Patent GB-1097989-A (GB)
Title: Apparatus for controlling the duration of pulses in a beam of charged particles
IPC: H05H7/00
Filed: 1965-09-27
Abstract: 1,097,989. Linear particle accelerators. VARIAN ASSOCIATES. Sept. 27, 1965 [Sept. 28, 1964], No. 41044/65. Heading H1D. An apparatus for producing pulses of charged particles, e.g. electrons or positrons, of variable length or time duration comprises a pulsed beam source and two successive pairs of deflector plates, the first pair deflecting the beam away from an aperture until de-energized to allow the beam to pass through the aperture and the second pair energized subsequently to deflect the beam from a position in which it passes through the aperture to a position away from the aperture. As shown applied to the injection system of a linear particle accelerator, the two pairs of deflector plates 51, 52 and 61, 62 and aperture electrode 47 are preceded by beam chopper, i.e. deflecting cavity resonator 34 and beam clipping aperture electrode 37 and followed by a pre-buncher cavity resonator 71. The beam chopper cavity resonator 34 may be rectangular, operating in the TE 102  mode, or cylindrical, operating in the TM 110  mode, to produce an oscillating magnetic deflecting field, but a zero electric field at the centre of the cavity. The aperture in each electrode 37, 47 is associated with an energy-distributing slot of V-shaped cross-section for collecting the beam over a large surface area. A magnetic lens 46 is provided between the pairs of deflector plates and to allow for rotation imparted to the beam by the lens the second pair of deflector plates 61, 62 may be rotated by the provision of a rotatable vacuum joint 57. Magnetic lenses are also provided at 36, 73 and 78 and beam steering rods 42 energized by coils 43 are provided around the aperture in anode 33. Anode 33 and aperture electrode 37 are provided with water-cooling channels 39, 41. The deflecting plates 51, 61 are held at high positive potentials but separately de-energized by thyratrons 55, 65 to control the pulse length of the beam. The opposing plates 52, 62 are earthed, a magnetic biasing field being provided in the region of the second pair of deflector plates 61, 62 by means of coils 67 in order to counter-balance the effect of the electric field in the situation where the second pair of deflector plates does not deflect the beam. In an alternative embodiment, Fig. 6 (not shown), beam-induced fields in the prebunching cavity resonator 71 due to the passage through it of clipped beam pulses are avoided by having only one beam clipping aperture electrode 47 and by positioning the pre-bunching cavity resonator 71 between the deflector plates 61, 62 and the aperture electrode 47. The pre-bunching cavity resonator is cylindrical, operating in the TM 010  mode, and has beam input and output slots in its end walls. In other modifications: the counterbalancing magnetic field provided by coils 67 may be omitted and both plates 61, 62 maintained at the same positive potential until one plate is discharged to earth to deflect the beam; the chopping cavity resonator may be placed downstream of the pre-bunching cavity resonator; a pair of chopping cavity resonators orientated at right angles or 90 degrees out of phase with one another may be provided to provide monitoring beam signals or an indication of pulse length.