US3047351A - Electron discharge device - Google Patents

Description

July 31, 1962 F. sALlsBURY 3,047,351

ELECTRON DISCHARGE DEVICE Original Filed March 25, 1954 United @tastes This invention relates to electron discharge devices and, more particularly, to an electron discharge device of the velocity modulation type arranged to operate at microwave frequencies as an amplifier.

As the operating frequency of electron discharge devices or tubes has increased, a corresponding increase has been noted with respect to the critical nature of the alignment of the elements thereof, the geometric shape and tolerances of such elements, and the losses of energy resulting from any deviation in these tube design factors. While the design problems presented have been severe with tubes such as reflex klystrons and two-cavity klystron amplifiers, they have been further accentuated, for obvious reasons, with multi-cavity klystrons, particularly when frequencies of the order of 20-30 kmc. are approached. In such cases, since a wavelength is in the neighborhood of l centimeter, even such minor structural aberrations as pro-duced by brazing can affect the losses and the tunability of the amplifier.

Accordingly, the object of the present invention is to provide a multi-cavity microwave amplifier of relatively simple construction and having optimum gain characteristics.

One feature of the present invention is to provide a microwave amplifier of the multi-cavity type wherein, in manufacture, the drift tube member may be accurately aligned in the tube body.

These and other features will become more apparent from the following description of a preferred embodiment of the present invention as shown in the accompanying drawings wherein:

FIG. 1 is an elevational View of a multi-cavity microwave amplifier, parts being cut away to illustrate features of construction,

FIG. 2 is a section taken along line 2--2 of FIG. l, and

FIG. 3 is an enlarged View partially in section illustrating interior construction of the resonator cavities of the amplifier.

The microwave amplifier generally includes a beamproducing section followed by a central section 11 wherein the interaction between the beam and the applied radio frequency wave takes place to provide the amplification and a collector section 12 where the electrons of the spent beam are collected.

In accordance with the invention, the amplification section 11 of the tube preferably has a body formed from a substantial-ly semi-cylindrical block 13 of copper, as best illustrated in FIG. 2. A cylindrical bore 14 which extends longitudinally through the semi-cylindrical block 13 is intersected by a number of spaced transverse bores 15 extending from the flattened side 21 of the block 13 and is adapted to receive a plurality of drift-tube members 17. Each member 17 comprises a copper cylinder which is provided with small cylindrical extensions 18 at its ends and has a central bore extending therethrough and through such extensions to provide a drift space 19 for the electron beam. When the mem-bers 17 are inserted in the bore 14 in properly spaced relation, as determined by the spacing of the transverse bores 15, resonator cavities 16 of the desired frequency are formed between adjacent members. Prior to insertion of the atent ice copper drift-tube members 17 into the bore 14, the latter is provided by a flash-plating technique with a thin layer of silver. The exterior dimension of the drift tube members 17 is slightly greater than the diameter of the plated bore 14 through the described semi-cylindrical block 13 so that upon insertion a pressed-fit is obtained. After the insertion of the drift tube members 17, a single heating operation will cause the copper and silver to alloy and the members to be fused within the bore. It should be noted that the same fusing technique and resulting assembly can be employed with drift tube members 17 which are not of solid copper construction; for example, copper-clad molybdenum drift tube members can be utilized in the fabrication. This technique obviously avoids the mentioned difficulty of conventional brazing methods and a clean juncture is lformed between the drift tube members 17 and the bore 14 which provide, in effect, the walls of the resonator cavities 16.

An inner drift tube member 17 which is machined to provide the theoretically desired geometry to as close tolerances as possible is first inserted and, in turn, each of the adjacent `drift tube members 17 is inserted to an extent slightly less than that required to provide the spacing productive of the desired resonant frequency of the cavities 16. Subsequently, radio frequency energy is supplied to each cavity 16` between adjacent drift tube members through the respective one of the transverse bores 15 extending from the flattened-side 21 of the semicylindrical block 13. While radio frequency energy of the desired frequency is inserted through the transverse bore 15 a slight axial movement is imparted to the drift tube member 17 until the resonance condition is achieved, this then being the final disposition of the member. The above described technique has been found not only desirable but actually indispensable to produce the desired cavity dimensions when operating at frequencies between 20 `and 30 kmc. because at such frequencies normal mashining operations have insufficient accuracy to effect the desired structural tolerances.

As is shown clear-ly in FIG. 3, the first and last cavities 16 of the illustrated five-cavity amplifier are completed by members 17' which in effect constitute one-half of the complete drift tube members 17 previously described, in the intermediate portions of the bore 14.

To provide coupling of radio frequency energy into and out of the amplifier, the described semi-cylindrical block 13 is cut away in its lower portion for the reception of conventional waveguide sections 22 which communicate with the first and last resonator cavities 16 through bored iris openings 23. A mica. Window 24 is suitably secured at the end of each of the waveguide sections 22 to maintain the vacuum within the cavities 16 and the remainder of the turbe.

The 'previously mentioned fbores 15 are enlarged adjacent the flattened side 21 of the semi-cylindrical block 13 as indicated at 20 to receive in vacuum-tight relation a tuning diaphragm 25 secured at the end of a tuning screw 26 which is adjustably suspended from the lateral arm 27a of a bracket 27 secured to the flattened side Z1 of the lblock 13 by suitable screws 28. The diaphra-gms 25 are maintained in adjusted position by pairs of lock nuts 29, 30 on the tuning screws 26 which nuts engage respectively the upper and lower surfaces of the lateral bracket arm 27a.

The beam-producing section 10 includes a substantially cylindrical hollow body 31 having a cathode button 32 and associated heater element 33 and focusing ring 34 mounted centrally therein land axially aligned with a tapered bore 35 `formed in the base of an attached pole piece 36 of magnetic material and adapted to register with the aligned drift spaces 19 in the drift-tube members 17, 17 within the amplification section 11 of the tube. To assure that such alignment is achieved, the cathode button 32 is provided with a small central aperture 32a which enables the cathode 32 and the drift tube members 17, 17 of the tube to be aligned optically; that is, a light may be positioned at the end of the beamproducing section and the same laterally shifted until such light passes through the small central aperture 32a in the `cathode button 32, the registering bore 35 in the pole piece 36 and the drift tube members 17, 17' so Ias to be visible to a viewer whose eye is adjacent the drift space 19 at the output end of the amplification section 11. While so aligned the beam-producing section 10 or more particularly the attached pole piece 36 is annularly brazed or otherwise secured to the amplification section 11 of the tube so that the alignment will be maintained. The collector section 12 of the electron discharge device is preferably mounted on the second cup-shaped pole piece 38 having a tapered lbore 39 extending centrally through its base and adapted to register with the aligned drift spaces 19 when said cup-shaped element is brazed in position at the output end of the amplification section 11 of the tube so as to accommodate the electron beam. Such alignment is again attained by the optical technique described above. The collector section 12 includes an elongated member 40 having a deep cylindrical recess 41 machined therein and arranged so that its open end is adjacent and axial with the tapered bore 39 in the pole piece 38. The ltwo elements are secured in such relation through a yglass-to-metal seal 42 `formed between tubular stubs 43, 44 extending in opposite directions from an annular cap 4S on the cup-shaped pole piece 38 and a thick sleeve 46 telescoped onto the outer end of the elongated collector member 40. This construction and insulated mounting of the collector enables current readings to be taken when desired.

Tubulations 47 in the side of the two pole pieces 36, 38 enable evacuation of the tube after completion of the assembly, and are then pinched-off as shown in FIG. 1. To facilitate evacuation, eccentric axially-extending passages 48 are formed in the drift tube members since the actual `diameter of the drift spaces 19 will -be somewhat less than .02 at the mentioned operating frequencies. Because these passages 48 are relatively well below cutoff at the operating frequency, no radio frequency energy can be transmitted therethrough.

The tube is electrically connected quite conventionally, a battery `50 being arranged, as shown in FIG. 1 t0 provide heater current, and a second battery 51 to provide positive D.C. voltage for the central and collector sections 11, 12 of the tube. Conventional coils indicated at 52, 53 are disposed adjacent the pole pieces 36, 38 t0 provide yfor proper focusing of the beam during its traverse through the tube. In operation, the beam of electrons emitted from the cathode button 32 is accelerated through the bore 35 and into the small cylindrical bores through the drift tube members 17. The radio frequency signal to be amplified is fed into the first or buncher cavity resonator through the input waveguide 22. The radio frequency electric field produced across the resonator gap in this first cavity resonator velocity modulates the electrons, that is, the electrons are slowed down or speeded up depending on the phase of the radio frequency field across the gap at the time of transit of the electron. In the field-free drift space defined by the firstfdrift tube member 17 and extensions 18, the velocity modulation forms the beam into groups or bunches of electrons which, at their point of sharpest bunching, pass through the resonator gap in the second cavity resonator. All of the cavity resonators are so proportioned as to size and gap spacing as to be sharply resonant at the desired operating frequency of this cascade amplifier.

The cavity resonators are initially tuned vduring assembly by the proper positioning and brazing of the drift tube members 17 to establish correct resonator gap spacing, the cavities thereafter 'being fine-tuned by means of the diaphragms 25. All of the cavity resonators between the first or input resonator and the last or output resonator serve to improve the bunching of the electron beam initiated in the first cavity resonator so that optimum bunching is produced before the beam passes through the resonator gap in the last or output resonator. The amplified radio frequency energy is passed out 'from this cascade amplifier through the output waveguide 22. An actual tube which has been constructed and operated with 1500 volts on `the sections 11, 12 has produced a gain of as high as 74 decibels, as a result of the minimization of losses through the described arrangement in accordance with the invention. What losses do occur, of course, appear as heat in the amplification and collector sections 11, 12 of the tube and are easily dissipated because of the large volume of the semi-cylindrical block 13 and the thick sleeve 46 on the collector member 4t), no cooling fins being required.

The present application is a division of US. patent application 418,714, filed March 25, 1954, now U.S. Patent r the main section of the klystron and a beam-producing cathode, of aligning the cathode with the electron beam path through the central openings in the drift tube meinbers which comprises the steps of forming a small hole through the center of the cathode, longitudinally aligning a source of light with the openings in the `drift tubes adjusting the position of the cathode at the cathode end of the main section until the light is longitudinally projected through the central hole in the cathode and the drift tube openings, and fixedly securing said cathode in that position.

2. The method, in a multicavity klystron having a plurality of drift tube members longitudinally aligned in the main section of the klystron and a beam-producing cathode, of aligning the cathode with the electron beam path through the central openings in the drift tube members during assembly ofthe klystron which comprises the steps of forming a small hole through the center of the cathode, positioning the cathode at the cathode end of the main section of the klystron in approximate alignment with the electron beam path through the drift tube openings, aligning a source of light with the openings in the drift tubes, `the source of light being positioned behind the cathode button, adjusting the position of the cathode until the light is longitudinally projected through the central cathode opening and the drift tube openings, and fixedly securing said cathode in that position.

References Cited in the file of this patent UNITED STATES PATENTS 1,705,356 Bohner Mar. l2, 1929 2,426,697 Larson Sept. 2, 1947 2,603,550 Bloomsburgh July 15, 1952 2,611,676 Pohle Sept. 23, 1952 2,663,012 Beers Dec. 15, 1953 2,695,442 Klopping Nov. 30, 1954 2,814,090 C'heatle Nov. 26, 1957

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