Positron jitter and wakefield effects in the SLC injector linac

摘要

The positron beam in the SLC injector linac is a high current (7*10(sup 10) particles/bunch), large universe emittance ((gamma)(var epsilon) = .01 m-rad) and long bunch length ((approximately)4 mm) beam. A large 5% positron intensity jitter was observed and correlated with the accelerating phase of the RF cavities in the positron source linac. For high transmission, the positron jitter must be reduced and strong wakefield effects cannot be ignored. A code was written to study causes of the positron jitter and wakefields in the SLC injector linac. The tracking results show that when the bunch lengths are 1.5, 2.1, 3.0, 4.0 mm, the injection apertures (leading to 30% loss) are 1.8, 1,6, 1.2, 1.0 sigma of transverse size at the beginning of the sector respectively. For the long bunches, the nominal 20% of beam size transverse pulse to pulse jitter causes an additional 3% loss. Also the bunch energy spread is more sensitive to the accelerating phase of the RF cavities.