Patent GB-959674-A (GB)
Title: Focusing apparatus for beams of charged particles
IPC: H01J23/087
Filed: 1960-08-11
Abstract: 959,674. Particle accelerators; travelling-wave tubes. VARIAN ASSOCIATES. Aug. 11, 1960 [Aug. 19, 1959], No. 27878/60. Heading H1D. Apparatus for decreasing the cross-sectional area of a beam of charged particles by a factor of at least 10 over a length of beam travel in excess of one beam-focusing wavelength, comprises means for subjecting the particles of the beam to a focusing field whose intensity increases progressively along the length of the beam, resulting in a progressive decrease in the beam-focusing wavelength and in the crosssectional area of the beam. Focusing may be magnetic or electrostatic and may be transverse and/or longitudinal space periodic. The focusing force is increased throughout the length of section 8, Fig. 1, by giving the focusing electrodes progressively increasing potentials but constant transverse spacing (pairs of annuli), or decreasing transverse electrode spacing or polepiece spacing. The length of the magnetic lenses in conical section 8 is inversely proportional to the square root of the current density of the electron beam and the longitudinal gap spacing of the magnetic or electrostatic focusing elements is progressively decreased. A further focusing section 9 is provided at the interaction region of the tube. In another form, Fig. 8 (not shown), a conical iron yoke surrounds conical coils and has partitions, the spacing of which decreases along the electron beam. The magnetic field intensity is made to increase along the beam by decreasing the transverse extent of the partition gaps or by progressively increasing the focusing potential. In another form, Fig. 9 (not shown), the longitudinal spacing, thickness and aperture of a series of focusing electrodes decrease along the beam and the average potential increases. In another form, Fig. 11 (not shown), a bifilar helix is used, the radius and spacing of the turns progressively decreasing. In another form, Fig. 12 (not shown), a tapered solenoid of tapering cross-section is used, the field increasing progressively along the axis. With a sheet electron beam, Fig. 13, passes through a plurality of pairs of polepieces 45 or electrodes, Fig. 15 (not shown). In a slalom focusing device, Fig. 16, there are a plurality of pairs of curved deflecting electrodes 52. The length and transverse spacing of the electrodes decreases along the electron beam. In a modification, there are two flared outer electrodes and an inner series of wires whose diameter and spacing decreases along the beam. For annular beams disc and ring electrodes or polepieces are used, Figs. 18, 19 (not shown). Accelerating potentials can be applied to the focusing devices. The tube of Fig. 1 is stated to be a velocity modulated tube the electron beam from annular indirectly heated cathode 1 passing through unloaded coaxial line 13 fed by waveguide 11 and transducer 12 and the outlet being fed by a similar transducer 14 to coaxial line 15; 16 are alumina ceramic windows. Members 18 give cut-off. The invention is also applicable to linear accelerators. Specification 945,610 is referred to.