US3383545A

US3383545A - Supported drift tube klystron

Info

Publication number: US3383545A
Application number: US325857A
Authority: US
Inventors: Heil Oskar
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1963-11-26
Filing date: 1963-11-26
Publication date: 1968-05-14
Anticipated expiration: 1985-05-14

Description

May 14, 19 o. HEIL SUPPORTED DRIFT TUBE KLYSTRON Filed Nov. 26, 1963 INVENTOR. OSKAR HEIL ATTORNE Y5 Unite States Patent 3,383,545 SUKKGRTED TUBE KLYSTRGN ()slrar Heil, San Mateo, @alih, assignor, by means assignmeuts, to Vnrian Associates, a corporation of California Filed Nov. 26, 1%3, No. 325,857 7 (til. 335-539} AESTRAKIT ill DESCLGSURE This invention relates to klystrons and more particularly to a mechanically rugged, frequency stable, supported drift tube klystron.

It is an object of the present invention to provide an improved drift tube lilYStI'OIl.

Another object of this invention is to provide a frequency stable supported drift tube klystron oscillator.

A further object of this invention is to provide a mechanically rugged, supported drift tube ldystron.

Still another object of this invention is to provide about .OOl% frequency stability in a supported drift tube kiystron oscillator.

A still further object of this invention is to provide a high Q, mechanically rugged, frequency stabilizing cavity for a supported drift tube klystron.

These and other objects of the present invention are accomplished by a mechanically rugged, frequency stable klystron having a cathode for producing a stream of electrons, a collector located in the path of the electron stream for collecting the electrons, an output cavity surrounding the electron path and having a gap for extracting energy from the electrons, and another, frequency determining, cavity surrounding the electron path and located intermediate the cathode and the output cavity. A drift tube is retained within the other cavity, surrounds the electron path, and forms first and second gaps with n the other cavity. The first gap is located nearest the c hode and is characterized as being a controlling whereas the second gap is located nearest the output cavity and is characterized as being a combined controlling and working gap. A plurality of supports, preferably substantially conically shaped, retain the drift tube within and in electrical contact with the interior wall of the other cavity at least a portion of which is a spherical surface. The conical supports form an appreciable part of the circuit inductance of the other cavity and have substantial dimensions in order to keep the Q of the other cavity high.

This invention as well as other objects, features, and advantages thereof will be readily apparent from consideration of the following detailed description relating to the following drawings in which:

FIGURE 1 illustrates a partial cross-sectional view of a supported drift tube lrlystron in accordance with the present invention; and

FIGURE 2 illustrates, in schematic form, radio frequency voltages that appear across gaps utilized in the device shown in FIGURE 1.

Referring now to FIGURE 1, there is shown in partial cross-section a supported drift tube lilystron in accordance with the present invention. The klystron includes an electron gun, generally indicated by the reference character 11, for producing a stream of electrons that follow a path indicated by the dashed line 12. A collector 13 is located in the path 12. of the electrons for collecting the electrons. A first or output cavity 14, located adjacent the collector l3, surrounds the electron path 12 and has a gap 15 for extracting energy from the electrons. A frequency stabilizing cavity 16 also surrounds the electron path 12 and is located intermediate the output cavity 14 and the electron gun 11. A drift tube 17 is located within the cavity 16 and forms first and second gaps 18 and 19, respectively therewith and is rigidly retained within the cavity 15 by a plurality of hollow, conical supports 20.

Briefly, the operation of the supported drift tube illustrated in FIGURE 1 as an oscillator is such that the first gap 18 velocity modulates the electrons produced by the electron gun 11 and is therefore a controlling gap. When the velocity modulated or bunched electrons arrive at the second gap 19, they are further velocity modulated or bunched and also deliver a portion of their energy into the frequency stabilized cavity 16 in order to sustain oscillations therein. Accordingly, the second gap 19 is characterized as a combined controlling and working gap. The bunched electrons then cross the gap 15, associated with the output cavity 14, which extracts energy from the bunched electrons in a well known manner before the electrons are collected by the collector 13. RF. energy is coupled out of the output cavity 1e by any suitable means, such as a waveguide, or a coaxial output coupling loop 26 as illustrated in FIGURE 1.

The structure illustrated in FIGURE 1 results in a ght weight, mechanically rug ed, frequency stable, supported drift tube klystron oscillator. In order to conserve weight, an electron gun 11 was utilized that did not require magnetic field focusing. AM and PM distribution noise and frequency instability due to thermal effects or vibration was eliminated by not utilizing grids in the structure of the tube. Electron beam focusing without a magnetic field is accomplished with an over-rolling produced by introducing the proper amount of spherical aberration to cause the outer electrons of the electron beam to cross the beam axis. The axial defocusing effect exerted on the center of a bunch of electrons which tends to cause defocusing of the electrons near the axis is reversed to bring a focusing effect upon the outer electrons. Therefore, the electron beam cross-section at a point along its length, such as at the second gap 29, varies little, it at all.

Such an over-rolling electron beam was accomplished with an electron gun 11 including a cathode 27 having an elliptical emissive surface 28, a first beam forming electrode 29 which is preferably at cathode potential, and a second beam forming electrode 3%} which is preferably at 10 to 15% of the collector voltage. The second electrode 35 produces periodic field focusing within the electron gun and permits adjustment of the electron beam focus. The current in the second electrode 30 is practically zero, amounting to only a few micro amperes, and its potential can be obtained from a high impedance divider (not shown) that is connected between the cathode 27 and collector 13 potential.

The walls 31 forming the frequency stabilizing cavity 16 are made from any suitable material, such as lnvar or copper. The bottom portion of the frequency stabilizing cavity 16 contains a re-entrant section 32 that surrounds the electron gun assembly ll and the upper portion 34 of the cavity 16 is preferably thin walled to permit tuning of the cavity 16 in a manner described hereinbclow. At least a portion of the interior wall of the frequency stabilizing cavity 16, generally indicated by the reference character 33, is a spherical surface.

The drift tube 17 is rigidly supported within and in electrical contact with the interior walls of the frequency stabilizing cavity 16 by a plurality of thin, hollow, metallic, conical supports 20. The supported drift tube 17 forms within the cavity 16 a first gap 18 adjacent the electron gun 11 and a second gap 19 adjacent the output cavity 14.

The bases of the conical supports 20 rest on the spherical surface 33 and the supports 20 have their axis coinciding with the radius of the spherical surface 33 which makes the intercepting line of the spherical surface 33 and the bases of the conical supports 20 a circle. This greatly simplifies construction of the cavity 16 inasmuch as the base portion of the conical supports 20 can readily be cut perpendicular to the axis of the cones to fit against the spherical surface 33 of the cavity to which they are attached by any suitable means, such as brazing. The opposite end, or apex, of each. of the conical supports 20 is secured to the drift tube 17 by any suitable means, such as brazing. It was found that three substantially equally spaced conical supports 21) spread the magnetic field within the cavity 16 more uniformly than did two conical supports 20. The conical supports 20 form an appreciable part of the cavity 16 inductance and therefore are of substantial dimensions in order to keep the Q of the cavity 16 high. Such dimensions also provide a greater form rigidity and frequency stability of the cavity against vibrations. The length of each conical support is about a quarter wavelength of the resonant frequency of the cavity 16 which is determined by the volume of the cavity 16.

The frequency stabilizing cavity 16 can be tuned by changing the spacing of the second gap 19 by flexing the thin walled upper portion 34 of the cavity 16 by any suitable means (not shown). Inasmuch as the voltage ratio of the first 18 and second 19 gaps is not critical, it is not necessary to change the spacing of the first gap 18 according to some fixed relationship when the spacing of the second gap 19 is changed to tune the cavity 16. It was found that a tuning range of 100 megacycles could be obtained by varying the gap 19 spacing.

The output cavity 14 is an ordinary re-entrant type having a gap 15 for extracting energy from the bunched electrons and is made from any suitable material, such as Invar or copper. It was found that only when the output cavity 14 was excessively underloaded did returning electrons have any effect on the amplitude of the oscillations in the frequency stabilizing cavity 16. As long as the output cavity 14 was properly loaded, it had no influence on the frequency stability of the frequency stabilizing cavity 16.

The supported drift tube klystron illustrated in FIG- URE 1 may operate as an oscillator or an amplifier. First, considering the supported drift tube klystron of FIGURE 1 as an oscillator and assuming that oscil1a tions are present in the frequency stabilizing cavity 16, the electrons emitted from the emissive surface 23 are initially velocity modulated by the gap 18.

Referring now to FIGURE 2, wherein the sine wave 41 represents the RF. voltage existing across the first gap 18 as a function of time, assume that an electron from the emissive surface 23 crosses the first gap 18 at time t when the alternating voltage is zero going positive. Such an electron passing through the gap 18 travels toward the second gap 19 with its velocity unchanged and can be termed a reference electron. An electron that passes through the gap 18 at a time slightly later than t encounters an increasing positive alternating field and is accelerated and so travels toward the second gap 19 with an increased velocity and tends to overtake the reference electron. Likewise, an electron that passes through the gap 18 slightly prior to time t encounters a decreasing negative retarding field and is accordingly slowed down and tends to be overtaken by the reference electron. It is clear then, that as the electrons travel away from the first gap 18 they gradually bunch together, and although they arrived at the first gap 18 at a uniform rate, they leave the first gap 13 with velocities that are a function of time and as such are said to be velocity modulated.

In order to increase the efficiency of the supported drift tube klystron oscillator, the bunched electrons are more closely bunched together at the second gap 19 in a manner as discussed hereinabove in connection with the first gap 18. Referring to FIGURE 2 wherein the sine wave 42 is a schematic representation of the voltage existing across the gap 19 as a function of time, it is shown that the RF. voltage across the second gap 19 is out of phase with the RF. voltage appearing across the first gap 18. In order for the second gap 19 to most eifi ciently bunch the electrons, it is necessary for the electrons partially bunched by the first gap 18 to cross the second gap 19 at a time when the alternating voltage across the second gap is zero going positive. For example, the second gap 19 will most etliciently further bunch the partially bunched electrons arriving from the first gap 18 i if they arrive at the second gap 19 at time t The partially bunched electrons could also arrive at the second gap at time t However, for this operating condition the drift tube 17 is unduly short. Further, the partially bunched electrons could arrive at time t However, for this operating condition, the drift tube is unduly long. It is clear then, that the length of the drift tube 17 is relatedto the transient time of the electrons that pass therethrough.

If the supported drift tube klystron of FIGURE 1 is to operate as an oscillator, it is necessary to sustain the oscillations in the frequency stabilizing cavity 16. This is accomplished by causing the partially bunched electrons to arrive at the second gap 19 a short time prior to time t For example, at time i When the partially bunched electrons arrive at the second gap at time t they encounter a decreasing negative field and surrender some of their energy into the frequency stabilizing cavity 16 and thereby sustain the oscillations therein. Accordingly, the second gap 19 functions as both a controlling or bunching gap and a working or feedback gap.

As the Q of the cavity 16 is increased and/ or the electron beam power increased by increasing the beam voltage, the amount of feedback provided by the second gap 19 in order to sustain oscillations in the cavity 16 may be decreased, thereby increasing the controlling or bunching action of the second gap 19 and increasing the efficiency of the support drift tube klystron oscillator illustrated in FIGURE 1. Since the drift tube 17 is electrically connected to the interior wall 33 of the cavity 16, the electric fields in the cavity are anti-parallel thereby causing the high frequency currents to flow along the conical supports 20 to the drift tube 17. The high frequency current I in the cavity 16 at an instant in time is indicated in FIGURE 1. By utilizing conical supports 20 of substantial dimensions, they form an appreciable part of the cir-. cuit inductance of the cavity 16 and keep the Q of the cavity high. For example, a value of Q in excess of 5,000 was accomplished with the structure illustrated in FIG- URE 1 and a frequency stability of .001% was obtained.

The higher the Q of the cavity 16, the smaller becomes the working action of the second gap 19 which increases its bunching or controlling action resulting in a higher electron beam efficiency. In accordance with one mode of operation of the supported drift tube device illustrated in FIGURE 1 as an osciliator, the second gap 19 functions predominately as a controlling or bunching gap.

After leaving the second gap 19, the bunched electrons cross the gap 15, which transfers energy from the bunched electrons into the output cavity 14 in a well known manner, after which the electrons are collected by the collector 13. The RF. power in the output cavity 14 is extracted by any suitable means, such as by a waveguide or by the coaxial pick up loop 26 located adjacent the output Window 2.5 which is formed from any suitable material, such as a ceramic.

In order to operate the device illustrated in FIGURE 1 as a supported drift tube amplifier, it is only necessary to apply the signal to be amplified into the frequency stabilizing cavity 16. For amplifier operation, the Q of the cavity 16 is substantially less than 5,000 to provide a sufiicient bandpass, and the partially bunched electrons leav ing the first gap 18 are caused to arrive at the second gap 19 at time 1 corresponding to maximum bunching action by the second gap 19 thereby causing the second gap 19 to operate solely as a controlling gap.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is to be understood, therefore, that Within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What I claim is:

1. A supported drift tube klystron comprising means including a cathode for producing a stream of electrons, means including a collector located in the path of said electron stream for collecting said electrons, means ineludin an output cavity surrounding said electron path and having a gap for extracting energy from said electrons, another cavity surrounding said electron path and located near said cathode, a major portion of the interior Wall of said other cavity being a spherical surface, a drift tube located Within said other cavity and surrounding said electron path, said drift tube cooperating with said other cavity to produce first and second controlling gaps therein at opposite ends of said drift tube, and a plurality of substantially conical supports retaining said drift tube within and in electrical contact with said other cavity, said conical supports having their base portions secured to said interior spherical wall of said other cavity with the axis of each support coinciding with a radius of curvature of said wall and the opposite ends of said conical supports secured to said drift tube.

2. The combination defined in claim 1 wherein each of said plurality of conical supports have a length about equal to a quarter Wavelength of the resonant frequency of said other cavity.

3. The combination defined in claim ll wherein said other cavity includes a re-entrant conical section that surrounds said means for producing a stream of electrons.

4. A mechanically rugged, supported drift tube klystron comprising means including a cathode having an elliptical emitting surface for producing a stream of electrons, means including a collector located in the path of said electron stream for collecting said electrons, means including an output cavity of the re-entrant type located adjacent said collector and having a gap for extracting energy from said electrons, another cavity surrounding said electron path and located near said cathode, at least a portion of the interior wall of said other cavity being a spherical surface, a drift tube located within said other cavity and surrounding said electron path, said drift tube cooperating with said other cavity to produce first and second gaps therein at opposite ends of said drift tube, and first gap located nearest to said cathode and characterized as a controlling gap, said second gap located nearest to said output cavity and characterized as being at least a controlling gap, and a plurality of substantially equally spaced conical supports retaining said drift tube within and in electrical contact with said other cavity, said conical supports having their axis coinciding With the radius of said spherical surface of said other cavity and the base of said conical supports secured to said spherical wall and the opposite ends of said conical supports secured to said drift tube.

5. The combination defined in claim 4 wherein said second gap is a combined Working and controlling gap.

6. A mechanically rugged, supported drift tube klystron oscillator comprising means including a cathode for producing a stream of electrons, means including a collector located in the path of said electron stream for collecting said electrons, means including an output cavity located near said collector for extracting energy from said electrons, another cavity surrounding said electron path and located near said cathode, at least a portion of the interior wall of said other cavity being a spherical surface, a drift tube located Within said other cavity and surrounding said electron path, said drift tube cooperating with said other cavity to produce first and second gaps therein at opposite ends of said drift tube, said first gap located nearest to said cathode and characterized as a controlling gap, said second gap located nearest to said output cavity and characterized as a combined controlling and Working gap, and three thin walled conical supports retaining said drift tube within and in electrical contact with the interior wall of said other cavity, said conical supports having their axis coinciding with the radius of said spherical surface of said other cavity and the base of said conical supports being perpendicular to their axis and secured to said spherical wall of said other cavity and the opposite ends of said conical supports secured to said drift tube.

7. A mechanically rugged, frequency stable, supported drift tube klystron oscillator comprising means including a cathode having an elliptical emitting surface for producing a stream of electrons, means including a collector located in the path of said electron stream for collecting said electrons, means including an output cavity of the reentrant type located adjacent said collector and having a gap for extracting energy from said electrons, another cavity having a high value of Q surrounding said electron path and located near said cathode, at least a portion of the interior wall of said other cavity being a spherical surface, said other cavity having a re-entrant conical section that surrounds said means for producing a stream of electrons, a drift tube located Within said other cavity and surrounding said electron path, said drift tube cooperating with said other cavity to produce first and second gaps therein at opposite ends of said drift tube, said first gap located nearest to said cathode and characterized as a controlling gap, said second gap located nearest to said output cavity and characterized as a combined controlling and working gap, and three substantially equally spaced conductive conical supports each having a length about equal to a quarter wavelength of the resonant frequency of said other cavity retaining said drift tube within and in electrical contact with the interior wall of said other cavity, said conical supports having their axis coinciding with the radius of said spherical surface of said other cavity and the base of said conical supports being perpendicular to their axis and secured to said spherical wall and the opposite ends of said conical supports secured to said drift tube.

References Cited UNITED STATES PATENTS 3,254,265 5/1966 Heil BIS-5.46 XR HERMAN KARL SAALBACH, Primary Examiner.

M. NUSSBAUM, Assistant Examiner.