Electron Linac Instabilities,
- G A. Loew
- U. S. National Particle Accelerator Conference,
Introduction The purpose of this paper is to review the progress that has been made in the last six months in understanding and remedying the beam break-up effect (BBU) at SLAC. No attempt will be made to give a comprehensive summary of earlier work on this subject published elsewhere. 1,2,3,4 As described in these references , the BBU effect observed and studied at SLAC is of the multisection cumulative type. The first few cavities of each of the 960 SI2C accelerator sections can sustain a series of beam-induced resonances in the HEMII transverse deflecting mode, of which the lowest, at N 4140 MHz, is predominant (see Fig. 1). The effect of this two-mile chain of “resonant cavities” is that of an amplifier in which the electron bunches and the beam-induced RF wave interact cumulatively as a function of distance and time, resulting in both a growing transverse electron bunch deflection and growing HEMII resonant fields. When the transverse deflection of the electrons becomes equal to the accelerator aperture, the electrons strike the walls and the familiar effect of pulse shortening is observed. For long pulses (0.4 1.6 psec), the effect occurs first in the vertical direction. This is due to the fact that in the vertical plane, perpendicular to the input couplers, the Q of the resonant mode is somewhat greater than in the horizontal direction.