US 3227957 A
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n. 9 HANS'HELMUT FELDMANN 3,227,957
CYCLQTRON-TYPE PARTICLE ACCELERATOR Filed Aug. 6, 1962 in venzor: Z WJQZM W United States Patent 3,227,957 CYCLOTRON-TYPE PARTICLE ACCELERATOR Hans-Helmut Feldmann, Berlin-Hermsdorf, Germany, assignor to Licentia Patent-Verwaltungs-G.m.b.H., Frankfurt am Main, Germany Filed Aug. 6, 1962, Ser. No. 215,100 Claims priority, application Germany, Aug. 10, 1961, L 39,754 3 Claims. 01. 328-234) The present invention relates to a fixed-frequency cyclotron-type particle accelerator.
The orbital frequency of electrically charged particles is proportional to their charge-mass ratio and the magnetic induction, While the orbital frequency is independent of the particle velocity, the latter being proportional to the radius of the orbit. It is on these considerations that the Lawrence principle of the ion accelerating cyclotron is based. The ions, which are guided along circular paths by strong magnetic fields, pass through 211 (n=1, 2, 3 accelerating gaps, during which passage they periodically take over energy from the generator which feeds the accelerating system. The frequency of the high-frequency generator should match the orbital frequency of the ions in order that as much energy as possible is transferred to the ions every time they pass through an accelerating gap, which, in turn, will cause the ions to reach the final radius as quickly, i.e., with as few orbits, as possible. If the accelerating and orbital frequencies are not equal to each other, the cyclotron will lose phase stability, because every time an ion passes through the accelerating gap there will be an increased phase shift, as a result of which the acceleration per gap decreases until it finally may even become negative. In the case of fixed-frequency or so-called isochron cyclotrons, it is therefore of the utmost importance that the induction across the air gap and the frequency of the accelerating voltage be matched to each other.
When the ions are accelerated to such an extent that the relativistic mass increase can no longer be ignored, this must be compensated for by a corresponding decrease in the air gap induction. This, however, requires special measures in order to avoid defocussing the field; to this end, so-called fixed-field, alternating-gradient (FFAG) machines or strong focussing have been used, in which the beam will, throughout each orbit, pass through a plurality of strong and weak magnetic fields as a result of which strong focussing forces are brought to bear on the ions. This makes it possible to use fixed-frequency cyclotrons for accelerating ions well into the region where they undergo relativistic mass increases.
It has been found that only a small portion of the energy delivered by the high-frequency generator producing the accelerating voltage is effective insofar as the actual ion acceleration is concerned, the remainder being lost in the accelerating system as current-generated heat. Inasmuch as this heat loss increases with the square of the accelerating voltage, it has been sought to increase the efiiciency by making do with low accelerating voltages. However, inasmuch as the number of orbits is inversely proportional to the accelerating voltage, efforts have been made to reduce the number of orbits by using especially high accelerating voltages, the reason being that the greater the number of orbits, the more precise does the magnetic field have to be. Therefore, the number of orbits is limited by the practical considerations which, due to physical and technological reasons, cannot go beyond certain limits.
It is, therefore, an object of the present invention to overcome the above drawbacks, namely, to provide a cyclotron in which optimal acceleration at all of the accelerating gaps is obtained even though the ions will, due to low accelerating voltage, have to execute a large number of turns or orbits before attaining the final velocity, and even though the magnetic guide field is of relatively low accuracy.
With the above object in view, the present invention involves a fixed-frequency cyclotron having a magnet for producing the magnetic guide field which confines the particles and thus defines their path of travel, and a highfrequency generator for accelerating the particles along the confined path. The invention specifically resides in the fact that special additional magnetic fields, which may be referred to as vernier fields, are provided approximately along the paths of the particles, the configurations of which fields are automatically regulated as a function of the phase of the orbiting particles taken with respect to the phase of the accelerating voltage.
As will be more particularly described below, the instant invention thus comprises a fixed-frequency cyclotron-type particle accelerator having means for generating a magnetic guide field providing a path for the particles, a high-frequency generator for accelerating the particles along the path, a plurality of additional magnet means for generating additional magnetic fields at different places along the path, and a plurality of control systems associated with the plurality of additional magnet means, respectively, each control system being responsive to the phase difference between the orbital frequency of the particles at the place at which the respective additional magnet means is located and the frequency of the high-frequency generator for automatically controlling the respective additional magnet means to bring the orbital frequency of the particles into substantially in-phase relationship with the frequency of the high-frequency generator.
According to one embodiment of the present invention, the additional magnet means each comprise magnetic exciter windings arranged along the path, and each of the control systems comprises a probe located at the place at which the winding with which the respective control system is connected is located, as well as a phase comparison circuit having two inputs, one of which is connected to the probe and the other of which is connected to the high-frequency generator, and an output connected to the respective winding. In practice, it is expedient to interpose an amplifier between the output of the phase comparison circuit and the respective winding.
The present invention further resides in a method of controlling a fixed-frequency cyclotron-type particle accelerator having means for generating a magnetic guide field providing a path for the particles and a high-frequency generator for accelerating the particles along the path. The method comprises the steps of comparing the phase of the orbital frequency of the particles, at a place along the path, with frequency of high-frequency generator, and applying at such point a vernier magnetic field whose size is a function of the phase difference between the two frequencies for bringing the orbital frequency of the particles into substantially in-phase relationship with are shown. The first control system has a probe S connected to one of the two inputs of a phase comparison circuit S1 whose other input is connected to the highfrequency generator represented schematically as HF. The output of the phase comparison circuit S1 is connected to the input of an implifier V whose output is connected to a vernier winding W Similarly, the other two control systems have probes S S phase comparison circuits SJ S1 and amplifiers V V respectively, the various components parts of each system being connected to each other and to the respective vernier windings W W in the manner described in connection with the first system. The three windings are suitably mounted on a pole shoe of a guide field magnet which is otherwise not shown. The ion source of the cyclotron is indicated at 11, and the path of the particles is indicated by the arrow shown in dashed lines.
The windings W W W are fashioned, for instance, as single turn coils through which flow currents indicated by the arrows. The crosses represent the direction of the magnetic fields produced by the windings. Instead of the precise windings arrangement shown in the figure, the distance between the coils can be smaller; moreover, the present invention is not limited to specifically three windings since more can be used, as indicated by the winding W shown in dashed lines; indeed, the greater the number of windings, the more accurate will the fine adjustment or vernier effect of the windings be. Nor it is essential that the windings have the actual configuration depicted in the drawing. Also, each winding can be composed of a plurality of suitably connected subwindings, i.e., individual windings which together make up the total winding for any one control system.
As shown in figure, the three windings are depicted as being arranged at different selected radii of the pole shoe 10. Each winding is fed from its respective amplifier which, in turn, is controlled by its respective phase comparison circuit. The latter compares the phase of the accelerating voltage HF with the phase position of the orbiting ions on the respective portion of the spiral path. The latter will, of course, be obtained from the respective probe.
The operation of the cyclotron is as follows:
The frequency of the accelerating generator as well as the current for the main exciter coils of the guide magnet (not shown) are kept constant. When the ion source 11 is turned on, the ions cannot at once be accelerated to travel along the outermost or maximum radius orbit because the magnetic field produced by the main guide magnet is not sufliciently accurate, the reason being, as explained above, the lack of phase stability. After the first orbits of the ions, when the path of the latter reach the radial position corresponding to the probe S this prove will pick up the phase of the ions and feed it to the phase comparison circuit S1 which compares this phase with that of the high-frequency accelerating voltage HF. The amount of this phase difference is fed, via the amplifier V to the vernier winding W which acts as a booster in that it produces a supplemental magnetic field acting on the ions, thereby reducing the phase difference to a negligible value. It will be appreciated, therefore, that the winding W serves, in effect as an auxiliary accelerating ring.
The ions, after having been subjected to the action of winding W will after a number of further orbits, come under the influence of the second control system, i.e., the probe S will now pick up the phase of the ions, the phase comparison circuit S1 will compare this phase with that of HF, and the winding W receiving its signal via amplifier V will produce the necessary vernier booster field, thereby once again bringing the ions into phase with the HF. After a number of further orbits, the ions will be subjected to the action of the third control system which functions in precisely the same manner.
Thanks to the above-described arrangement of cooperating control systems, the ions will be kept in phase with the HF, even though there are relatively large deviations from the magnetic field strength which the main guide magnet theoretically should produce. Consequently, a cyclotron equipped with the fine adjustment according to the present invention will maintain a very high degree of phase stability while operating with relatively low accelerating voltages. This, as explained above, is desirable because low accelerating voltages involve low current heat losses. Moreover, it will be appreciated that the complexity of a fine adjustment means of the above-described type is far less than that of a magnetic field which, in some other way, is made to maintain the desired accuracy. According to the instant invention all that is required is that the frequency of the accelerating generator be kept constant with a high degree of accuracy, which can readily be done, for instance, by
conventional frequency control techniques involving the use of a standard reference frequency. Under these circumstances, the speed with which the frequency control system responds need not be particularly high because the electrical inertia of the magnetic field will inherently and effectively prevent any rapid fluctuations. In contradistinction thereto, the speed with which the orbital frequency of the ions can be measured by the measuring probes is exceeedingly high, as is the speed at which this measurement can be processed by phase comparison circuits. Therefore, there will be no dynamic difficulties involved, which, in turn, allows the use of high amplifiication factors by the amplifiers incorporated in the several control systems.
It will be understood that the above description of the present invention is susceptible to various modifications, changes, and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.
What is claimed is:
1. A fixed-frequency cyclotron-type particle accelerator, comprising, in combination:
(a) means for generating a magnetic guide field providing a path for the particles;
(b) a high-frequency generator for accelerating the particles along said path;
(0) a plurality of additional magnet means for generating additional magnetic fields at selected places along said path; and
(d) a plurality of control systems associated with said plurality of additional magnet means, respectively, each control system being responsive to the phase difference between the orbital frequency of the particles at the place at which the respective additional magnet means is located and the frequency of said high-frequency generator for automatically controlling said respective additional magnet means to bring the orbital frequency of the particles into substantially in-phase relationship with the frequency of said high-frequency generator,
(1) a probe located at the place at which the additional magnet means with which the respective control system is connected is located; and
(2) a phase comparison circuit having (i) two inputs, one of which is connected to the probe and the other of which is connected to said high-frequency generator,
(ii) an output connected to the respective additional magnet means.
2. A particle accelerator as defined in claim 1 wherein each control system further comprises an amplifier interposed between the output of said phase comparison circuit and said respective additional magnet means.
3. In a fixed-frequency cyclotron-type particle accelerator having a magnet for generating a guide field providing a path for the particles and a high-frequency generator for accelerating the particles along the path, the improvement which comprises: a plurality of Vernier magnets for generating Vernier fields at selected places along the path; and a plurality of control systems associated with said Vernier magnets, respectively, each control system including a circuit responsive to the phase difference between the orbital frequency of the particles at the place at which the respective vernier magnet is located and the frequency of the high-frequency generator for automatically controlling said vernier magnet to bring the orbital frequency of the particles into substantially in-phase relationship with the frequency of the high-frequency generator.
References Cited by the Examiner UNITED STATES PATENTS 2,193,602 3/1940 Penny 31362 X 2,898,456 8/1959 Christofilos 328233 2,942,106 6/1960 Bennett 31362 X 10 GEORGE N. WESTBY, DAVID J. GALVIN, Examiners.
R. E. DZIURGOT, R. L. JUDD, Assistant Examiners.