Transverse beam break-up study of SNS SC linac

Numerical simulation indicates that cumulative beam breakup (BBU) instability is not a concern to the SNS SC linac. First, simulation is carried out for the CW operation mode where the driving harmonics are those with frequency multiples of bunch frequency 402.5 MHz. Even when the median HOM frequency is exactly on resonance with multiples of bunch frequency of 402.5 MHz, the cavity-to-cavity HOM frequency spread can ensure operation of the linac. Second, in the case of the pulsed operation mode, additional driving harmonics of 1 MHz and 60 Hz are added on top of those of the CW mode. The shunt impedance of these additional modes is relatively small. BBU is not a concern also for pulsed mode operation, as is verified for a few most dangerous modes. More systematic analysis of BBU of pulsed mode operation is done by Sundelin et al. and presented at this conference.