Wakefields of Very Short Bunches in an Accelerating Cavity *

The behavior of the longitudinal and transverse wakefields of short bunches in an -accelerating cavity has been examined. Computations of short time wakefields have been carried out using. T. Weiland’s computer program TBCI,l which integrates Maxwell’s Equations in the time domain. We present also our version of the diffraction model of the high frequency impedance of the cavity, a model originally suggested by Lawson. 2 This model predicts a longitudinal impedance that varies as wm112 and a transverse impedance that varies as wm3i2 at high frequencies, with their ratio equal to wa2/(2c), where a is the radius of the beam tube. We find that as shorter bunches are used, the computations approach asymptotically the predictions of the diffraction model. These asymptotic wakes l -are also in accord with calculations of Dome3 and of Heifets and Kheifets.4 * work supported by the Department of Energy, contract DE AC03 76SF00515. Presented at the Workshop on Impedances Beyond Cut-off, Berkeley, Ca., August 18-21, 1987 TABLE OF CONTENTS Page