Patent GB-666202-A (GB)
Title: Apparatus for accelerating electrically charged particles
IPC: H01J27/02
Filed: 1946-11-14
Abstract: 666,202. Discharge apparatus. DALLENBACH, W. Nov. 14, 1946 [Oct. 8, 1945; Feb. 15, 1946], Nos. 33823/46 and 33824/46. Class 39 (i). [Also in Group XL (c)] A spiral accelerator for ions or electrons comprises a cavity resonator 1, Fig. 25, energized to oscillate at its fundamental frequency by a source 107 such as a triode oscillator (as shown) arranged in the vicinity of a current antinode, a source 88 for generating the ions and electrons in the vicinity of a potential antinode of the resonator, a plurality of guide tubes 89, 90... 98 and separate magnetic field means associated with each guide tube for focusing and guiding the charged particles therein, the arrangement being such that the electrons successively traverse the guide tubes, being accelerated during the traversal of the gaps between the guide tubes, and finally emerge through opening 93 to impinge on a target. The guide tubes of circular, rectangular or trapezoidal cross-section are staggered in a plane perpendicular to the plane of the paper and pass around spherical cups 86 attached by capacitor plates 85 to re-entrant parts of the resonator wall. The particle source 88 generating positive ions such as deuterons, helium nuclei or protons, is connected to an impulse generator 171 so that particles are only emitted in short bursts at intervals equal to a period of the resonator oscillations. The pulse generator 171 is energized by the resonator field via coupling loop 172. The particle transit time through each guide tube is one half period of the resonator oscillations; due to the appreciable transit time across the first accelerating gap the first guide tube may be so long as to give a total particle transit time of three half periods to avoid a prohibitively large value of magnetic field which would be required to give a total transit time of one half period, Fig. 32 (not shown). If the magnetic fields in all the guide tubes are equal the radii of successive guide tubes are as 1 : #2 : #3 ... &c. Alternatively this ratio may be abandoned and the odd numbered tubes positioned parallel to one another and equal distances apart, Fig. 29 (not shown); in this case the particles are only approximately in phase with the resonator field. The magnetic fields in the guide tubes may progressively increase to compensate for the relativistic increase of mass of the particles. Two separate guide-tube systems may be associated with a single cavity resonator, Fig. 31 (not shown). The magnetic field in each guide tube may be made to decrease dB B outwardly so --/-=¢ where B is magnetic dr r induction and r radius by flaring the magnetic pole-pieces, Fig. 33 (not shown), the conditions for radial and axial stability of the particle orbits being discussed. Alternatively the field may be homogenous in which case the polepieces are not flared. A practical magnetic focusing system for a guide tube of slightly trapezoidal; cross-section is shown in Fig. 36. It comprises two magnetic shells 98, 98<SP>1</SP>, each enclosing a respective coil 101. The magnet systems of the guide tubes are placed side by side. Each coil 101 comprises a main portion carrying most of the flux and a minor portion serving for fine focusing adjustment. The main coil portion may be replaced by a permanent magnet. All the coils of all the guide tubes are connected in series to a common battery. The magnet systems are secured to the annular resonator plates 85, Fig. 38, by non-magnetic brackets 104, pilot pins 177 serving as locating means. For very high particle energies, the device includes multiple guide tubes 204, 205, Fig. 41, i.e. guide tubes through which the particles pass more than once. The time between two consecutive accelerations is a half period of the resonator oscillations or any odd multiple thereof. The two magnets comprise cores 210, 211 of U-shaped cross-section energized by coils 208, 209 respectively, the coils being disposed wholly at the inner and the outer circumference of the guide tubes respectively. Thus the inner circumference of guide tube 205 and the outer circumference of guide tube 204 are left free for the entrance and exit respectively of the particles. The magnetic field in each guide tube may decrease outwardly by flaring the pole-pieces as described in connection with the previous arrangement. In a modification, Figs. 43, 44 (not shown), a multiple guide tube is arranged so that there is only one acceleration of the particles per complete revolution. The time between two consecutive accelerations is a period of the resonator oscillations or any multiple thereof. Practical embodiments for accelerating particles to velocities approaching that of light, and using arrangements of single and/or multiple guide tubes are described with reference to Figs. 45-60 (not shown). The cavity resonator and triode system for energizing it are described with reference to Figs. 1-13. The cavity resonator, Fig. 1, has only one voltage antinode (between capacitor parts 3) and one current antinode namely the equatorial line around the cylindrical shell 1, or it may have two current antinodes. The edges of the capacitor parts 3 are rounded and metal discs 4, 5, 6, each supported by three metal arms 7, 8, 9 are inserted between the capacitor parts, to prevent flashovers. The discs may alternatively be insulated from the resonator wall so that D.C. potentials may be applied to them. The inner resonator walls except the capacitor halves 3 to the boundaries 10 are formed of gold, silver, or copper to a depth of 1.6 times the skin depth. The toroidal space may be circular or square in section. The capacitor parts 3 are of tungsten, molybdenum or chromium or coated with glass, ceramic or titanium dioxide. The insulation layer may have a low ohmic resistance to dissipate surface charges. The vacuum is maintained by an oil diffusion pump 26 backed by an auxiliary vacuum pump 105, a parallel rod grid or slotted sheet 28 maintaining the continuity of the resonator wall at the pump connection. During manufacture the inside walls of the resonator are cleaned, degassed, and then heated to incandescence by a glow discharge to an auxiliary anode introduced therein in an atmosphere of two parts argon and one part hydrogen at 20 mms. pressure. The bottom of the resonator 1 and the cover 2 are reinforced by ribs (not shown). The resonator is energized by a triode oscillator which may be constructed in a number of different manners, Figs. 3-11 (not shown), according to whether the cathode, grid and anode are conductively connected to or capacitatively coupled to the resonator wall. In the preferred form of triode oscillator, Fig. 12, the cathode 40 is connected to loop 38 by a plurality of rods 46 passing through resonator wall 1, the grid 41 to a blocking condenser formed with the resonator wall by metal plate 43 and mica 108, and the anode 42 to loop 39 terminated in a similar blocking condenser 44, 108 by a plurality of rods 46 also passing through the resonator wall. The cathode has a heater coil 109, one end of which is connected to the cathode and the other end of which is connected to insulated heater lead 109<SP>1</SP> enclosed within loop 38. A metallic cap 45 maintains the vacuum. Loops 38, 39 are ribbon shaped, and left-hand limb of loop 38 has a row of openings near resonator wall 1 so as not to disturb the high frequency surface current flow. A positive ion source Fig. 16 (not shown), has a cup-shaped anode and an annular oxide coated thermionic cathode separated by a diaphragm to form two chambers. The gas for supplying the positive particles which may be hydrogen, deuterium or helium is pumped by oil diffusion pump from the cathode chamber to the anode chamber. An arc discharge maintained between the anode and cathode causes part of the gas flowing through an aperture in the anode chamber to be ionized and the ionized particles pass as a beam through the apertured diaphragm, the non-ionized gas being withdrawn by the pump and re-used. A storage flask replenishes the gas as it is used up. The anode-cathode potential may be D.C. pulses to get an intermittent beam or constant D.C. If D.C. pulses are used, they are controlled by the resonator oscillations. A cyclotron arrangement not in accordance with the invention is described, Figs. 19, 24, in which the capacitor plates 3<SP>1</SP> are formed as truncated cones supporting hollow dees. The source of positive particles is mounted in the median plane between the two dees and is supported by arms as is disc 5 in Fig. 1. The gas supply and D.C. leads for the electrodes of the positive ion source are accommodated within the support arms. The finally accelerated particles are withdrawn through tube 79 by means of deflecting plate 155. The dees are provided with separate electromagnets each having pole shoes 81, Fig. 24, with semi-circular faces, two prismatic yokes 83, a circular core 82 and an exciting winding 84. The two electromagnets are mounted on a common baseplate (not shown) upon which the resonator is also supported. One of the magnet cores 82 is eccentrically arranged so that outlet tube 79 is easily accessible, as indicated with reference to the lower magnet in Fig. 19. The magnetic field decreases radially outwardly to obtain axial focusing, and additional focusing is effected by the peripheral magnetic field variation. The Specification as open to inspection under Sect. 91 sets out the invention broadly as comprising arrangements for generating high potentials. The capacitor parts may be of gold, silver or copper coated with glass or ceramic. One of the capacitor halves has a yielding wall portion controlled by set screws arranged in two groups of three for adjustment in two perpendicular directions. The surface of the capacitor part exposed to the interior of the resonator is a cap secured to a support by an elaborate bayonet joint, Figs. 3, 4, 4A (Cancelled, not shown). A similar