Pre-Compression of Bunch Length

After extraction, the damping ring (DR) bunch length is eom@essed in the RTL (Ring-To-Linac) transport line to achieve the desired linac bunch length. For this, an energy spread is induced proportional to the DR bunch length, producing large, aperture filling, horizontal beam sizes in the dispersive regions of the RTL. There, nonlinear magnetic fields and wakefields cause higher order dispersion at the beginning of the linac, which increases the effective beam emittance. More important, in the presence of DR extraction jitter this causes linac launch jitter which makes linac operation unstable at high currents. A pm-compression of the DR bunch length would reduce the horizontal beam size in the RTL and thus relieve the problems mentioned above. It can be achieved by a modulation of the damping ring RF amplitude with a socalled ‘bunch muncher’ shortly before extraction. A principle limitation is the inevitable increase in energy spread of the extracted bunch which sets the lower limit for the achievable bunch length in the linac. For SLC parameters the possible reduction in DR bunch length is about 35%. The routinely operation of the ‘bunch muncher’ consists of two .‘munches’, cancelling the effect of the beam loading and keeping the extraction phase unchanged.