US 3638127 A
A stabilization system for resonant cavity excitation of a particle accelerator or particle storage ring includes a pair of cavity current control tubes coupled in parallel. One tube is coupled to a beam pickup in the accelerator to provide a cavity current which is a function of the beam intensity. The current through the other tube is controlled by a comparison between the voltage across the accelerating gap and a desired voltage. The currents through each of the pair of tubes are combined to provide excitation for the cavity. Each of the pair of tubes may consist of a number of tubes in parallel. The cavity is constructed so that energy at undesired frequencies is absorbed.
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Description (OCR text may contain errors)
[451 Jan. 25, 1972 United States Patent Kerns 3,529,259 9/1970 Holmes et al..L.........................33l/l2  STABILIZATION SYSTEM FOR RESONANT CAVITY EXCITATION Primary Examiner-John Kominski Attorney-Roland A. Anderson  Inventor: Quentin A. Kerns, Glen Ellyn, Ill.
 Assignee: United States of America as represented by the United States Atomic' Energy Commission Jan. 29, 1970 cle accelerator or particle storage ring includes a pair of cavity  Filed:
current control tubes coupled in parallel. One tube is coupled to a beam pickup in the accelerator to provide a cavity current ] Appl. No.:
which is a function of the beam intensity. The current through the other tube is controlled by a comparison between the volt-  U.S.  Int.  Field of 5 Claims, 2 Drawing Figures UNITED STATES PATENTS 1/1961 Hansen et a1.
MODULH 7 0)? P0 WER INTEGER 7'01? REFEPEA E VOL 756E MODULfl 70/? STABILIZATION SYSTEM FOR RESONANT CAVITY EXCITATION CONTRACTUAL ORIGIN OR THE INVENTION The invention described herein wasmade in the course of, or under, a contract with the UNITED STATES ATOMIC ENERGY COMMISSION.
BACKGROUND OF THE INVENTION Advances in particle accelerator and storage ring technology have led to continued increases in the magnitude of beam current to be accelerated or stored. Accordingly, the disturbing effect of the charged particle beam in traversing the accelerating cavity gap has become greater with each increase in particle beam current. A further problem is that the beam current often is not continuous but occurs in interrupted pulse trains. The beam-loading effect is thus not constant and leads to RF cavity voltage and phase fluctuations.
Changes in real loading at the gap will tend to change the amplitude of the voltage, while changes in reactive loading will tend to change both amplitude and phase by introducing detuning of the resonant system. Thus there is a requirement for rapid automatic control of both real and reactive effects. In addition, it is necessary to provide a constant radiofrequency current sufficient to balance the total resonant circuit losses exclusive of the beam. These losses include conductor losses, dielectric losses, power tube and transmission line losses and are characterized by being constantly present independent of beam load.
As is well known, all resonant cavities can resonate not only at a lowest mode frequency but at a large number (theoretically infinite) of higher frequencies. The standing wave patterns of the various high-order mode resonances are each unique and distinguishable such that a separate individual attack on each undesired resonance is prohibitively costly.
It is therefore an object of this invention to provide an improved stabilization system for resonant cavity excitation.
Another object of this invention is to provide a controlled RF voltage across a gap despite random or periodic variations in real or reactive loading at the gap.
Another object of the invention is to provide a constant radiofrequency current to balance constant resonant circuit losses of a particle beam acceleration system.
Another object of the invention is to provide -a control system which will cause the cavity gap voltage to followa prescribed program of frequency and voltage with time.
Another object of the invention is to limit the resonantbuildup of RF voltages in the cavity at high-order mode frequencies.
SUMMARY OF THE INVENTION In practicing this invention, a resonant cavity is used to provide an RF voltage across an accelerating gap of a particle accelerator or storage ring. A stabilization circuit forthe cavity is provided to counteract beam loading effects and to regulate the accelerating voltage as desired. A pair of cavity current control tubes are coupled in parallel and the current through each tube is combined and coupled to the cavity-amplifying tube. The voltage developed at the accelerating gap is a function of the combined currents from each of the cavity current control tubes. One of the cavity current control tubes is coupled to a beam pickup in the accelerator to provide a cavity current which is a function of the beam intensity. The current through the other cavity current control tube is controlled by an error signal which is derived by comparing the voltage across the accelerating gap with a desired reference voltage. Each of the cavity current control tubes can include more than one tube connected in parallel in order to provide the desired transconductance.
The cavity is constructed as a' section of a transmission'line with one end terminated at the accelerating gap and the other end terminated'in a resistance such as a ferrite. By correctly choosing the type of ferrite terminating the transmission line,
the energy at frequencies higher than the desired frequency is absorbed.
BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, there is shown a partial schematic and partial block diagram of the circuit of this invention. A cavity structure includes a cavity gap 11 and a beam pickup 13.
, These structures are known and examples are illustrated in FIG. 2. Power is supplied to the cavity from power supply 14 through modulator 16, choke 17 to anode 19 of tetrode 20. A plurality of control tetrodes 122 and 23 are coupled in parallel between cathode 24 of tetrode 20 and ground. Control grid 25 of tetrode 23 is connected to the beam pickup 13 of cavity 10.
A voltage sample from the cavity gap 11 is coupled to comparator 29 through capacitor 28. In comparator 29 the voltage sample is compared with the output of frequency synthesizer 31 in a known manner to develop an error signal which is a function of the difference between the voltage at cavity gap 11 and the desired voltage from frequency synthesizer 31. The error signal is integrated in integrator 32 to remove short-term fluctuations and the resulting integrated error signal is applied to modulator 34.
A signal from frequency synthesizer 31 is coupled to modulator 34 and the output of modulator 34 is coupled to linear amplifier 35. The output of linear amplifier 35 is coupled to the control grid 26 of tetrode 22 to control the current through tetrode 22. With no error signal present, the current through tetrode 22 is controlled by the output from frequency synthesizer 31. With an error signal present, the error signal acts to modulate the signal from modulator 34 in a manner to counteract the error. This modulated control signal is applied to linear-amplifier 25 of control grid 26 of tetrode 22. Program generator 36 connected to frequency synthesizer 31 and modulator l6 acts to control the operation of the resonant cavity excitation.
In operation, tetrodes 22 and 23act to stabilize the resonant cavity system. Fluctuations in beam current are detected at the beam pickup l3 and act to control the current flow through=tetrode 23. Voltage fluctuations at the cavity, gap act to control-the current flow through tetrode 22. Since normally there are desired voltage fluctuations across cavity gap 11, the voltage atthis point must be compared to the desired voltage in order to detect errors. Any detected error is used to control the current flow through tetrode 22.
The currents through tetrodes 22 and 23 are combined at cathode 24 of tetrode20 and thus the flow of current through tetrode 20 is controlled by both the beam current and departures from norm of the voltage at the cavity gap 11. The beamloading effect on cavity 10 is neutralized by controlling the flow of current through tetrode 23 and the cavity gap voltage is made to follow a prescribed program of frequency and voltage with respect to time and any errors in this program are automatically'corrected.
Referring to FIG. 2, there is shown a drawing of the mechanical arrangement of the cavity and the portion of the particle accelerator associated therewith. The cavity 40 is designed as a section of a uniform coaxial transmission line. A power tetrode 41 acts to supply the accelerating voltage to gap 49 in the particle accelerator. A plurality of tubes 43 and 44 connected in parallel are connected in series with power tetrode 41. Thus the current flowing through tubes 43 and 44 is combined in the power tetrode 41 to providethe necessary drive current for the cavity. Vacuum tube 41 corresponds to vacuum tube'20, vacuum tubes 43 correspond to vacuum tube 22, and vacuum tubes'44 correspond to vacuum tube 23 of FIG. 1. Tubes 22 and 23 can each be a single tube or a number of tubes in parallel in order to provide the necessary transconductance.
The particle accelerator tube 46 has an accelerating gap 49 herein. This gap acts as a termination for one end of the cavity and the voltage developed across this gap is the accelerating voltage for the particle accelerator. A probe 52 corresponding to capacitor 28 of FIG. 1 senses the voltage across the accelerating gap. A beam pickup probe 47 inserted in the particle accelerator tube 46, by developing a voltage proportional to beam current, senses the intensity of the particle beam in the accelerator tube. Beam pickup probe 47 is connected to vacuum tubes 44 and an RF drive voltage from linear amplifier 35 of FIG. 1 is connected to tubes 43. These connections are shown in schematic form in FIG. 1. The other end of the cavity 40 is terminated with ferrite damper blocks which provide a resistive termination to absorb energy at frequencies above 'the fundamental frequency.
The cavity is designed as a section of uniform transmission line. For its fundamental frequency the length of the transmission line is such that it acts as if it were terminated in a short circuit at one end with the accelerating gap at the other end. For higher frequencies, the cavity is no longer terminated in a short circuit but is terminated in a resistance equal to the characteristic impedance of the coaxial line forming the cavity. This terminating resistance is provided by the ferrite damper blocks. Thus higher frequency energy introduced by disturbances at the accelerating gap flows along the transmission line and is absorbed by the terminating resistance. If the ferrite is selected to have at the higher mode frequencies a constant surface impedance in ohms/square w/ 377 ohms the complete spectrum of higher mode energy will be absorbed. That is, the terminating resistor has the characteristics of a black body, absorbing energy at all wavelengths.
1. An IF stabilization circuit, including in combination, an RF resonant system having a cavity and a beam tube with a particle beam therein, said beam tube including a gap coupled to said cavity with said gap having a gap voltage thereacross for acting on said particle beam, first cavity current control means including beam pickup means positioned in said beam tube and responsive to the intensity of said particle beam to develop a first control signal, second cavity current control means including voltage pickup means positioned at said gap and responsive to said gap voltage to develop a second control signal, said first cavity current control means being responsive to said first control signal to develop a first supply current and said second cavity current control means being responsive to said second control signal to develop a second supply current, combining means coupled to said first and second cavity current control means for combining said first and second supply currents to develop a combined supply current, said combining means further being coupled to said RF resonant system to provide said combined supply current to said cavity, said cavity acting to develop said gap voltage as a function of said combined supply current.
2. The stabilization circuit of claim I further including, means for generating an accelerating control voltage coupled to said cavity, said cavity being responsive to said accelerating control voltage and said combined supply currents to provide said gap voltage, said second cavity current control means including comparator means coupled to said gap and said means for generating an accelerating control voltage and being responsive to said accelerating control voltage and said second control signal to develop an error signal as a function of the difference therebetween, said second cavity current control means being responsive to said error signal to control the magnitude of said second supply current.
3. The stabilization circuit of claim 3 wherein, said second cavity current control means includes integrating means for integrating said error signal, said second cavity current control means being responsive to said integrated error signal to control the magnitude of said second supply current.
4. The stabilization circuit of claim 3 wherein, said first and second cavity current control means include first and second vacuum tubes respectively each having a cathode connected to a reference potential, an anode and a control electrode, said control electrode of said first tube being coupled to said beam pickup means for receiving said first control signal, said control electrode of said second tube being coupled to said integrating means for receiving said integrated error signal, said combining means being a third vacuum tube having a cathode coupled to said anodes of said first and second vacuum tubes and an anode coupled to said cavity.
5. The stabilization system of claim 4 wherein, said cavity is in the form of a section of a uniform coaxial transmission line terminated by said gap at one end and a resistance at the other end, the length of said transmission line and the magnitude of said resistance being chosen so that said other end thereof acts as short circuit at the frequency of interest, and so that at frequencies higher than said frequency of interest said transmission acts as a line terminated in a resistance equal to the characteristic impedance of said transmission line.