Patent GB-1143200-A (GB)
Title: Apparatus for shortening electron pulses emitted from an electron gun
IPC: H05H7/08
Filed: 1966-08-03
Abstract: 1,143,200. Pulse circuits. CSF-COMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. 3 Aug., 1966 [3 Sept., 1965], No. 34771/66. Heading H3P. [Also in Division H1] A short burst of electrons is produced by passing a pulse of electrons successively through a first and a second waveguide, the first of which is energized to bunch the pulse into a sequency of electron bunches distributed along the pulse length, and the second of which is energized to alter the velocities of the bunches so that the trailing bunches catch up and merge with the leading bunches by the time they reach the end of the second waveguide, to provide an electron burst of shorter duration than the original pulse. In a first embodiment, Fig. 1, a pulse of electrons of duration T from an electron gun 1 is shortened by passing it through a slow wavestructure 4 to which a signal of frequency f 1  is applied, of sufficient intensity to produce bunching by trapping at a fixed phase of the wave, and passing the resultant sequence of bunches through a waveguide 6 excited at a frequency f 2 , and of insufficient intensity to produce phase trapping. A phase shifter 9 is adjusted to ensure that the central bunch of the sequence enters the waveguide when the signal f 2  is at maximum accelerating amplitude. Since f 2  is chosen slightly less than f 1 , the phase of the signal f 2  at the times of arrival of successive electron bunches varies such that the initial bunch lags its corresponding f 2  intensity maximum, while the final pulse leads its correspondingf 2  maximum. Since, in the waveguide, the signal f 2  is propagated faster than the electron bunches, and the length of the waveguide is chosen suitably, successive bunches of the sequence are accelerated by increasing amounts while passing through the waveguide, causing them to close up upon one another and thereby compressing the sequence into a time interval shorter than T. When employed as the source for an electron linear accelerator (Fig. 3, not shown), the compressed sequence is passed in succession through a conventional buncher device (10), on accelerator (11), and a regrouper device (12) of the type disclosed in Specification 1,054,237. to provide a final high energy output burst of shortened duration. In a second embodiment, Fig. 4, the electron pulses from the gun 1 enter a bunching section 14 prolonged by an accelerating section 17, the section 14 being fed by a frequency f 4  sinusoidally amplitude modulated at a frequency F locked to the gun pulse repetition frequency. The length of the section 14 corresponds to Vg/2F, where Vg is the group velocity of the modulated wave in the buncher section. and the relative phase of the signals are so arranged that the first part of an electron pulse arrives at the buncher during the amplitude distribution A, Fig. 5, and is therefore comparatively weakly accelerated, and groups itself at /2 from the wave peak, while the second part arrives during the distribution B, is accelerated more strongly, and groups itself at the wave peak. In the accelerating section 17 only the second group has a sufficiently high velocity to be phase trapped by the propagating wave, whereby the second group closes up upon the first group to provide an output burst of a duration shorter than that of the original pulse from the gun.