US2885552A - Particle beam tracking circuit - Google Patents

Description

O. A. ANDERSON PARTICLE BEAM TRACKING CIRCUIT May 5, 1959 Filed July 27, 1956 2 Sheets-Sheet 1 INVENTOR.

OSCAR4 A. ANDERSON mPDaEOo ATTORNEY.

O. A. ANDERSON PARTICLE BEAM TRACKING CIRCUIT May 5, 1959 2 sheets-sheet v2 Filed July 27, 1956 INVENTOR. OSCAR A. ANDERSON ATTORNEY.

nted States Patent PARTICLE BEAM TRACKING CIRCUIT Oscar A. Anderson, Oakland, Calif., assignor to the YUnited States of America as represented by the United States Atomic Energy Commission Application `Iuly 27, 1956, Serial No. 600,643

l 4 Claims. (Cl. Z50-27) The present invention relates to an electronic circuit for use with a charged particle accelerator and more particularly to a control circuit for maintaining an optimum beam orbit or track in a particle accelerator.

^ In the field of nuclear physics, an important aid to research is the charged particle accelerator, wherein high energy is imparted to charged particles by magnetic or electrical fields or a combination thereof. Generally, such particles are caused to impinge on some selected material to create nuclear events for study and analysis. f the various classes of accelerators, the synchrotron is probably the most widely used and appears in a range of sizes adapted for various research and applied uses. The synchrotron principle is described in detail in United States Patent No. 2,624,841, A Method of and Apparatus for Accelerating to High Energy Electrically Charged Particles, issued to E. M. McMillan, January 6, 1,953. In general, a synchrotron particle accelerator is comprised of a closed particle orbit defining magnetic field with a radio frequency excited accelerating electrode toimpart additional energy and speed to such particles during each revolution. The intensity of the magnetic field is increased during the beam acceleration cycle so that as the speed and energy of the beam particles increase, the magnetic field through which the particles pass is always just sufficient to hold the particles in an orbit of substantially constant radius.

A high degree of correlation is required between the magnetic field strength and the frequency of potentials applied to the accelerating electrode. Improper synchronization results in loss of all or part of the particle beam. In the class |of synchrotron for which the present invention is most readily adapted, the magnetic field is allowed to build up at a rate determined by the inherent characteristics of the magnetic field coil. The frequency of the potential at the dritt tube is then altered at a rate matching the build up of magnetic field. Need for accurate synchronization has been the cause for the building of lcomplicated and expensive control equipment, such as described in United States Patent No. 2,683,807, issued to G. D. Paxson on `Tuly 13, 1954, entitled Variable Voltage Wave Form Generator.

Approximate synchronization between the magnetic field and the drift tube frequency may be achieved by utilizing the saturating effects of the magnet field current t'o alter the inductance of a frequency controlling saturable reactor. Considerable correction is still required for the resulting frequency modulation curve before usable correlation is obtained. The present invention may be utilized to provide the required synchronization for such a system without requiring complicated and extensive adjustment of controls.

In the present invention a pair of beam induction electrodes are placed on opposite sides of a particle beam orbit. The beam consists -of a hunched pulse of charged particles which, when passing, induce a potential in the induction electrodes, such potential having a frequency harmonically related to frequencies applied tothe drift lce tube. If the beam passes midway between the electrodes, the potentials induced in the induction electrodes are of equal amplitude, but at any other beam position the induced potentials will be unequal. The amplitude of the signals induced in the induction electrodes are compared in a voltage comparator or discriminator, producing an error or correcting signal by which the frequency modulation curve of the voltage applied to the drift tube is altered to return the beam orbit to the preferred position.

In effect, the device operates as a negative feedbacksystem, creating a correcting signal related to the physical position of the particle beam.

It is therefore an object of the present invention to' provide a means for obtaining maximum output from a charged particle accelerator.

It is a further object of the invention to provide a new and improved means for detecting the orbit radius of a charged particle beam.

Another object of the present invention is to provide an automatic control system for maintaining a particle beam in a selected orbit.

Still another object of the invention is to provide a synchronization system for maintaining accurate correlation between the growth of a magnetic field and the frequency of the accelerating voltage in a charged particle accelerator. v v

Other objects and advantages of the invention will be apparent in the following description and claims considered together with the accompanying drawing, in which:

Figure l is a diagram showing the circuitry in block form with a pictorial view of a synchrotron.

Figure 2 is a sectional view of a synchrotron taken along a line 2 2 in Fig. l.

Figure 3 is a schematic wiring diagram of a noise filter, amplifier, voltage comparator and amplitude regulator circuit illustrated in Fig. 1 in block form.

Referring now to Fig. l, there is shown a synchrotron particle accelerator 11 arranged with four curved vacuumy envelope segments 12 and four connecting straight segments 13. A particle defiecting magnetic field` is produced across each of the curved segments 12 by a set of four field coils 14. The straight segments 13 are provided for accomplishing auxiliary functions such as injection of particles through an input coupling manifold 16, connecting to a particle injector 15.`

In another one of the straight segments 13 is placed a cylindrical accelerating electrode 17 Where energy is irn-4 parted to correctly phased particles each time they pass. The accelerating electrode 17 is impressed with an alternating voltage having a frequency derived from a master oscillator 18 and amplified by an output amplifier 19.

The frequency of the master oscillator rises from a minimum to a maximum frequency during each accelerating cycle. Such frequency change is controlled by a coarse frequency control 21 which is, in turn, linked (not shown) to the magnetic field or field current.

If the signal frequency applied to the accelerating electrode 17 is too high in relation to the correct value, the particle beam impinges against the outside wall of the vacuum chamber formed by the curved and straight segments 12 and 13. Similarly, the beam impinges on the inner vacuum tank wall, if the frequency is too low. To prevent loss of the beam by such action, the present invention acts to hold the beam in a preferred median orbit. Such orbit path may be held constant or be changed during vthe synchrotron cycle as hereinafter discussed. The genw are located in another one of the straight-vacuum segments 13(see Fig. 2). The-exact shape of the induction electrodes-.23 and 24 is not critical, being in this embodiment a pair of metal plates on either side of the beam orbit with the upper and lower extremities extended inwardly. Suitable' support and electrical isolation for the induction electrodes 23 and 24 are provided by a iirst and a second insulator-'26 and 27. Such insulators may also provide a convenient passage for lead wires 25' and 30 from each ofthe induction electrodes to external circuitry.

Referring now again to Fig. 1, there is shown a channel A noisetilter 2S and ank identical channel B noise filter 29 connected to the inner and outer deflection eleetrodes23 and 24, respectively. The circuitry included inthe noise filters will vary with the` particular installation; however, the principal function is to reject low frequency signals induced in the induction electrodes incident to the synchrotron power supply. The outputs of the noise lters 28 and 29 are respectively connected to an A amplifier 31 and an identical B amplifier 32. The amplifiers V31 and 32 increase the amplitude of signals induced in the induction electrodes 23 and 24 to a value suitable for use in a voltage comparator 33. Since the amplitude of signals induced in the induction electrodes 23v and 24 varies during the synchrotron cycle and with -various operations, an amplitude regulator 34 connected to the voltage comparator 33 applies a gain control signal tothe amplifiers 31 and 32 through the noise filters 28 and 29. The aforementioned amplifiers 31 and 32, ratio comparator 33, and amplitude regulator 34 will be discussed in more detail hereinafter.

The output of the voltage comparator 33 generally comprises a direct current signal, the polarity of which dependsupon beam orbit deviation from a median path and the amplitude upon the amount of deviation of the beam 22 from the median path. Thus, when the beam 22 is at the median path, the output of the voltage comparator is zero Volts.

In many cases it is desirable to program the beam position; i. e., to vary the beam orbit at different times during a synchrotron cycle. For instance, it may be desired to deect or spill the beam onto a target at the end of an acceleration period. Such spilling may be accomplished by either raising the frequency of the master oscillator 18 out of proportion to the change in magnetic field and spilling the beam outwardly or by decreasing` the rate of frequency change to spill the beam inwardly. Itis also sometimes necessary to vary the beam orbit as the synchrotron cycle progresses because-of changes in magnet characteristics caused by iron saturation at high fields. Such considerations will, of course, vary with the particular application of the synchrotron. For producing such variations in the beam path, the output of the voltage comparator 33 may be medied by a computer 36A synchronized with the synchrotron acceleration cycle.

The output signals from the computer 36, or voltage comparator if no computer is used, is passed through an anti-hunt network 37. Such anti-hunt network 37 does noteffect slowly changing signals, but lters out any fast changes in the applied voltage. Such a network, control lingthe overall frequency response around the feedback loop, is generally necessary in a complex system with large amounts of feedback to prevent instability and resultant oscillation or hunting.

The corrective output signal from the anti-hunt network 37 is applied to the master oscillator 18, changing the frequency modulation curve as required. Such frequency corrections may be accomplished by various conventional means, for instance, by saturable inductive or capacitive. reactors in the master oscillator circuit, voltage controlled multivibratcrs, reactance tube modulators, etc.

Up to now, the various circuits and components asso.-l ciated with the invention have been described in general.

terms.. Theparticular configuration ofV thecircuitry associated with some rctA the components will dependv largely upon the particular installation and use of the synchrotron. For instance, the noise lters 28 yand 29 willv be adapted to eliminate the particular type of interference present, which in turn will depend largely upon the magnet power supply. Similarly the design of the computer 36 and anti-hunt network 37 will be varied to meet the particular application, or may be omitted in some instances. The master oscillator 18, coarse frequency control 21, and output amplier 19 design will vary between installations and still operate in conjunction withthe present invention.

The A and B ampliers 31` and 32, Voltage comparator 33, and amplitude regulator 34 comprise the more critical and novel portions of the inventionand will be described in greater detail, the circuitrybeing shown in Fig. 3.

Referring now to Fig. 3, there is shown the A noise filter 28, having an input terminal 43, such input terminal being provided for convenient connection to the induction electrode 24. A filter resistor 44, in parallel with an inductor 4S, is connected in series with a bias filter'` capacitor 46 from the input terminal 43 to a grounded or neutral bus 47. The filter-resistor 44 and the inductor 45 combine with the capacitance of the induction elec-y trode 24 to form a filter to remove low frequency signals. A first amplifier tube 48 in the A amplifier has a con trol electrode connected to the input terminal 43. TheI cathode of the first amplifier tube 48 is connected'to thev ground bus 47. Terminal means 49 is provided forv frequencies is provided by the first coupling capacitor 54' and the control electrode resistor 56. Anode potential is coupled from the B plus bus 52 by an anode resistor 58 and screen electrode potentials are provided by a connection to the screen potential terminal 49.

Anode signal voltages are coupled through a series connected coupling capacitor 59 and a control electrode capacitor 61 from the anode of the second amplifier tube 53 to the control electrode of a first cascode tube 62. A

diode resistor 63 is connected in parallel with a diode 64,.

from the juncture of the coupling capacitor 59 and the control electrode capacitor 61 to the ground bus 47. The diode 64 has the anode side connected to ground. Signals passed by the coupling capacitor 59 are then referenced to a zero bias level.

across the diode resistor 63, the pulse type signal voltages appear above a at zero base line. Such reference is necessitated by the distortion in the signal voltages created. by the small capacity value of the coupling capacitors 54 and 59, the necessity for the small value being hereinafter explained. A cascode grid resistor 66 couples the control., electrode of the first cascode tube 62 to the ground bus. 47. A bias potential is created by a cascode cathode ref` sistor 67 and cascode cathode capacitor 68 connected from the cathode of the irst cascode tube 62 to theA ground bus 47. The anode of the first cascode tube 62 is.

connected to the cathode of a second cascode tube 69.

First and second voltage divider resistors 71 and 72 are series connected from the B plus bus 52 to the groundv bus 47 and the juncture of such resistors is connected to the control electrode of the second cascode tube 69. A

capacitor 73 is connected in parallel with the second volt-- age divider resistor 72, thereby holding the control'eleo trode of the. second cascode tube 69 at a fixed potential.

The anode ofthe second cascode tube 69 is. connected;

Since the conduction of theA diode 64 prevents a negative voltage from being createdv through a casc'ode anode resistor 74 to the BA plus bus 52. A coupling capacitor 76 is connected in series with a coupiling resistor 77 from the anode of the second cascode tube 69 to the ground bus 47. A diode detector 78 is connected in parallel with the coupling resistor 77, the cathode side being connected to the ground bus 47. Likewise, a series-connected filter resistor 79 and filter capacitor 81 are coupled from the juncture of the coupling capacitor 76 and the coupling resistor 77 to the ground 'bus 47. The coupling capacitor 76 and resistor 77 average the input signals, producing an output related to the summation of signal voltages in a given time period rather than peak values. The diode detector 78 sets a zero base voltage, all, output potentials being negative with respect thereto. The filter resistor 79 and capacitor 81 remove high frequency variations in signals appearing across the coupling resistor 77.

In the voltage comparator 33 an A comparator tube 101 has a control. electrode connected to the juncture of the filter resistor 79 and the filter capacitor 81 in the A amplifier 31. Similarly, the output from the B amplifier 32` is connected to the control electrode of a B comparator tube` 102. The anodes of the A and B comparator tubes 101, 102 are coupled to a source of B plus potential 103 through an- A anode resistor 104 and a B anode resistor 106, respectively, While the cathodes are connected together. Direct coupling from the anode of the B comparator tube 102 to the control electrode of a cathode follower output tube 107 is made through a coupling resistor 108. Bias voltage for the control electrode of the cathode follower output tube 107 is supplied through a bias resistor 109 connected to a negative voltage bus 111. The output tube 107 has the cathode coupled to the negative voltage bus 111 through an output resistor 112, output signals being conveniently obtainable at a cathode connected output terminal 113.

The amplitude regulator 34 has a bias level potentiometer 114 with one end connected to the cathodes of the A and B comparator tubes 101, 102 and the other end connected through a resistor 116 to the negative voltage bus 111 in the voltage comparator 33. The movable arm on the potentiometer 114 is connected through a first bias resistor 117 to the juncture of the bias filter resistor 44 and the bias filter capacitor 46 in the A noise filter 28. A bias resistor 118 is connected from the movable arm on the potentiometer 114 to the control electrode capacitor 57 in the A amplifier 31. Additional bias resistors 119, 121, respectively, are connected to equivalent points in the B noise filter and amplifier 29, 32, respectively, such resistors minimizing mixing of signals between amplifiers.

Considering now the operation of the circuitry shown in Fig. 3, the device as shown here is designed to Work with accelerated particles having a positive charge although the circuit configuration may obviously be readily adapted to work with negatively charged particles. With positive particles, a series of positive pulses are received at the input of the noise filters 28 and 29, each pulse corresponding to one revolution of the charged particles around the synchrotron. During the acceleration cycle, the particle beam usually becomes less dense as individual particles are lost for various reasons, sometimes being purposely decreased to a low value for a particular application, or the beam injected may be decreased. Therefore, the pulses received by the amplifiers 31 and 32 vary greatly as to amplitude, as well as frequency. Also, the shape of the pulses changes during the synchrotron cycle as the density and dispersion of particles in the particle pulse changes. The circuitry of Fig. 3 is designed to mitigate such variable efiects so that a consistent output is produced dependent only on the radial position of the beam.

Input pulses are amplified in the amplifier tubes 48 and S3 and coupled through the coupling capacitor 59 to the' first diode 64. The coupling capacitors 54 and 59 are purpos'ely made l'ow in value so that low1 frequency signals from the amplitude lregulator 34 will not be amplified by bias controlled amplification stages. Increased stability and freedom from oscillation or hunting is obtained by preventing low frequency Asignals from being re-applied to the amplitude regulator 34 in amplified form. The first diode 64 then restores to the signal the zero level. which will have been lost by passage through the shorttime constant circuits.

The control electrode capacitor 61 and the coupling capacitor 76 both are sufficiently large in value to pass the pulse type signals without distortion. From the amplifiers 31 and 32, the comparator tubes 101 and 1.02 each receives a signal, the relative amplitudes depending on the beam orbit position relative to the induction electrodes 23 and. 24. Since the comparator tubes 101 and 102 have a common cathode load, a filtered signal which represents` the average value of the two control electrode signalsk may be taken therefrom for an amplitude regulation signal. Such signal', though filtered, Will have a small time delay compared to rate of beam loss', making the device virtually independent of beam intensity variations.

The plate signal of the B comparator tube 102 is proportional to the difference in the twocontrol' electrode signals; Output signals are. then taken from the cathode of the output tube 107 at the output terminal 113 to control the synchrotron acceleration electrode frequency.

As shown here, the invention functions as a fine tuning control for the master oscillator 18, although it is possible `for the invention to assume complete control once the start frequency has been established. The invention might also be adapted for use with accelerators where the magnetic field is held constant and only the accelerating electrode signal frequency varied or where the accelerating electrode signal frequency is held constant and the magnetic field varied. It is apparent that other modifications and variations may be made within the spirit and scope of the invention, and it is therefore not desired to limit the invention to the exact details shown and described except insofar as they may be defined within the following claims.

What is claimed is:

l. In an oscillator frequency control for a synchrotron, the combination comprising a first and second induction electrode located respectively radially inward and outward from the particle beam orbit of said synchrotron, a rst and second amplifier connected to said first and second induction electrode respectively, a voltage comparison circuit having a first and second input coupled to said first and second amplifier respectively and having a first output signal proportional to the difference of input signals from said first and second amplifier and a second output signal proportional to the summation of input signals from said pair of amplifiers, said first output signal being applied to said oscillator frequency control, said second output signal being applied to said first and sec ond amplifier and providing a gain controlling bias signal.

2. In amplification and voltage comparison circuitry for a synchrotron charged particle beam tracking device, the combination comprising a rst and a second beam detector, a rst and a second amplifier having inputs connected to said first and second beam detector respectively, said amplifiers having a generally restricting low frequency response, a first and second diode zero voltage level restorer connected to said first and second amplifiers respectively, a first and second low pass filter connected to said first and second zero level restorer respectively, a voltage amplitude comparison circuit having a first and second input terminal connected to said first and second low pass lter respectively, said voltage comparison circuit having a first output signal proportional to the difference of signals at said first and second input terminal and a second output signal proportional to the sum of signals at said first and second input terminal, said second ,output signal being applied 'to said ampliers as a gain controlling jbias, a-modulator controlling ,the frequency of the oscillator in said synchrotron, said rst output signalicontrolling said modulator.

-3. In an oscillator frequency control for maintaining a preferred charged particle orbit in a synchrotron particle accelerator, the combination comprising a rst and second inductionfelectrode disposed proximal to the orbit of said particle beam and on opposite sides thereof, a first and second high frequency amplifier each having an .input connector coupled to said rst and second electrodes respectively, a rst and second tube having a control electrode coupled to the output of said tirst and second amplifier respectively, a power supply having a negative `and a positive violtage supply terminal, a common cathode resistor connected from the cathodes of said rst and second tubes to said negative voltage supply terminal, a rst andvsecond anode resistor coupled from the -anode of said rst and second tube respectively to said positive voltage supply terminal, an automatic gain controlling biasconnection from the cathodes of said irst and second tubes to the inputs of said iirst and second ampliiers, and a connection from the anode of one of said tubes to said oscillator frequency control.

4. In a frequency controlier in a synchrotron oscillator for maintaining a. preferred charged particle beam to the`outputs of said rst and second high frequency Vpass lters respectively, a voltage comparison circuit hav- `ing inputs coupled to the outputs of said rst and second amplifiers and having a first output terminal signal proportional to the difference in input signal voltages and yhaving a second output signalproportional to the sum? mation of input signal voltages, said second output signal being applied to said first and second a plier asa gain control signal, a low frequency pass lt r coupling said second output signal from said comp rison circuit to said frequency controller.

References Cited in the le of this patent UNITED STATES PATENTS 2,491,918 De Boer et a1 Dec. 20, 1949 2,530,081 Ross Nov. 14, 1950 2,567,904 Christolos Sept. 11,1951 2,644,082 Avins June 30, 1953 2,761,969 Blancher Sept. 4, 1956 2,812,431

Adler Nov. 5, 1957'

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