GB1268714A - Magnetic lens - Google Patents

Abstract

1,268,714. Discharge apparatus. BROWN, BOVERI & CO. Ltd. 25 June, 1969 [27 June, 1968], No. 32112/69. Heading HID. A magnetic lens for an electron beam, e.g. in a linear accelerator, betatron or microtron, comprises a toroidal coil 1 surrounding a toroidal chamber coaxial with the lens axis, and which is formed of a number of turns for connection to a current source to produce in the chamber an axially symmetric, azimuthally orientated magnetic field to focus outer electrons of the beam passing through the chamber. Adjacent turns of the coil may be separated only by an electrically insulating compound and the coil may be a ribbon-like aluminium conductor supported in a housing 2. The frusto-conical walls 3 of the trapezoidal cross-section toroidal chamber through which the peripheral electrons 18 pass are made thinner than the remaining parts 4, 5 of the coil. Electrons within the toroidal chamber are deflected proportionally to the magnetic field strength and path length within the chamber; thus since the field strength is inversely proportional to the distance from lens axis the path length should decrease with a square law relationship to the decreasing distance from the lens axis, to obtain precise focusing. These conditions are approximated by the arrangement shown. When used in conjunction with accelerators which produce electron beams in pulses preferably the magnetic lens is energized only when electron beams are present, Fig. 3 (not shown). Two such individually adjustable magnetic lenses may be used in series in a linear accelerator delivering electron beams with small divergence angles, the first being employed as a dispersion lens while the second lens, disposed on the object to be irradiated, focuses the beam. The coil may be cooled by blowing air into an annular duct between the toroid coil and the housing wall and into which cooling fins extend.