Creation of Hollow Bunches by Redistribution of Phase Space Surfaces

The creation of hollow bunches in longitudinal phase space, in order to decrease the peak current and hence the transverse direct space charge forces, is an old idea. A new method to create such a hollow distribution at high energy has been simulated and tested experimentally at the PS Booster synchrotron. It is based on are distribution of surfaces in the longitudinal phase space by using a double harmonic RF system. During the process, the beam is transferred from one second harmonic sub-bucket to an other. Low density phase space surfaces from the periphery and high density regions from the centre are exchanged, leading to flat profiles, even after switching off the second harmonic RF system. During the process, the peak current is temporarily increased, which makes it suitable only to improve the situation in a receiving machine (in our case the PS) after transfer. In practice, the set-up of this new scheme turned out to be fast and simple and to yield reliable and reproducible results