Beam loading compensation for optimal bunch lengthening with harmonic cavities

Emittance growth and short beam lifetime due to intra-beam scattering in extremely low emittance storage rings can be mitigated if harmonic RF cavities are used for the purpose of lengthening the longitudinal bunch size. The “flat potential condition” can be achieved with harmonic cavities, lengthening the bunch by a factor of ~5. However, the performance is limited by the transient beam-loading effect in the RF cavities, which is induced by gaps in the ring fill pattern. As a counter-measure of this effect, we have proposed the active compensation techniques by using a single kicker cavity having a bandwidth of several times the revolution frequency. The transient effect can be compensated when the kicker cavity voltage is determined to suppress bunch phase shifts along the train.