US2492324A

US2492324A - Cyclotron oscillator system

Info

Publication number: US2492324A
Application number: US793680A
Authority: US
Inventors: Winfield W Salisbury
Original assignee: Collins Radio Co
Current assignee: Collins Radio Co
Priority date: 1947-12-24
Filing date: 1947-12-24
Publication date: 1949-12-27
Anticipated expiration: 1966-12-27

Description

CYCLO'IHON OS C ILLATOR SYSTEM Filed D90. 24, 1947 2 Sheets-Sheet l A 77 E/Vfy ee/e556 2254 A y Dec. 27, 1949 Filed Dec. 24, 1947 w. w. SALISBURY 2,492,324

CYCLOTRON OSCILLATOR SYSTEM 2 Sheets-Sheet 2 Patented Dec. 27, 1949 CYCLOTRON OSCILLATOR SYSTEM Winfield W. Salisbury, Cedar Rapids, Iowa, as-

signor to Collins Radio Company, Cedar Rapids, Iowa, a corporation of Iowa Application December 24, 1947, Serial No. 793,680

This invention relates to orbital electron devices and more particularly to magnetic-electrostatic resonance systems ofv which the cyclotron system is typical.

A principal object of the invention is to provide a more eflicient system employing electron accelerators of the cyclotron type.

Another object is to' provide a more efficient cyclotron system which is energized with frequency-modulated high frequency waves.

A further object is to provide a more efficient manner of coupling a high frequency potential source to a cyclotron of the type which is energized by frequency-modulated waves.

A feature of the invention relates to a frequency modulation cyclotron system, wherein the high frequency oscillations are coupled to the dees of the cyclotron by a special coupling arrangement, as a result of which the oscillator tubes and their adjuncts can be located at some convenient place and distance with respect to the cyclotron chamber proper.

Another feature relates to a coupling transmission line between a frequency-modulated oscillator and a cyclotron chamber, whereby the proper phase relations are obtained at the oscillator throughout its frequency-modulated range.

A further feature relates to a frequency-modulatedcyclotron system employing grounded grid electron discharge tube oscillators having a feedback loop comprised of symmetrically-connected one-quarter wavelength transmission lines in series with lumped inductances, whereby the delay throughout the feedback loop is maintained substantially uniform over the entire frequency modulation range.

A still further feature relates to the novel organization, arrangement and relative interconnection of parts which cooperate to provide an improved and more efiicient cyclotron system.

Other features and advantages not particularly enumerated will be apparent after a considera-' tion of the following detailed description and the appended claims.

In the drawing,

Fig. 1 is a composite schematic and structural diagram of a cyclotron system according to the invention.

' Fig. 2 is a detailed view of the cyclotron chamber of Fig. 1.

9 Claims. (Cl. 25027) Fig. 3 is a detailed wiring diagram of the oscillator and transmission line coupling system and showing the cyclotron dee electrodes in equiva lent analogue form.

Fig. 4 is a graph explanatory of the operation of Fig. 3.

Referring to Figs. 1 and 2, there is shown insomewhat schematic form a cyclotron comprising the hollow semi-circular metal dee electrodes l, 2, which are spaced apart to provide a gap 3 within which is centrally located the electronemitting cathode 4. The dees I and 2 are suitably supported within a closed chamber 5 which can be connected to a vacuum pumping system for maintaining a predetermined vacuum within the chamber. Lead-in wires 6 and 1 for heating the cathode 4 are insulatingly sealed through the chamber wall, and lead-ins 8 and 9 are provided for applying high frequency oscillating potentials respectively to the dees. The peripheral wall of dee 2 has an exit window or slit H] from which the accelerated electrons or particles emerge and thereafter are deflected by the negatively charged plate II so as to follow a path passing through the electron permeable window l2 of the sample bombarding tube l3. Usually the chamber 5 is connected to a source of hydrogen (not shown) at a predetermined pressure.

In the well-known way, the dees are mounted between the opposite pole pieces l4, l5, of a powerful electromagnet so that the interaction between the electrons and the magnetic field causes the electrons to follow a somewhat spiral path, that is one of increasingly greater orbital extent until they emerge from the slit l0. Since the electrons travel over increasing orbital paths around the cathode, if the dees are energized by a single frequency wave, there will, at different portions of gap 3, be lack of synchronism between the wave reversals and the instants when the electrons leave one dee and pass over the gap 3 to the other dee. It has been proposed heretofore to meet this difliculty, by frequen-cy-modw lating the high frequency potentials which are applied to the dees. Thus, it has been proposed to bridge the lead-ins 8, 9, within the chamber 5, by a rotating capacitor l6 which rotates at a uniform rate and varies the shunt capacitance across the lead-ins 8, 9, at a regular frequency. This change in capacitance loading of the high frequency input line feeding the dees, changes the resonant frequency of the supply oscillator l1. Thus the charged particles which follow the orbital paths around cathode 4 are accelerated during that half of the modulation cycle wherein the oscillator frequency is decreasing.

However, one of the difiiculties encountered with such frequency-modulated cyclotrons is that of supplying sufiicient oscillating energy over the entire modulation range, and the concomitant difficulty of maintaining the phases at the electrodes of the oscillator generator tube in the proper relation for generating sustained frequencies by feedback action. Thus, in one cyclotron arrangement, the oscillator was required to deliver a considerable amount of power in the frequency modulation range between 12 and megacycles per second. This necessarily required os cillator-generator tubes capable of dissipating large powers. Because of this power dissipation requirement, the tubes and their adjuncts necessarily must be physically very large; and because of the configuration of the cyclotron chamber proper it is well nigh impossible to couple the oscillator tubes directly into the resonant dee chamber.

In accordance with one phase of this invention, there is provided a coupling arrangement which will allow the oscillator tubes to be located conveniently at some distance from the deechamber of the cyclotron. Where the cyclotron is operated at a fixed frequency, as distinguished from frequency modulation, there is .no problem in using the transmission lines for coupling, because the phase shift of the transmissionline can be controlled by fixing the electrical length so that the over-all phase shift satisfies .the requirement for sustained oscillation. However, in the case of a frequency-modulated supply where it is necessary to vary the frequency of the oscillator 11, the problem of maintaining the desired phase .shift in the feed back circuit of the oscillator becomes very difficult.

One arrangement that was found to be satisfactory for this purpose is shown in Figs. land 3. In Fig. 3, the dee structure of the cyclotron is represented by the half-wave shorted coaxial transmission line .IB, this being .possible because the dee structure of a cyclotron forms .a high Q resonant tank circuit having distributed constants. In accordance with the invention, the oscillator-generator tube I9 .is of the grounded grid type, that is its control grid 20 is substantially at ground potentialfor .the high frequency waves. While the drawing shows .a single oscillator tube 19, preferably it consists of four or more such tubes connected in parallel. For purposes of simplicity, only one tube is illustrated in the drawing and its comprises an electronemitting cathode-2i, control grid 20, and plate or output anode 22., The tubes [9 may each be of the type designated 880 by the Radio Manufacturers Association,- and capable. .of delivering a useful output of 45 kilowatts. In that case the plate power supply .23 may have its positive terminal grounded and capable of supplying 10,000 volts negative potential to the cathode 2!, at 6 .amperes output.

In .Fig. 3, the resonant circuit of the cyclotron proper, that is the .dee structure, is represented by the half-wave coaxial transmission line 18, which is shorted at its opposite ends and the electrical length .of this transmission line being variedby the continuously variable capacitor Ifia which is analogous to the capacity l6 of Fig. .-1.

Since this dee structure is a high Q resonant tank circuit, the coupling to the oscillator can be taken therefrom adjacent each end thereof so that there is substantially zero phase shift between the voltage at one end of the line [8 as compared with the voltage at the other end of this line l8. For this purpose, the dee structure is provided at each end with lead-in

lines

24, 25, each consisting of a coaxial wave transmission line of onequarter wavelength at the midpoint of the operating range of the frequency modulations, that is at 18 megacycles in the example assumed above. The outer conductors of

lines

24, 25, and the resonant chamber l8, are grounded as shown.

- The center conductor 26 of line 24 is connected in series with the lumped inductance 21 to the anode 22 of the oscillator. Likewise, the center conductor2-8 of line 25 is connected through the lumped inductance 29 and thence through the v capacitor 30 to the cathode 2 I.

In Fig. 3, the capacitance C represents the grid-to-anode capacitance of tube I9 plus the stray capacitance of the circuit, and in the particular example mentioned above, equals approximately 177 mmf. Capacitance Cl represents the cathode-to-grid.capacitance of the tube l9 and the stray capacitance of the circuit and'in the example assumed was equal to mmf. The capacitance 31 is an additional capacitor connected effectively across the grid 20 and cathode 2|, and in the example above mentioned, is equal to 885 mmf. The capacitor 32' is a filtering capacitor of .01 mf., which cooperates with the cathode choke coil 33 to isolate the radio frequency energy from the .conductorsfleading to the power supply 23. The capacitor .34 is a by-pass capacitor having a value of .02 inf to maintainv the grid 20 effectively at radiofrequency ground potential. The capacitor 30, above described, is a coupling capacitor of .012 mf., and serves to isolate the negative high voltage terminal 35 of the power supply so that it is not short-circuited to ground :in the cyclotron dee. The anodejinductance 21 may have .an, inductance of 0.42 microhenry, and the cathode inductance 29 may have a value of 050763 microhenry, while inductance 33 may have a value of .5 microhenries. The feedback path for tube I9 is as follows. The high frequency voltages at the anode 22 pass through the inductance 21, coaxial transmission line 24, the cyclotron dees as represented by the half -Wave,line l8, thence through the one-quarter wave coaxial transmission .line '25. andinductance 29 to the cathodes 21 of the oscillator tube. The coaxial transmission lines 24 and25 are adjusted to be one-quarter Wavelength long at 18 megacycles, which in the assumed example is the midpoint of the operating frequency-modulated range of the oscillator.

In order that the grounded grid oscillator I19 can operate properly, it is necessary that the cathode and anode circuits operate with their voltages close to and, in-phase relationship. Since at 18 megacycles there is approximately 90 phase shift .across eachof the'l'ines 24., 25., it .is necessary to provide an additional phase shift so as to maintain the iii-phase relationship between the cathode and anode circuits. This is made possible through the delays provided by the lumped inductance 21' and the shunt capacitance C in the anode circuit and by the lumped inductance 2.9 and the combined .capacitances Cl and 30 in the cathode circuit. By the proper selection of the valuesof inductances .21. 29,.and capacitance 30, it is possible to maintain substantially 360" shift throughout the range of 12 to 27 megacycles as is illustrated in the graph of Fig. 4. Following is a, chart of specific values for the phase delays involved for both 18 and 25 megacycles.

Summary of phase calculations Delay in lumped-anode net (21, C) 77.5 Delay in output line 24 90,0

Dee lags anode 22'by 167.5 Delay in cyclotron Zero Delay in feedback line 25-- 90.0 Delay in lumped cathode net Cathode lags dee by 167.5"

Hence cathode lags anode by 335.0 or cathode leads anode by 25.0

Delay in lumped anode net (21, C) 173.8 Delay in output line 24 s 5.6

Dee lags anode 22 by 179.4 Delay in cyclotron Zero Delay in feedback line 25 5.0 Delay in cathode lumped net Cathode lags dee by 178.8 Hence cathode lags anode by 358.2 or cathode leads anode by 1.8

It can be seen from the above tabulation and the graph of Fig. 4, that the sum of the phase delays of the

coupling transmission lines

24, 25, and the lumped networks consisting of inductance 21 and the capacitance C on the one hand, and inductance 29 and the capacitance 30 on the other hand, is substantially 180 throughout the entire range of 12 to 27 megacycles. The circuit as described and as disclosed in the drawing, therefore, provides a convenient method by which a high powered electronic tube oscillation generator can be coupled to a cyclotron which is to be energized with frequency-modulated high frequency waves over a wide range of modulation.

While one particular embodiment has been illustrated, various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

l. A system for producing high velocity electrons traveling in orbital paths of increasing radius, comprising a pair of electrodes for accelcrating electrons in resonance with applied high frequency waves, electron discharge tube oscillator means for exciting said electrodes, means to vary the capacitance across said electrodes and thereby to vary the frequency of the oscillator in timed correlation with the increase in said orbital path radius, and a feedback circuit for said oscillator comprising a first transmission line and an inductance in series therewith and connected to the anode of said oscillator, a second wave transmission line and an inductance in series therewith and connected to the cathode of said oscillator, said transmission lines being symmetrically connected to said electrodes, each of said transmission lines being a one-quarter wave line at the midpoint of the operating freill) 8, quency range of said oscillator, said inductances and transmission lines being proportioned to maintain the excitation of said anode and cathode in substantially like phase and substantially re gardless of the frequency of said oscillator.

2. A cyclotron system comprising a pair of cyclotron electrodes for accelerating electrons in orbital paths of increasing radius and in resonance with applied high frequency waves, electron discharge tube oscillator means for exciting said electrodes, means to vary the effective capacitance across said electrodes and thereby to vary the frequency of the oscillator in timed correlation with the increase in said orbital path radius, a feedback path connected between the anode of the oscillator and. one of said electrodes and including a lumped inductance and a coaxial wave transmission line section, another feedback path connected between the cathode of said oscillator and the other of said electrodes and including a lumped inductance and a coaxial wave transmission line section, each of said coaxial transmission line sections being a onequarter wave line at the midpoint of the operating frequency range of said oscillator.

3. A cyclotron system comprising a pair of cyclotron electrodes for accelerating electrons in orbital paths of increasing radius and in resonance with applied high frequency waves, electron discharge tube oscillator means for exciting said electrodes, said oscillator having means to maintain its control grid at substantially ground potential for high frequency waves, a feedback path from the anode of the oscillator to one of said electrodes, a feedback path from the cathode of the oscillator to the other of said electrodes, each path including a lumped inductance in series with a coaxial Wave transmission line, each transmission line being of one-quarter wavelength at the midpoint of the operating range of said oscillator, and means to vary the effective capacitance across said electrodes to vary the operating frequency of said oscillator between predetermined limits and in timed correlation with the said increase in said orbital path radius.

4. In combination, an evacuated device having an electron emitter and a pair of electron accelerating electrodes, at least one high frequency oscillator-generator tube for exciting said electrodes for progressively and cumulatively accelerating the electrons from said emitter in trajectories of increasing electron transit time, means to frequency-modulate the oscillations from said generator in timed correlation with the increase in the electron transit time of said trajectories, said device having the electrical characteristics equivalent to a coaxial wave transmission line which is of one-half wavelength and short-circuited at opposite ends, a one-quarter wave coaxial transmission line connecting one of said electrodes to the anode of said oscillator tube, another one-quarter wave coaxial transmission line connecting the other electrode to the cathode of said oscillator tube, and lumped reactances connected in circuit with each of said one-quarter wave transmission lines to maintain the anode and cathode of said oscillator tube in substantially like phase with respect to thegenerated oscillations and substantially independent of the frequency of said frequency-modulated oscillations.

5. In combination, an evacuated device having an electron emitter and a pair of electron accelerating electrodes, at least one high frequency oscillator-generator tube for exciting said elecauaeac trodes to progressively :and cumulatively accelcrate the electrons in trajectories of increasing electron transit time, said device having the electrical characteristics equivalent to a high Q resonant cavity, means to frequency-modulate the oscillationsfromsaidgenerator in timed correlation with the said increase of the electron transit time of said trajectories, a, one-quarter nected across said electrodes to vary the irequency-ofthe oscillator.

. 7. In combination, a grid controlled electron discharge tube oscillator, means to maintain the control grid of said oscillator at substantially ground potential forhigh frequency waves, a tank circuit connectediacross the anode and cathode of saidoscillatorand including a'variable loading capacitor, said ,tank circuit constituting a resonant cavity short-circuited at opposite ends, a one-quarter wave transmission line connected adjacent one endof the cavity and inseries with an inductance to the anode of said tube, and a one-quarter Wave transmission line connected adjacent the opposite end .of said cavity and in series with an inductance to'the cathode of said oscillator tube, and means to vary the resonant frequency of said cavityand thereby to vary the frequency of the oscillator tube while maintaining the excitation of the anode and cathode of said oscillator in substantially like phase.

8. The combination according to claim 6 in which said inductances are proportioned with respect to the said one-quarter wave lines to maintain the anode and cathode of said oscillator tube in proper feedback phase over the entire operating frequency range of said oscillator.

9. The combination according to claim 7 in which said resonant cavity is constituted of two cyclotron'dee electrodes.

WINFIELD W. SALISBURY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,267,520 Dow Dec. 23, 1941 2,300,315 Puhimann Oct. 27, 1942 OTHER REFERENCES Physical Review, vol. 71, No. 7, April 1, 1947. (Copy in-Scientific Lib.)

Physical Review, vol-68, pages 143-144, Sept. 5, 1945. (Copy in Scientific'Library.)

Physical Review, vol. '69, pages 669-670, June 3, 1946. (Copy in Scientific'Library.)

Electronics, F. M. Cyclotron, vol. 20, No. 3, page 119, Mar. 1947.