US3043985A

US3043985A - Reflex klystron

Info

Publication number: US3043985A
Application number: US800178A
Authority: US
Inventors: Rowland W Haegele; Heil Oskar; Carl H Turnblom
Original assignee: Eitel Mccullough Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1959-03-18
Filing date: 1959-03-18
Publication date: 1962-07-10
Anticipated expiration: 1979-07-10
Also published as: DE1860687U; CH371189A; GB908186A

Description

July 10, 1962 R. w. HAEGELE ET AL 3,043,985

REFLEX KLYSTRON Filed March 18, 1959 2 Sheets-Sheet 2 INVENTORS ROWLAND w. HAEGELE OSKAR HEIL BY CARL 1. TURNBLOM ATTORNEYS Statea 3,0433% REFLEX KLYSTRON Rowiand W. Haegeie, Manic Park, and Oshar Heil, San

lilateo, Calif., and Carl H. Turnblom, Salt Lake City,

Utah, assignors to Eitel-McCullongh, Inc, San Bruno,

Calif., a corporation of California Filed Mar. 18, 1959, Ser. No. 890,178 7 Claims. (Cl. SIS-4.21)

' netic oscillations in the cavity surrounding the gap. The

electrons proceed on toward the repeller electrode, where they are reflected back across the interaction gap during the course of which the fast electrons will catch up with the slow electrons to form bunches. The bunches of electrons will add to the electromagnetic oscillations in the cavity as they recross the interaction gap. Output power is coupled from the cavity through a high frequency output window in the cavity well.

Since the operating frequency of klystrons in general is determined by the resonant frequency of the cavity surrounding the interaction gap and the resonant frequency of the cavity is determined by the cavity volume, smaller volumes will resonate at higher frequencies. Reflex klystrons operate in the range of 10,000 megacycles, and therefore the volume of the resonant cavity is small, in the order of one cubic inch. Since it is diflicult to change a small volume with any degree of fineness, reflex klystrons are made to operate at a fixed frequency. The frequency may be changed slightly in order to match a given system by such means as deforming the cavity box by applying a force to the cavity box and not exceeding the yield strength of the material the cavity is made from.

It is an object of the present invention to provide a reflex klystron with a broad-frequency range and with fine tuning over the frequency range.

Since the reflex klystron is small and compact, no axially aligned magnetic field is used for electron beam guidance. Consequently the electrons tend to drift away from each other and become ineffective in producing power output. This is one of the features which make a reflex klystron a low efficiency apparatus.

It is another object of the present invention to provide a reflex klystron with higher efiiciencies.

It is a further object of the present invention to increase the number of bunched electrons reflected back through improved focusing characteristics, improved flat, fixed,

radial-vane interaction gap grids, and a reflector electrode with an improved repeller surface contoured to return the greatest number of bunched electrons in phase with each other through the interaction gap.

These and other objects of the invention will become atent apparent from the following description when taken in conjunction with the accompanying drawings. The invention is not limited to the embodiment disclosed, as variant embodiments may be adopted within the scope of the claims.

FIGURE 1 is a sectional view of the reflex klystron taken on line 1-1 of FIGURE 2;

FIGURE 2 is a sectional view of the reflex klystron taken on line 2-2 of FIGURE 1.

Referring to FIGURE 1 of the drawing, there is shown a reflex klystron tube with a cathode header assembly 12, a repeller header assembly 14, a tubular body 16, a tuning assembly 17, an output waveguide assembly 18, and a resonant cavity 20.

The cathode header assembly 12 has a disk-shaped ceramic header 22 with eight metal prongs 24 disposed in a circle protruding through and brazed to the header 22. A larger center prong 26 with an index key 28 is disposed at the center of the array of the eight prongs 24, and on the klystron axis. A sealing ring 3% of U-shape cross-section is brazed to the periphery of the ceramic header 22.

The electron gun for the reflex klystron is assembled on the cathode ceramic header in the following manner: a cathode mounting bar 32 with upturned end flanges is attached to two opposite prongs 24 and disposed flat against the inside face of ceramic header 22. A stepped tubular cathode support 34 is welded :to the mounting bar 32 by an inward flange disposed on the small end of the support 34 and is disposed coaxially with the klystron axis. A thin tubular heat shield 36'which supports at one end a dished cathode 38 is welded at its other end to the cathode support 34. The concave side of the cathode 38 is coated with an electron emissive oxide such as barium oxide and faces the repeller assembly 14. The cathode is an indirectly heated type and is heated by a helical filament 40 disposed within the cathode 38 and heat shield 36. The filament 40 is' supported by two L-shaped rod lead-ins 42 which protrude through two ports 44 in the side of the cathode support 34. The leadins are welded to two other adjacent prongs 24. For more efficient cathode heating a ceramic disk 46 is disposed transversely within the cathode support 34 and fixed to the stepped section, as illustrated. An outwardly flanged tubular metallic heat conductor 43 is disposed around the filament and fixed to the convex side of the cathode by its flange.

A stepped tubular electron beam focusing support 50 is welded by its smaller end to heat shield 36 around the cathode support 34. Two rings 52 and 54 of L- shaped cross-section are welded, respectively, internally and externally of the focusing support 50 spaced from each other and disposed coaxially on the klystron axis. The internal diameter of the focusing support is larger than the cathode diameter. The first ring 52 has an internal diameter equal to the cathode diameter and is spaced axially from the cathode 38. The second ring 54 having the same internal diameter as ring 52 is spaced a greater axial distance from the cathode 88. This spacing, the shape of the anode, and the location of the anode determine the beam focusing and beam perveance.

On the body 16 are machined external circumferential cooling fins 56. The internal cylindrical surface of the body 16 ismachined to the concentric stepped bores for accurately disposing of internal parts of the reflex klystron tube. A first bore 58 having a diameter larger than the focusing support 50 is disposed at the cathode end of body 16. The second larger bore 60 is located at the center of the body 16. A drift tube anode 62 with an external flat ring 63 is press fitted Within bore 60 against the step thereof. A larger third bore 64 extends into 3 the body 16 at the repeller end. A disk 66 with a center port 67 is pressed therein against the step, completing the resonant cavity 20. Sealingrings 68 and 68' of Z-shaped cross-section are brazed to the ends of body 16 which is counterbored at each end to accommodate the flanges on the

sealing rings

68 and 68.

The waveguide assembly 18 is mounted perpendicularly to the body 16 through a machined rectangular port 70. A waveguide'71 is brazed to and protrudes through the port 70 so that a flat plate 72 welded to the inner end lies totally within the bore of the body 16. The flat plate 72 has an output port 74 communicating with the resonant cavity 20. A flat ceramic output window 75 is brazed to plate 72 over the port 74 making a vacuum seal. A sandwich type metal-to-ceramic bond 76 is used for this seal to withstand high thermal stresses. The sandwich bond 76 is made by machining an annular step on the window 75 and brazing a metal sealing ring 78 of- L-shaped cross-section to this stepped portion. An annular ceramic backing ring 82 is brazed on the opposite side of the sealing ring 78. As seen in the drawing, both faces of the ceramic windows are flat and the external face is flush with the ceramic ring 82. A flange 83 on the sealing ring 78 is then brazed to the plate 72. The waveguide 71 has a standard coupling flange 84 on its external end for attaching more waveguides and has a matching plug 86 to match the circuit.

Directly opposite the waveguide and on the waveguide center. line is disposed the tuning assembly 17. A rectangular port 88 is machined in body 16 to accommodate a rectangular ceramic tuning slug 90. I Port 88 communicates with the resonant cavity 20. A counterbore .92 is machined coaxially with the port 88 and a tubular guide plug 94 is welded in the counterbore 92. The ceramic tuning slug 90 is brazed to the end of a control rod 96 which slides through the guide plug 94. The other end of the control rod has a thick flange 98 and a spring 100 is disposed between the flange 98 and guide plug 94, urging the tuning slug 90 out of the body' 16. A vacuum seal is made with the aid of a metal bellows 102 between flange 98 and guide plug 94. A tubular support 104 is held in place by screws 105 bearing against plug 94. The support 104 keeps the rod 96 in line and supports a ball bearing 106 in the external end. A tuning shaft 108 is pressed into the inner bearing race. A screw thread 110 on the inner end of shaft 108 engages mating threads on nut 112 which is pressed tightly in an axial well 114 bored in the outer end of the control rod 96. As the shaft 108 is rotated, the nut 112 moves axially on the threads 110', urging the control rod 96 and ceramic tuning plug 90 with it.

The repeller header assembly 14 has 'an annular disk ceramic header 116. Anexhaust tubulation 118 protrudes through the header 116. Since the tubulation 118 has a thick wall, it is not brazed directly to the header as the header will have temperature changes which will cause excessive thermal stresses. A conical metal sealing ring 120 is brazed on its periphery to the inside surface of the header 116. The tubulation 118 is welded to an upturned internal flange on the sealing ring 120 Also, on the inner surface of header 116 is brazed a tubular repeller support v122 by its outward turned flange. The support 122 makes electrical contact with the ring 120. On the other end of the support 122 is brazed a repeller 124 with a novel contoured repeller surface 126 which will be described more fully hereinafter. The tubulation 118 is protected by a flanged tube 128 brazed in the hole of the header 128. A contact cap 130 is brazed on the tube 128. The cap and tube 128 provide the external terminal for the repeller 124. A sealing ring 30' similar to sealing ring 30 is brazed to the ceramic repeller header 116 in the same manner as sealing ring 30 is brazed to the ceramic cathode header 22.

It is observed that the cathode header assembly 12 63 towards the cathode.

was made of two diameters with the smaller diameter and the repeller header assembly 14 are accurately assembled individually and then sealed to body 16 by arc welding together the adjacent edges of sealing

rings

30 and 68 and of sealing rings 30' and 68. The design of the body 16 and header assembly 12 and 14- insures accurate positioning of the electrode within the body 16.

In operation, the filament 40 heats the cathode 38, and electrons are emitted from the coated concave surface. The electrons are attracted by the positive potential on drift tube anode 62. The spacing of the rings 52 and 54, cathode 38 and anode 62 from each other forms the focusing electric field which concentrates or focuses the electrons near the beam axis. One end 132 of the drift tube anode 62 is placed close to the cathode to attract the greatest number of electrons without aflecting the focusing field, thus producing a high perveance beam. The higher the perveance the greater is the available output power of the klystron for a given beam voltage.

In this embodiment of the invention, a simple geometric shape machined anode having a drift tube section as shown produces the best practical beam. The end 132 of the drift tube 62 was made to protrude through ring The bore of the drift tube at end 132. With this arrangement the greatest number of electrons from the cathode will pass through the open ing of the drift tube and not be intercepted by the positive potential thereon because the larger diameter at the exit of the drift tube accommodates the defocusing electron beam. Port 67 in plate 66 is made larger than the drift tube bore for the same reason.

The electrons pass through the cavity 20 and the interaction gap where they are influenced by a velocity modulating force which accelerates certain electrons and decelerates other electrons. The electrons after passing across the interaction gap are repelled by the repeller 124 back across the gap. During this action the fast electrons tend to catch up with the slow electrons so that the electrons in recrossing the gap are in bunches producing the power output of the klystron. This power is directly proportional to the compactness of the bunches. Since electrons traveling on the tube axis travel a shorter distance than the electrons removed from the tube axis,

the electrons on the axis will tend to be out of phase with the outer electrons so that compact bunches are not produced.

According to this invention all the electrons are returned in phase across the interaction gap by a contoured repeller surface 126, which has a novel configuration.

The preferred configuration on repeller surface 126 is of a hollow conical frustum with its small flat base 135 recessed in the repeller 124. A sharp center point 134 protrudes coaxially from the smaller base. The length of point 134 is made shorter than the depth of the frustum. The surface 126 is symmetrical on the tube axis. The center point 134 deflects radially the electrons traveling on the klystron axis, allowing hte electrons on the outer edge of the beam time to catch up and bunch with them. In other words, the electrons on the klystron axis are caused to disperse radially, and the conical surface of the repeller surface 126 then helps to focus the reflected electron beam radially so that the electrons pass across the interaction gap in phase with each other.

The space between port 67 and the adjacent end of the drift tube 62 is known as the interaction gap. Since the gap spacing is small in comparison to the port size, the electrons which are on the axis would not be afiected by the cavity field. Therefore, grids 137 and 136 are placed in the ports. The grids each have sixteen molybdenum radial vanes suspended from the port periphery leaving the center open. The radial type grids with the center opening were used because this arrangement offered the least interference with the electron flow. Sixteen vanes were used to keep the space between the vanes to less than one electron wavelength and to provide adequate electrostatic coupling between the gap. An electron wavelength is defined as the distance an electron Will travel at the operating voltage during one cycle of the operating frequency. Grid 137 is disposed in port 67, and grid 136 similar to grid 137 but of a smaller diameter is disposed in the adjacent end of drift tube 62 with the radial vanes aligned with grid 137 vanes. With this arrangement grid 136 does not add interference to the electron beam.

Referring to FIGURE 2, there is shown the novel ceramic tuning slug 90 disposed Within cavity 20. This view shows the inner end of the ceramic slug 90 with a semi-cylindrical concave surface 138. The slug 90 is positioned by shaft 108 to its inmost position towards the klystron axis, and the drift tube 62, shown in crosssection, is disposed Within the concave surface 138. The ceramic slug 90 has a rectangular cross-section and is made to fill the tuning cavity as much as practical when fully inserted. The lower frequencies are obtained with the ceramic slug fully inserted. The rectangular slug 90 has been found to give a more uniform tuning rate with each turn of the shaft 108 than any other simple geometric shape.

The control rod 96 is made round, and the inner end of the rod stops on the body 16 (FIGURE 1). This prevents the ceramic slug from bearing on the drift tube 62. Upon retraction of the slug 90 the outer end of rod 96 rests against bearing 106, which is the outer stop.

Thus, a reflex klystron having more efficiency, a higher output power and broadband tuning is provided. The efficiency and output power is increased by combining a gridless electron gun, an elongated step-bored drift tube anode, and a contoured repeller face. The broadband tuning is obtained by a ceramic tuning slug which is movable in and out of the resonant cavity.

We claim:

l. A reflex klyston comprising an electron gun, a repeller electrode, body means forming an interaction gap surrounded by a resonant cavity disposed between said electron gun and repeller electrode and coaxial therewith, said resonant cavity having a drift tube section one end of which forms a boundary of said interaction gap, a ceramic tuning slug having a rectangular cross-section disposed within said resonant cavity and insertable through a port formed in said body means, said ceramic tuning slug having a concave semi-cylindrical surface on its end adjacent said drift tube, mechanical means including a metallic bellows covering said port formed in said body means provided for withdrawing and inserting said ceramic tuning slug within said resonant cavity and sealing said port and tuning said resonant cavity from the exterior of said reflex klystron tube.

2. A reflex klystron tube comprising a tubular conductive body having a cathode header disposed at one end and a repeller header disposed on the other end forming a vacuum envelope, a gridless electron gun disposed on said cathode header and within said body for emitting a beam of electrons along said tubular body axis, said body including means forming an interaction gap coaxial Within said body and surrounded by a resonant cavity, a repeller electrode disposed on said repeller header within and coaxial with said tubular body on the opposite side of said interaction gap from said electron gun, said repeller electrode having a contoured repeller surface comprising a hollow conical frustum shape recessed into said repeller electrode and disposed with its smaller flat base recessed into said repeller electrode coaxial with said tubular body, and a sharp point coaxial with said tubular body and protruding from said smaller base towards said electron gun, said resonant cavity having a drift tube section one end of which forms a boundary of said interaction gap and the other end of said drift tube section being disposed adjacent said gridless electron gun and forming the anode of said gun, said drift tube section having a smaller inner diameter at said other end thereof, a ceramic tuning slug disposed in said resonant cavity formed by said first means forming said interaction gap surrounded by said resonant cavity, mechanical means provided for withdrawing and inserting said ceramic tuning slug within said resonant cavity and for tuning said resonant cavity from the exterior of said tubular body, and a vacuum-tight ceramic window in said resonant cavity. for radiating high frequency electromagnetic power from said tubular body.

3. A reflex klystron comprising a tubular conductive body, a first ceramic header disk hermetically closing one end of said tubular body, a plurality of contact prongs extending through said first header disk, a gridless electron gun mounted on said first header disk connected to said contact prongs, said electron gun being disposed coaxially within said tubular body for emitting a beam of electrons along said tubular body axis, a second ceramic header disk heremetically closing the other end of said tubular body, a repeller electrode mounted on said second ceramic header disk coaxially within-said tubular body, said body including means forming an interaction gap coaxial within said body and surrounded by a resonant cavity, said' repeller electrode having a contoured repeller surface comprising a hollow conical frustum shapedisposed with its smaller fiat base recessed into said repeller electrode coaxial with said tubular body, and a sharp point coaxial with said tubular body and protruding from said smaller base towards said electron gun, said resonant cavity having a drift tube section one end of which forms a boundary of said interaction gap and the other end of said drift-tube section being disposed adjacent said gridless electron gun and forming the anode of said gun, said drift tube section having a smaller inner diameter at said other end thereof, *a ceramic tuning slug disposed in said resonant cavity, mechanical means provided for withdrawing and inserting said ceramic tuning slug within said resonant cavity and for tuning said resonant cavity from the exterior of said tubular body, and a vacuum-tight ceramic window in said resonant cavity for radiating high frequency electromagnetic power from said tubular body.

4. A reflex klystron comprising a gridless electron gun,

a repeller electrode, and body means forming an interaction gap surrounded by a resonant cavity disposed between said gridless gun and said repeller, said gridless electron gun comprising a heating filament disposed within a cylindrical heat shield, said heat shield supporting a concave cathode at one end, a cylindrical focusing electrode surrounding said heat shield and spaced therefrom, said focusing electrode extending beyond said cathode from said filament, a first flat ring having an inner diameter equal to said cathode diameter disposed within said focusing electrode adjacent said cathode, and a second flat ring having an inner diameter equal to said cathode diameter disposed on the end of said focusing electrode spaced from said first flat ring and said cathode.

5. A reflex klystron comprising a tubular conductive body, a first ceramic cathode header disk heremetically closing one end of said tubular body, eight contact prongs evenly spaced in a circle coaxial with said tubular body and extending through said first header disk, an indexing prong disposed coaxial with an external of said tubular body and disposed on said first header disk, a gridless electron gun mounted on said first header disk and connected to said contact prongs, said electron gun disposed coaxial within said tubular body for emitting a beam of electrons along said tubular body axis, a second ceramic header disk heremetically closing the other end of said tubular body, a repeller electrode mounted on said second ceramic header disk coaxially within said tubular body, said body including means forming an interaction gap coaxial within said body and surrounded by a resonant cavity, said repeller electrode having a contoured repeller surface comprising a hollow conical frustum shape disposed with its smaller [flat base recessed into said re- 7 peller. electrode coaxial with said tubular body and a sharp point coaxial with said tubular body and protruding from said smaller base towards said electron gun, said resonant cavity having a drift tube sectionone end of which forms a boundary of said interaction gap andthe other end-of said drift tube section being disposed adjacent said gridless electron gun and forming the anode of said gun, said drift tube section having a smaller inner diameter at said other end thereof, a ceramic tuning slug disposed in said resonant cavity, mechanical means provided for withdrawing and inserting said ceramic tuning slug within said resonant cavity and for tuning said resonant cavity from the exterior of said tubular body, and a vacuum-tight ceramic window in said resonant cavity for radiating high frequency electromagnetic power fiom said tubular body.

6. .A reflex klystron as set forth in claim wherein said gridless electron gun comprises a heating filament disposed within a cylindrical heat shield, said heat shield supporting a concave cathode at one end, a cylindrical focusing electrode surrounding said heat shield and spaced therefrom, said focusing electrode extending beyond said cathode from said filament, a first flat ring having an inner diameter equal to said cathode diameter disposed within said focusing electrode adjacent said cathode, and 'a second fiat ring having an inner diameter equal to said cathode diameter disposed on the end of said focusing electrode spaced from said first flat ring and said cathode, and said ceramic tuning slug having a rectangular cross- 8 section and having a semi-cylindrical concave surface on its end facing the tube axis for fitting around said drift tube section Whensaid ceramic tuning slug is fully inserted Within said cavity.

7. A reflex klystron tube comprising an electron gun, said gun including cathode means for producing a solid beam of electrons, body means forming an interaction gap surrounded by a resonant cavity, and a repeller electrode disposed in that order and coaxial with each other, said repeller electrode having a contoured repeller surface comprising a hollow conical frustum shape with its smaller base recessed into and coaxial with said repeller electrode, and a sharp point protruding from said smaller base to- Ward said electron gun and disposed coaxial therewith.

References Cited in the file of this patent UNITED STATES PATENTS 2,411,913 Pierce et a1. Dec. 3, 1946 2,466,062 Str atton Apr. 5, 1949 2,468,152 Woodyard Apr. 26, 1949- 2,570,289 Touraton et a1 Oct. 9, 1951 2,581,408 Hamil-ton Jan. 8, 1952 2,777,969 Svensson Jan. 15, 1957 2,861,213 Dalman Nov. 18, 1958 2,878,415 Gormley et a1 Mar. 17, 1959 FOREIGN PATENTS 739,716 Great Britain Nov. 2, 1955 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,043,985 July 1O 1962 Rowland W. Haegele et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 57, for "hte" read the column 6, 11ne 62, for "an" read and line 68, for "heremetically" read hermetically Signed and sealed this 30th day of October 1962.

EA (S L) ERNEST w. swmsn DAVID LADD Atteeting Offieer Commissioner of Patents