| abstract |
1,270,619. Particle accelerators; ion sources. CYCLOTRON CORP. 28 July, 1970 [20 Jan., 1970], No. 36588/70. Heading H1D. In a method of producing a beam of high energy negative ions, a plurality of randomly charged ions is generated outside the magnetic field of a cyclotron; the negative ions are separated out and migrated, in a direction parallel to the magnetic field, to the centre of the cyclotron, during which migration they are velocity bunched in phase relationship with the cyclotron radio frequency field; and the velocity bunched migrating ions are deflected into the median plane at the centre of the cyclotron, to be accelerated therein to the extraction radius, where they are electrostatically deflected from orbit into a linear beam. The output beam may subsequently be linearly accelerated e.g., by a Von de Graaff accelerator, stripped of its negative charge by a foil stripper, and finally further linearly accelerated to provide a high energy beam of positive ions. Ion source.-The negative ion source 40, Fig. 10, and Figs. 6 and 8, which is held at minus 12,000 volts and is mounted above the cyclotron, is formed by an arc chamber 46 which is cooled by coolant tube 56 and is supported at one end by a cathode support tube 42 and at the other end, which has a restrictive arc defining orifice 47, by a filament support tube 41, hydrogen being supplied to the arc chamber by the central conduit 48. The filament 52 is supported between copper tubes 49a and 50a, which also act as leads, and which house inner coolant tubes 49b. and 50b; cathode insert 55 is mounted on copper tube 53a which maintains it at cathode potential, and is cooled by coolant tube 53b. Negative hydrogen ions are produced in the arc chamber by collision of electrons from the filament with neutral hydrogen molecules, and are extracted from the resulting plasma column through a slit 60 by an earthed puller electrode assembly 61, Fig. 10, formed by a carbon block 62, on arm 64 which is adjustable to permit accurate alignment with the slit 60, and a conductive shield 66. The ion source is mounted between the poles (one only, 32b, shown in Fig. 10) of an electromagnet and supports a graphite electron dump block 59, the pole 32b adjacent the smaller end of block 59 being faced with an earthed copper electron discharge plate 67 which is cooled by a coolant tube 68. Cyclotron beam injection arrangement.-The issuing ions are deflected by the electromagnet into a linear tube system 72, 83, 85 containing a ball valve 73 operable from outside the vacuum system to permit the ion source and cyclotron evacuated regions to be separated from one another; a buncher formed of two pairs of grids 94a, 94b and 95a, 95b supplied with r.f. synchronous with the cyclotron supply; and two pairs of quadrupole focusing lenses (Fig. 12, not shown). The focused beam is introduced axially into the cyclotron through a plug 87, Fig. 17, and is deflected into the median plane by an assembly consisting of a positive electrode 109 and an earthed tube 112 apertured at 114, Fig. 16, and having a grid 113 which, like the surface of electrode 109, is inclined at an angle of 42À5 degrees to the horizontal, the complete assembly being eccentrically disposed with respect to the cyclotron axis. Unwanted r.f. perturbation of the initial orbits of the ions is prevented by the earthed shield 115 and the dee shields 116 and 117, all these shields having an increasing radius of curvature, and shields 116 and 117 being provided with focusing grids 120 and 121. The cyclotron (Fig. 2, not shown) is isochronous, and has a pair of 120 degree dees and an azimuthally varying magnetic field provided by shaped iron pieces (18, Fig. 18, not shown) mounted on the pole tips, and water-cooled electromagnets. The beam is extracted through a channel of increasing radius of curvature defined by a positively charged deflection bar and an earthed tungsten septum, the mounting of which assembly on the cyclotron is described in detail (Figs. 18, 19, 20 and 21, not shown), and may thereafter be passed through a focusing magnetic channel of the type disclosed in Specification 1,238,583, before entering a Van de Graaff accelerator in which the ions are successively accelerated in a first stage, stripped of their negative charge to produce positive ions, e.g., protons, in a foil stripper, and then further accelerated in a second stage. |