A study of fast bunch rotation in the negative mass region

Fast bunch rotation of high-intensity proton or ion bunches above transition is-in principle-supported by the self-bunching effect of the attractive space charge force ("negative mass instability"). Due to the broad-band nature of the space charge impedance, the highest harmonics of this negative mass mode grow fast and inhibit compression, unless the bunch rotation is accelerated by a sufficiently high rf-voltage. Using a particle-in-cell simulation we establish the threshold below which an effective compression is still possible. We find that the required rf-voltage for the compression of a given bunch above transition can be reduced at most by a factor of 2 compared with a compression below transition, where the space charge requires an extra voltage