US3427557A - Device for accelerating particles - Google Patents

Description

filed April 11, 1966 Feb. 11, 1969 R. A. SPECIALE 3,427,557 I DEVICE FOR ACCELERATING PARTICLES Sheet f of INVENTOR. ROSARIO ASPECIALE AGENT Feb. 11, 3969 R. A. SPECIALE DEVICE FOR ACCELERATING PARTICLES Sheet Filed April 11, 1966 United States Patent Ofice US. Cl. 328-234 Int. Cl. H05h 13/00 6 Claims ABSTRACT OF THE DISCLOSURE A particle accelerating device of the cyclotron type employing at least three accelerating electrodes having equal aperture angles and comprising two opposing sections substantially in the form of sectors of a circle located on each side of a median plane. The electrodes are connected together in such manner that each electrode is connected, at its periphery, to two other electrodes located symmetrically relative to one of the electrodes. The connections, together with the electrodes constitute a delay line whose length is a whole multiple of a highfrequency wave on it and in which high frequency can circulate either in the same direction as, or in a direction opposite to the particles being accelerated. The delay line is coupled at a number of points not exceeding half the number of electrodes to a high-frequency generator.

This invention relates to a device for accelerating particles of the cyclotron type comprising at least three accelerating electrodes having equal aperture angles and each comprising two opposing parts substantially in the form of sectors of a circle located one on each side of the median plane and having applied to them a multiphase system of voltages. In such a device the particles are accelerated whenever they pass along a gap between the sequential electrodes due to the potential difference between the electrodes.

In such a device a multiple of successively active electrodes have been used which are energized by a multiphase alternating voltage the phases of which are correlated to the transit time of the particles between the sequential electrodes so that each electrode provides a maximum field gradient exactly at the instant when the particle becomes subjected to its influence. In one embodiment thereof having three electrodes, the sectors are connected at the centres of their peripheries to corresponding phases of a three-phase oscillator. In another embodiment having three electrodes, the sectors are connected at their central parts to corresponding phases of a three-phase oscillator, the sectors constituting three transmission lines which erminate at the periphery in three impedances connected to the corner points of the sectors. Energy is dissipated in said impedances. In the last-mentioned case it is believed to alleviate the problem that, due to the relativistic increase in mass of the particles upon reaching high energies, the period of circulation of the particles becomes longer so that the particles would get out of phase. In either case the energy supplied to each sector is derived directly from the oscillator.

The invention is based upon the discovery that certain advantages may be obtained in such a device if the high-frequency voltage is applied to the electrodes in a particular manner. According to the invention each accelerating electrode is connected at its periphery to two other accelerating electrodes located symmetrically relative to the first accelerating electrode, the arrangement being such that said connections, possibly together with the accelerating electrodes, behave as a delay line 3,427,557 Patented Feb. 11, 1969 in which high-frequency energy can circulate in the same direction or in an opposite direction to the particles to be accelerated, the delay line being coupled to a highfrequency generator at a number of points at most equal to half the number of the accelerating electrodes. This coupling to a high'frequency generator may be effected either on an accelerating electrode or on one of said connections. The coupling will preferably be made on a connection since the voltage in situ can be lower. The couplings may be made to separate generators, or to one multiphase generator. The mutual phases of the separate generators in the one case, and the phases of the multiphase generator in the other case, must be such as to correspond to the phase of the circulating high-frequency energy. In the geometry of the delay line the equivalent impedance between each accelerating electrode and earth appears as a shunt impedance of the delay line at its node points. The delay line is closed in itself and may behave as a ring resonator on which an unreflected advancing high-frequency wave propagates azimuthally with no losses other than the intrinsic attenuation in the delay line and the load resulting from the particles to be accelerated. In order for it to behave as a ring resonator, the total electrical length of the delay line must be a whole multiple of the wave-length of the high-frequency wave on it. Thus the amplitudes of the voltages on the accelerating electrodes are equal with high accuracy, or this may at least be realized in a simple manner. Further, the delay line may be such that the phases of the voltages on the accelerating electrodes automatically correspond to the desired pattern which is correlated to the period of circulation of the particles. In the device according to the invention all these advantages may be obtained with a generator which provides a number of phases at most equal to half the number of the accelerating electrodes. The energy provided by the generator must be at least equal to the energy extracted from the unreflected, azimuthally propagating high-frequency wave due to the intrinsic attenuation in the delay line and the load, as aresult of the particles to be accelerated and the energy provided, need not exceed the energy extracted. The energy of the high-frequency wave which is lost due to the intrinsic attenuation in the delay line and the load resultingfrom the beam constitutes only a very small portion of the energy of the circulating high-frequency wave. In the device according to the invention only a very small portion of the azimuthally propagating energy need therefore be provided by the generator during operation. If more than one generator is used, this holds good for the total power provided by the generators. Furthermore, the device affords the advantage that the entire high-frequency accelerating system is relatively small so that it can be incorporated almost completely in the accelerating chamber. This applies at any rate to the connections between the accelerating electrodes. Further, vacuum-tight lead-through devices at most equal to half the number of the accelerating electrodes are required coupling with a generator and because only a small portion of the energy of the high-frequency wave need be provided, said lead-through devices may have comparatively small dimensions. The accelerating chamber thus becomes more readily accessible for the connection of pumping apparatus and other necessary steps to be taken. Furthermore, the total surface area occupied by the device is smaller and no high-frequency shortcircuit connection to earth is required. Insofar as the voltage is concerned, the assembly comprising the accelerating electrodes and their mutual connections is floating with respect to earth. It is therefore easier to provide a bias connection in order to avoid the known multipactor effect. There is no need for a large number of high power capacitors to provide a high-frequency shortcircuit while retaining a D.-C. voltage insulation.

Each accelerating electrode is connected at its periphery to two other accelerating electrodes. As viewed in the axial direction, said connections to each accelerating electrode need not be made in the same manner. Thus, a connection to an accelerating electrode may be made at one side of the median plane, while the corresponding connection to another accelerating electrode is made at the other side of the median plane. In this case a connection may extend from one side of the median plane, on one electrode, to the same side of the median plane on the other electrode, or from one side of the median plane, on one electrode, to the other side of the median plane on the other electrode. These connections are the same in azimuthal respect. Preferably each accelerating electrode is symmetrically connected at its periphery in azimuthally the same manner to two other accelerating electrodes located symmetrically with respect to the first accelerating electrode. This means that either one connection is present at the periphery at the area of the radius dividing the accelerating electrode into two equal parts, or two connections are present at the periphery which are located symmetrically in azimuthal respect relative to said radius. It is thus readily possible for the phase to be correlated to the period of circulation of the particles. In the case of two connections symmetrical relative to the radius, azimuthal currents flow along the peripheries of the parts of the electrodes, which are substantially in the form of sectors of a circle, and these currents can render the voltage in situ as great as in the rest of said parts. This is important especially in cases where the dimensions of the electrodes become approximately a quarter of a wavelength.

Each accelerating electrode comprises two parts substantially in the form of sectors of a circle located one on each side of the median plane. The two parts need not be connected together throughout their peripheries. It is even advantageous if these parts are connected together over a small azimuthal distance only at a limited number of areas since in this case there is a greater possibility for the accelerated particles to emerge from the device and then also at several areas. In this case the connection between the accelerating electrode and another accelerating electrode is usually made at the area of a connection between the two substantially circle-sectorshaped parts.

It is advantageous if the number of couplings to a generator is as small as possible. For this purpose it is primarily desirable that as many accelerating electrodes as possible be connected together directly or indirectly. ThlS is not always the case since in the presence of, for example, six accelerating electrodes, if each accelerating electrode is connected to two other electrodes located symmetrically with respect to the first electrode, a structure may result in which the first, the third and the fifth accelerating electrodes and the second, the fourth and the sixth accelerating electrodes are connected together. Both systems must then be connected to a generator in such manner that the energy in either of them circulates in the desired direction and the mutual phases are correct. If the first accelerating electrode is connected to the second and the sixth, all of the accelerating electrodes belong to one system. If the number of accelerating electrodes is a prime number, each method of connection results in one system. More particularly all of the accelerating electrodes are connected together directly or indirectly and the delay line is coupled to a highfrequency generator at two points which do not diametrically oppose each other. By correct choice of the phases of the generators it is then achieved that the power circulates in the desired direction. If these were coupled at two points which diametrically oppose each other, there would be no certainty that the high-frequency wave on the delay line is a travelling wave since it would become in the first instance a standing wave.

Preferably all of the accelerating electrodes are connected together directly or indirectly and the delay line is coupled at one area to a monophase generator with the use of a directional coupler. In this case it is achieved by the method of coupling that the energy circulates in the desired direction. This results in the advantage that a monophase generator can be used.

The particles are accelerated whenever they pass along the gap between two sequential electrodes, the width of the gap depending upon the voltage used. A device for accelerating particles of the cyclotron type comprising accelerating electrodes each formed by two opposing parts substantially in the form of sectors of a circle located one on each side of the median plane affords in itself the advantage that the azimuthal distance between sequential gaps is shorter as the number of electrodes is larger. This results in better axial focussing with the result that a larger proportion of the particles emerging from the particle source is captured and accelerated. The number of phase steps of the voltages on two sequential accelerating electrodes increases with an increasing number of accelerating electrodes. Since each phase step corresponds to a frequency, the number of frequencies employed increases, it then being advantageous that their ratio may become smaller. Further, the driving frequency of the device generally is a multiple of the beam circulation frequency and hence the micro-structure of the beam is of a greater fineness. The larger the number of electrodes, the higher the energy to which the particles are accelerated during each revolution with the voltage between two sequential electrodes unchanged. The sequential paths of the particles then lies more remote from one another and this is an advantage in extracting the beam and in forming gaps which determine the radial width of the paths. The total number of internal paths is smaller, thus reducing the risk of beam oscillations and resonances. On the other hand, if the number of electrodes is increased, the voltage may be reduced so that the particles during each revolution are still accelerated to the same energy. The loss of power then decreases and this by the second power of the voltage. Due to the lower voltage a higher capacity of the electrode is also permissible so that the distance between the poles of the magnet can be smaller and the beam may be extracted more readily and further the power of the magnet may be reduced, as also the dimensions of the coils.

In practice an upper limit is set to the number of the electrodes. The accelerating electrodes comprise parts substantially in the form of sectors of a circle which parts must not have an acute angle at the center but rather a circular boundary. Thus a central aperture results which is larger as the number of accelerating electrodes is larger. If the particles to be accelerated are provided by a particle source arranged substantially at the center, the central aperture must not be unduly large since the particles would then have to cover an unduly large distance before becoming subjected to the influence of the field of the accelerating electrodes. So in this case no more than, for example, twelve electrodes will be used. If the particles to be accelerated are first accelerated to a given energy in another device and subsequently injected into the device according to the invention which then has a magnetic system of an annular shape; a larger aperture at the center is permissible and the number of electrodes may be, for example, from twenty to thirty. In this way, for example, protons are accelerated to produce masons.

The connections, possibly together with the accelerating electrodes, behave as a delay line in which high-frequency energy can circulate in the same direction or in the opposite direction to the particles to be accelerated and which may behave as a ring resonator at a frequency so that its total electrical length is a whole multiple of the wavelength. Especially for this reason the generator is an amplifier in which part of the circulating high-frequency energy is amplified after being coupled out. The frequency of the generator is thus matched to the requirement imposed for the total electrical length. Preferably the part of the circulating high-frequency energy is coupled out with the use of a directional coupler. If coupling-in is also effected with the use of a directional coupler both the frequency of the generator and the direction in which the energy circulates are determined. If the circulating energy in the ring line is, for example, 100 megawatts, this energy may be maintained, for example, by a supply power of 1001 kilowatts which may be obtained by amplifying a control power of; for example, from 2 to 3 kilowatts possibly derived elsewhere from the ring line.

In the device according to the invention the frequency of the electrical field cannot be matched without particular steps to the increased period of circulation of the particles resulting from the increase in their mass, but the larger the number of the accelerating electrodes, the smaller the necessity for such matching on the ground of the small number of internal traces. The device according to the invention may thus be built up as a classic cyclotron having plane pole-pieces. Further, the cyclotron in the device according" to the invention may be an isochronous cyclotron in which the geometry of the pole pieces is such that the means value of the density of the flux increases in the radial direction in proportion with the increase in mass of the particles so that the period of circulaion of the particles remains constant. If special steps are taken, notably if variable capacitors or movable panels are used, the device may become a frequency-modulated synchrocyclotron.

The invention will now be described with reference to the accompanying drawing, in which:

FIGURE 1 isa plan view of part of the device;

FIGURE 2 shows an azimuthal section of FIGURE 1;

FIGURE 3 shows a radial section along the line III-III of FIGURE 1, and

FIGURE 4 is a plan view of part of another device.

FIGURE 1 is a plan view, partly cut open of part of a device according to the invention in which those elements only are shown which are interesting in this connection. There are nine accelerating electrodes 1, 2, 3, 4, 5, 6, 7, 8 and 9, of which the sector-shaped parts located above the median plane of the device are indicated by 10, 11, 12, 13, 14, 15, 16, 17 and 18. The accelerating electrode 1 is connected to two accelerating electrodes 3 and 8 which are located symmetrically with respect thereto. A connection 19 is made to the electrode 1 at 28 and to the electrode 3 at 29; and a connection 27 is made to the electrode 1 at 30 and to the electrode -8 at 31. The connections 19 and 27 lie symmetrically with respect to electrode 1, since the points of connection 28 and 30 lie symmetrically relative to the radius dividing the substantially circle-sector-shaped part into two equal parts. The connection 27 is made to electrode 1 azimuthally in the same manner as the connection 19 to electrode 3 since the point of connection 30 on electrode 1"has, in azimuthal respect, the same position as the point of connection 29 on electrode 3. The same remark applies to the other connections 20, 21, 22, 23, 24, 25 and 26. The connections may be in the form of rods but preferably are hollow tubes since internal water cooling may then be provided. The assembly operates as a cavity resonator the outer side of which is bounded by a metal wall 32. The electrodes, the connections and the metal wall may be of copper which need have only a small thickness since the currents flow along the surface. The connection 25 is coupled to a directional coupler 33 comprising an inner conductor 34, an outer conductor 35 and a hole 36 (a so called Bethe hole). One end of the directional coupler 33 is provided with a. generator 37 shown diagrammatically and its other end with a matched load 38 shown diagrammatically. Since the directional coupler 33 lies in part inside the vacuum space bounded by a wall 39, lead-through devices 40 and 41 are provided in the wall 39. It is also possible to arrange the directional coupler entirely outside the vacuum chamber and the metal wall forming the boundary of the cavity resonator may then be provided as a coating on the wall of the vacuum space. In this case the hole is provided with a window of insulating material which can maintain the vacuum. With optimum coupling the full power of the generator will be transferred to the delay line and no power will be dissipated in the matched load. Such a directional coupler 33 operates in such manner that the power in connection 25 travels via accelerating electrode 4 to accelerating electrode 6.

FIGURE 2 shows an azimuthal section of the portion located inside the wall 32. The accelerating electrode 1 comprises substantially circle-sector-shaped parts 10 and 50 and at the peripheries thereof vertical parts 51 and 52. Connection 27 is coupled to the vertical part at 30 and connection 19 is coupled to the vertical part 52 at 28. Similar connections are made for the other accelerating electrodes. The connections 19, 20, 21 and 22 between the accelerating electrodes 1 and 3, 3 and 5, 5 and 7, 7 and 9 respectively extend above the median plane of the device. The connections 23, 24, 25 and 26 between the accelerating electrodes 9 and 2, 2 and 4, 4 and 6, 6 and 8 respectively extend below the median plane of the device. The connector 27 between the accelerating electrodes 8 and 1 extends from below the median plane of the device to above this plane.

FIGURE 3 is a cross-sectional view taken along the line of FIGURE 1. The upper and lower substantially circle-sector-shaped parts of accelerating electrode 8 are indicated by 17 and 53 and one of its vertical parts by 55. Accelerating electrode 3 comprises the two substantially circle-sector-shaped parts 12 and 54. Connection 26 is made to the vertical part 55 of accelerating electrode 8. In a particular case the metal wall 32 is provided as a copper layer on the magnet poles (not shown) and on the inner side of the accelerating chamber.

FIGURE 4 is a plan view of a device comprising nine accelerating electrodes in which the feeding takes place, instead of with a directional coupler, by means of three generators. Accelerating electrodes 61, 62, 63, 64, 65, 66, 67, 68 and 69 are connected together in the same manner as in FIGURE 1 by means of connections 70, 71, 72, 73, 74, 75, 76, 77 and 78, the assembly being arranged inside a wall 79 of the accelerating chamber. The connections 75, 72 and 78, which are relatively shifted by are each coupled at the center to a generator shown diagrammatically. Connection 75 is coupled through a capacitorr and a lead-through device 81 in the wall 79 to a generator '82. Connection 72 is coupled through a capacitor 83 and a lead-through device 84 to a generator 85 and connection 78 is coupled through a capacitor 86 and a leadthrough device 8-7 to a generator 88. The phases of the generators 82, '85 and 88 correspond to the phase of the circulating high-frequency energy.

What is claimed is:

1. A device for accelerating particles of the cyclotron type comprising at least three accelerating electrodes having equal aperture angles and each comprising two opposing sections substantially in the form of sectors of a circle located one on each side of a median plane, means connecting each accelerating electrode at the periphery thereof to two other accelerating electrodes located symmetrically relative to an intermediate electrode and constituting therewith a delay line having an electrical length equal to a whole multiple of the wave-length of a highfrequency wave thereon and in which high-frequency energy can circulate in a given direction relative to the particles accelerated, the delay line being coupled to a high-frequency generator at a number of points at most equal to half the number of the accelerating electrodes.

2. A device as claimed in claim 1, in which each accelerating electrode is symmetrically connected at its periphery in azimuthally the same manner to two other accelerating electrodes located symmetrically with respect to the intermediate accelerating electrode.

3. A device as claimed in claim 1 in which all the accelerating electrodes are connected together, and the delay line is coupled to a high-frequency generator at two points which do not diametrically oppose one another.

4. A device as claimed in claim 1 in which all the accelerating electrodes are connected together, and the delay line is coupled to a monophase generator through a directional coupler.

5. A device as claimed in claim 4, in which the generator is an amplifier in which part of the circulating highfrequency energy is amplified after being coupled out.

6. A device as claimed in claim 4, in which part of the circulating high frequency energy is coupled out through a directional coupler.

References Cited UNITED STATES PATENTS 2,531,384 11/1950 Bailey 328234 2,683,216 7/1954 Wideroe 313-62 X ROBERT SEGAL, Primary Examiner.

E. R. LA ROCHE, Assistant Examiner.

US. Cl. X.R.

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