referring now to Ithe drawing, the present invention includes a cylindrical main body portion 11 as of, for example, copper having a multi-diameter longitudinal bore 12 extending therethrough. Two hollow drift tube supporting cylinders 13 and 14, as of copper, are positioned adjacent one another and axially aligned within a reduced diameter portion of the longitudinal bore 12. Axially aligned hollow cylindrical drift tubes 15 and 16 are positioned in the outer ends of cylinders 13 and 14 by annular headers 17 and 18, respectively. The drift tubes 15 and 16 project outwardly from the cylinders 13 and 14 thereby providing a long, accurately aligned drift region from the outer end of drift tube 15 to the outer end of drift tube 16. This construction provides a long uniform drift region which is extremely dilcult to provide by means of machining a single tu-be. Also by opening lup the drift region within cylinders 13 and 14 the reduced palsma radian frequency is increased and thus the length of the required drift region is reduced. The cylinders `13 and 14 could project in the opposite direction from that shown or could be limited 'in length to the Width of the headers 17 and 18 as long as the drift tubes 15 and 16 are rigidly supported. A drift tube assembly such as that illustrated passed 99.7% of the electron -beam therethrough.

Fixedly secured, as by brazing, within one end of the longitudinal bore 12 of the main body 11 is a radially tapered annular header 19, as of copper, with a short input drift tube 21 projecting from the aperture therethrough toward the drift tube 15. Axedly secured, as by brazing, within the other end of the longitudinal bore 12 of main body 11 is a narrow annular header 22, as of copper, with a short output drift tube 23 projecting toward drift tube 16 from the aperture therethrough. The main body 11 and the annular headers 19 and 17 define a re-entrant input cavity resonator 24, and the main body 11 and the annular headers 1S and 22 define a re-entrant output cavity resonator 25.

A bore 26 is provided through the main body 11 into cavity resonator 24 for positioning a coaxial input assembly 27 therein.

Each of the cavity resonators 25 and 26 can be provided with a tuner assembly 31 including a flexible tuner diaphragm 32 sealed in a bore in the main body 11 and actuated by a tuner rod 33 for changing the resonator frequency of the cavity resonator, but inasmuch as the tuner assembly does not constitute part of the present invention it will not be described.

Afiixedly secured, as by ybrazing, to the main body 11 and sealing oft one end thereof adjacent the input cavity resonator 24 is a beam generating assembly 34. The beam generating assembly 34 includes a cathode 35 made up of an emissive member 36 as of, for example, impregnated porous tungsten and a cup-shaped mounting member 37 as of, for example, molybdenum. The emissive portion 36 comprises a cylindrical pellet 36a with a proboscis 36b projecting forwardly therefrom having a flaredout cathode face 36e. The pellet 36a is held within a sleeve 37a projecting from the bottom of the mounting member 37. The cathode is supported from a shoulder 37b projecting outwardly from the bottom of the mounting member 37. Three first mounting stilts 38 of, for example, molybdenum support the shoulder 37 on a first mounting ring member 39 which is in turn supported by three second mounting stilts 41 of the same material as the stilts 38 on a second mounting ring member 42, the cathode 35 and the second mounting ring member 42 being disposed in the same direction from the first mounting member 41 so that the position of the cathode with respect to the second mounting ring member is maintained substantially fixed over a wide temperature range. A focus electrode 43, a first anode electrode 44, a second anode electrode 45, and a third anode electrode 46 are positioned in front of the cathode in the direction opposite from the first mounting ring member 39, and an insulator spacer ring 47 as of, for example, quartz is positioned on each side of the electrodes and of the second mounting ring member. The shapes for the electrodes were determined by use of an electrolytic tank as customary in the art for electron guns designed to be operated with a negative focus electrode potential of about 20% of the anode or rst electrode potential.

A first retaining ring member 48 is positioned against that insulator spacer ring on the back of the second mounting ring member, and a second retaining ring member 49 is positioned against that spacer 47 on the front of the forwardmost electrode. These retaining ring members 48 and 49 are held together with the second mounting member 42, the electrodes 43, 44, 45, and 46, and the spacers 47 positioned under compression therebetween by three mounting screws 51 of, for example, stainless steel which pass through apertured projections of the first retaining ring member 49 and thread into tapped holes in the second retaining ring member. A hollow cylindrical expansion member 52 as of, for example, copper is positioned between the heads of the screws 51 and the apertured projections of the first retaining ring member 48 so that the compression on the second mounting ring member 42, the electrodes, and the spacers 47 is not reduced when the mounting screws 51 expand after heating up.

A hollow cylindrical inner heat shield 53 is supported by a flange on one end fixedly secured, as by brazing, to the first mounting ring member 39, projects toward and surrounds the back end of the cathode 35, shield 53 being slotted to permit passage of the first mounting stilts 38 therethrough. An outer heat shield 54 is supported by a flange on the end thereof ixedly secured, as by brazing, to the second mounting ring member and projects rearwardly of the cathode 35 to surround the front end of the cathode 35. A mounting cup 55 surrounds the rst mounting ring member 38 and the heat shields 53 and 54 and is held in place by an outwardly projecting flange on an end thereof, the outwardly projecting flange being positioned between the second mounting ring member '4Z and the spacer 47 therebehind.

The second retaining ring member 49 is axially aligned with a hollow conical gun mounting block 56 by being positioned on an annular shoulder on the end thereof and is tixedly secured to the gun mounting block 56 by means of screws 57 which thread into tapped holes in the block 56. The annular shoulder on the block 56 is notched to permit the mounting screws 51 to extend through the second retaining ring member 49. The gun mounting block 56 is fixedly secured to and axially aligned with one end of a gun mounting sleeve 58 which has a conical interior surface which matches the conical exterior surface of the gun mounting block 56. The other end of the gun mounting sleeve 59 is fixedly secured to the end of the main body 11 adjacent cavity resonator 24 by means of a braze between mating flanges on the sleeve 58 and the main body 11.

The vacuum seal around the beam generating assembly 34 includes a hollow cylinder 59 which is fixedly secured at one end, as by brazing, to an outwardly projecting shoulder on the gun mounting sleeve 58 and is xedly secured at the other end, as by brazing, to a hollow cylindrical gun sealing member 61 which surrounds the cathode, the electrodes and their respective mounting members. The other end of the gun sealing member 61 is secured to a stem cup 62, the end of which is sealed closed by a stem sealing disk 63 as of, for example, ceramic. The stem sealing disk 63 is provided with a aoeasr U plurality of apertures 64 therethrough which are in tu-rn covered externally by stern hats 65. An apertured electrode positioning plate 66 is positioned on the rearward end of the mounting cup 55 and lead wires 67 attached to each of the individual electrodes 43, 44, 45 and 46 pass through standoff insulators 68 as of, for example, ceramic and are then xedly secured to one of the individual stem hats 65'. Positioned within the cupped end of cathode 35 is a heater element 69, the lead wires 71 of which pass through an aperture in plate 66 and are connected to separate stem hats 65. A circular spray plate 72 is positioned over the central aperture in plate 66 by means of support rods 73 which pass through plate 66 and are atiixedly secured to the outside surface of the mounting cup E5. Annular iange splash shields 74 are positioned between the second mounting ring member 42 and the focus electrode 43 and the iirst anode electrode 44 adjacent the spacers 47 to prevent cathode material from being deposited upon the spacers 47 which would thus cause arcing between the members insulated by spacers 47. The klystron tube is evacuated through a tube 70 which is then pinched off.

Fixedly secured to the annular header 22 at the end of the main body 1l and sealing off that end thereofis a collector assembly 75 (see FG. 5) including a hollow cylindrical outer adapter '76 as of cupronickel, one end of which is fixedly secured to the header 22 surrounding the aperture therethrough and the other end of which is secured to a sealing cup '77 as of platinum. The cup 77 is in turn secured, as by brazing, to one side of an insulator ring 7S as of, for example, ceramic, the other end of the ring 77 being secured to an eyelet 79 as of platinum. Also secured to the eyelet 79 and positioned axially within the outer adapter 76 is a hollow cylindrical inner adapter 8l as of cupronickel, the inner end of which is secured to an outwardly projecting iiange on the inner end of a hollow cylindrical collector S2 as of copper positioned axially within and spaced from the inner adapter 81. The outward end of collector 32 is closed by a collector plug `33 as of copper, the inner surface of which is tapered in order to help prevent secondary electron emission. A hollow cylindrical field distorting member Sd as of, for example, steel provided with a taper on one end thereof is adapted to tit around the collector 82 Within the inner adapter 81 for distorting the magnetic field within the collector 82 to thereby prevent secondary electrons from retraversing the drift region and inducing undesired noise currents in the input cavity. This asymmetrical piece of steel appeared to be mandatory, for without it the noise figure of a low noise klystron amplifier which had achieved a noise tigure of 6.7 db was about db.

Since a section of an electron beam is like a section of a transmission line, by analogy the electron beam can act as an impedance transformer. It has been shown that under usual conditions of operation a diode is essentially an exponential impedance transformer. Therefore, a low noise electron gun which includes a plurality of electrodes with different voltages applied thereto for minimum noise `figure acts as a section of non-uniform transmission line which exponentially transforms the existing impedance of the beam at the virtual cathode to the required impedance of the beam at the input cavity gap.

Referring now to lFIG. 6 the top of the graph shows a schematic diagram of a low noise klystron tube embodiment of the present invention showing a cathode, a three region low noise gun, and a drift space followed by a klystron circuit. Because of the various electrode voltages, it is shown in the middle of the graph that the characteristic beam impedance 'which exists as a constant value through the drift space and the klystron circuit is not constant in the region between the cathode and the last electrode on the low noise electron gun. The gun, then, transforms, the impedance of the beam at the cathode to the impedance of the beam in the drift space.

At the bottom of the graph in FIG. 5 is shown an approximate estimate of curves for the square of the different noise currents. The solid line is the overall noise current wave. Up to the klystron circuit region the overall noise current is the sum of the uncorrelated noise currents excited at the potential minimum by the full shot-noise fluctuations in the current designated by the line of short dashes and the Rack uctuations in the electron velocity designated by the line of long and short dashes. A minimum of overall noise current is believed to occur in the vicinity of the second anode, and the drift space is made long enough to place the next minimum at the input cavity gap. In the klystron circuit region noise from the input coupli-ng antenna, indicated by the dotted line, and ampliiied noise current created at the klystron input cavity gap, indicated by the line of long dashes, add to the overall noise current wave.

Several two-cavity, low noise klystron amplifiers of the present design have been built `for operation in both the S-band and C-band frequency ranges, with a noise igure of 6.7 db being achieved for an S--band ampliiier and 9 db for a C-band amplifier.

The construction of the low-noise electron gun which performed satisfactorily incorporated a type B impregnated cathode comprising a 0.100 diameter tungsten pellet with a 0.030 proboscis on the end thereof flared out to a 0.040 cathode face. The spacing of the first anode from the end of the cathode was equal to the diameter of the cathode face, namely, 0.040; of the second anode from the -rst anode was 0.096, and of the third anode from the second anode was 0.122. This made the total length of the low noise gun 0.258". None of these dimensions were necessarily optimized as far as obtaining noise reduction is concerned; hence, it appears there is room in this area for improved noise performance. The design of the cathode with the flare on the end thereof was aimed at reducing emission from the sides of the cathode proboscis and may well have helped reduce the noise figure by the manner in which the noise space charge waves were excited in the vicinity of the cathode.

Experimental data proved that for a low noise klystron amplifier of the construction` herein described optimum operation is achieved with a filament voltage of 9.5 volts, a focus electrode voltage of zero volts, a iirst anode voltage of 52 volts, a second anode voltage of 120 volts, a third anode voltage of 300 volts, a cavity voltage of 1400 volts and a beam current of 2 milliamperes. The optimum drift space between the low noise electron gun and the klystron circuit was found to be approximately 3" for an S-band amplifier and 11/2 was used for a C- band amplifier, although the length of this latter drift space was not necessarily optimized. A comparison of gain versus cavity voltage showed that the maximum gain occurred at a cavity voltage of 1700volts, and that at 1400 volts there would be only one db less gain, while going from 1700 volts down to 1400 volts lowers the noise figure four db. Consequently, one would obviously choose 1400 rather than 1700 for the cavity voltage since there is more to be achieved in reduction of noise figure than is to be lost in gain.

Dynamic range is defined as the range of variation of power output fromV the levell of that power due to noise alone up to the saturation level.` Traveling wave tubes and backward wave ampliers have a dynamic range of approximately db. Recently it was reported that parametric amplifiers have exhibited a dynamic range of about db. Experimental data taken on a low noise'klystron amplier of the present construction showed a dynamic range of 118 db above'the minimum output level due to noise' alone.

Since many changes could be made in the above construction and many apparently widely different embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A cathode assembly for an electron discharge device comprising in combination a cathode, a rst mounting member, a plurality of stilts supporting said cathode on said first mounting member, a second mounting member, a plur-ality of stilts supporting said first mounting member on said second mounting member, said cathode and said second mounting member being disposed in the same direction from said first mounting member and the stilts supporting the first member on said second member adapted to compensate for expansion and contraction of the stilts supporting said cathode on said first mounting member whereby the position of the cathode with respect to the second mounting member is maintained substantially fixed for a Wide temperature range, and supporting means for positioning said second mounting member in the electron discharge device.

2. A cathode assembly of claim 1 including a hollow gun mounting block, means for mounting said second mounting member on said gun mounting block axially aligned therewith, and a hollow gun mounting sleeve adapted for mounting on the electron discharge device, a surface of said gun mounting block and a surface of said gun mounting sleeve -being mating conical surfaces whereby said gun mounting sleeve is mounted on the electron discharge device and aligned with the axis thereof and said gun mounting block is mounted on said gun mounting sleeve via said mating conical surfaces thereby to mount the cathode assembly axially of the electron discharge device.

3. The cathode assembly of claim l wherein said cathode includes an emissive member comprising a pellet with a proboscis projecting forwardly therefrom and outwardly flared on the forward end thereof and a mounting mem-ber provided with a sleeve projecting from an end thereof and adapted to hold said pellet, said mounting member being provided with an outwardly projecting shoulder for mounting the cathode in the electron discharge device.

4. A cathode assembly of claim 1 including a plurality of electrodes mounted in front of said cathode in the direction of said first mounting member, a plurality of in sulator spacers one positioned on each side of said electrodes and said second mounting member for keeping said electrodes and said second mounting member in insulated, spaced-apart relation, a first retaining member positioned against the spacer on the back of said second mounting member, a second retaining member positioned against the spacer on the front of the forwardmost electrode, a plurality of mounting screws adapted for connecting said first -and said second retaining members and with said second mounting member, said electrodes and said spacers positioned under compression therebetween, and a plurality of hollow expansion members one positioned between the head of each of said mounting screws and the adjacent retaining member whereby each of said expansion members expands when heated and compensates for the expansion in the respective mounting screw thereby keeping said second mounting member, said electrodes and said spacers under com-pression.

5. A cathode assembly of claim 4 including a hollow gun mounting block having a flange on one end adapted for mounting said second retaining member thereto and a hollow gun mounting sleeve adapted for mounting on the electron discharge device, a surface of said gun mounting block and a surface of said gun mounting sleeve being mating conical surfaces whereby the second retaining member is mounted on said gun mounting block and aligned with the axis thereof, said gun mounting sleeve is mounted on the electron discharge device and aligned with the axis thereof, and said gun mounting block is mounted on said gun mounting sleeve via said mating conical surfaces thereby to mount the cathode assembly axially of the electron discharge device.

6. In an electron discharge device including means for forming an electron beam, means for collecting the electron beam land cavity resonator means disposed between the means for forming the electron beam and the means for collecting the electron beam, a drift t-ube assembly comprising two axially aligned drift tubes and an annular header supporting each of said drift tubes within the body of the electron discharge device and axially aligned with the electron beam, whereby said two drift tubes provide a single uniform drift region accurately aligned with the axis of the electron beam.

7. The apparatus of claim 6 including two hollow cylinders axially mounted adjacent one another, each closed at its outward end by one of said annular headers providing an enlarged opening between the adjacent ends of said drift tubes whereby the reduced plasma radian frequency is increased and thus the required length of the drift space is reduced.

8. In an electron discharge device including means for forming an electron beam, means for collecting the elcotron beam, and cavity resonator means disposed between the means for forming the electron beam and the means for collecting the electron beam, a magnetic field distorting member adapted to surround the means for collecting electron beam and provided with a taper on the end thereof which surrounds the inner end of the means for collecting the electron beam whereby a magnetic field within the means for collecting the electron beam is distorted thereby preventing secondary electrons from returning to the cavity resonator means from the means for collecting the electron beam.

9. A cathode for a low noise electron discharge device including an emissive member comprising a pellet with a proboscis projecting forwardly therefrom and outwardly flared on the forward end thereof and a mounting member provided with a sleeve projecting from an end thereof and adapted to hold said pellet, said mounting member provided with means for supporting the cathode in the electron discharge device.

l0. In a low noise electron discharge device including means for Iforming an electron beam, means for collecting the electron beam and cavity resonator means disposed between the means for forming the electron beam and the means for collecting the electron beam, a cathode assembly including a cathode, a first mounting member, a plurality of stilts supporting said cathode on said first mounting member, a second mounting member, a plurality of stilts supporting said first mounting member on said second mounting member, said cathode and said second mounting member being disposed in the same direction from said first mounting member whereby the position of the cathode with respect to the second mounting member is maintained substantially fixed over a wide temperature range; supporting means `for positioning said second mounting member in the electron discharge device; and a drift tube assembly axially aligned and disposed from said cathode assembly including two axially aligned drift tubes and an annular header supporting each of said drift tubes Within the body of the electron discharge device and axially aligned' with the electron beam, whereby said two drift tubes provide a single uniform drift region accurately aligned with the axis of the electron beam.

ll. In a low noise electron discharge device including means for forming an electron beam, means for collecting the electron beam and cavity resonator means disposed between the means for forming the electron beam and the means for collecting the electron beam, a cathode assembly including a cathode, a first mounting member, a plurality of stilts supporting said cathode on said first mounting member, a second mounting member, a plurality of stilts supporting said first mounting member on said second mounting member, said cathode and said second mounting member being disposed in the same direction from said first mounting member whereby the position of the cathode with respect to the second mounting member is maintained substantially fixed over a wide temperature range; supporting means for positioning said second mounting member in the electron discharge device; and a magnetic field distorting member adapted to surround the means for collecting the electron `beam and provided with a taper on the end thereof which surrounds the inner end of the means for collecting the electron beam whereby a magnetic field applied to the means for collecting the electron beam is distorted thereby preventing secondary electrons from returning to the cavity resonator means from the means for collecting the electron beam.

12. The low noise electron discharge device ot claim 11 including a drift tube assembly in said cavity resonator means axially aligned within the electron discharge device and comprising two axially aligned drift tubes, an annular header supporting each of said drift tubes within the body or the electron discharge device and axially aligned with the electron tube, whereby said two drift tubes provide a drift region accurately aligned with the axis of the electron beam.

13. The low noise electron discharge device of claim 11 including a plurality of electrodes for mounting in front of said cathode in the direction opposite said first mounting member; a plurality of insulator spacers one positioned on each side of said electrodes and' said second mounting member for keeping said electrodes and said second mounting member in insulated, spaced-apart relation; a rst retaining member positioned against the spacer on the back of said second mounting member; a second retaining member positioned against the spacer on the front of the forwardmost electrode; a plurality of mounting screws adapted for connecting said rst and said second retaining members and with said second mounting member, said electrodes and said spacers positioned under compression therebetween; and a plurality of hollow expansion members, one positioned between the head of each of said mounting screws and the adjacent retaining member whereby each of said expansion members expands when heated and compensates for the expansion in the respective mounting screw thereby keeping said second mounting member, said electrodes, and said spacers under compression.

14. The low noise electron discharge device of claim 13 including a hollow gun mounting block having a ange on one end thereof adapted for mounting said second retaining member thereto and a hollow gun mounting sleeve adapted for mounting on the electron discharge device, a surface of said gun mounting block and a surface of said gun mounting sleeve being mating conical surfaces, whereby the second retaining member is mounted on said gun mounting block and aligned with thc axis thereof, said gun mounting sleeve is mounted on the electron discharge device and aligned with the axis thereof, and said gun mounting block is mounted on said gun mounting sleeve via said mating conical surfaces thereby to mount the cathode assembly axially of the electron discharge device.

15. In an electron discharge device including means for forming an electron beam, means for collecting the electron beam and cavity resonator means disposed between the means for forming the electron beam and the means for collecting the electron beam, a hollow gun mounting block adapted for mounting of the means for forming the electron beam thereon and a hollow gun mounting sleeve adapted for mounting on the cavity resonator means, a surface of said gun mounting block and a surface of said gun mounting sleeve being mating conical surfaces whereby the means for `forming the electron beam is mounted on said gun mounting block and aligned with the axis thereof, said gun mounting sleeve is mounted on the cavity resonator means and aligned with the axis thereof, and said gun mounting block is mounted on said gun mounting sleeve via said mating conical surfaces thereby to mount the means for forming the electron beam axialiy of said cavity resonator means.

16. in an electron discharge device including means for forming an electron beam, means for collecting the electron beam, and cavity resonator means disposed between the means for forming the electron beam and the means for collecting the electron beam, means Afor positioning a plurality of electrodes in front of the mounting member which supports the cathode in. the means for forming the electron beam comprising a plurality of insulator spacers, one positioned on each side of the electrodes and the mounting member for keeping the electrodes and the mounting member in insulated, spacedapart relation; a first retaining member positioned against the spacer on the back of the mounting member; a second retaining member positioned against the spacer on the front of the forwardmost electrode; a plurality of mounting screws adapted tfor connecting said first and said second retaining members with the electrodes, the mounting member, and said spaces positioned under compression therebetween; and a plurality of hollow expansion members one positioned between the head of each of said mounting screws and the adjacent retaining member whereby each of said expansion members eX- pands when heated and compensates for the expansion in the respective mounting screw thereby keeping the mounting member, the electrodes and said spacers under compression.

17. A cathode assembly for an electron discharge device comprising in combination a cathode, a rst mounting member, a support for said cathode on said rst mounting member, a second mounting member, a support for said first mounting member on said second mounting member, said cathode and said second mounting member being disposed in the same direction from said first mounting member and said support for .said first mounting member 0n said second mounting member adapted to compensate for expansion and contraction of said support for said cathode on said first mounting member, whereby the position of the cathode with respect to the second mounting member is maintained substantially xed during temperature fluctuations, and supporting means ifor positioning said second mounting member in the electron discharge device.

18. An electron discharge device including means for forming an electron beam, means for collecting the electron beam, electron utilization means disposed between said means for forming the electron beam and said means for collecting the electron beam, mating conical mounting means on both said means for forming the electron beam and said electron utilization means for axially aligning said means for forming the electron beam with said electron utilization means.

References Cited in the file of this patent UNITED STATES PATENTS 2,335,818 Trumbull et al. Nov. 30 1943 2,458,167 Homey Jan. 4, 1949 2,680,209 Veronda .Tune 1, 1954 2,740,913 Majkrzak Apr. 3 1956 2,900,561 Gormley Aug. 18j 1959 2,914,694 Chin NOV. 24, 1959

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