COHERENT SIGNALS FROM PROTON BUNCHES BEYOND THE STATIONARY BUNCH SPECTRUM

The beams in the LHC will be permanently excited by ground motion. Spe cial feedback will reduce the emittance blow-up due to this excitation to acceptable levels. However, coherent excitation will remain at a l evel much higher than the incoherent Schottky noise. This phenomenon m ay be exploited advantageously for continuous tune measurements, but r eal Schottky observations at frequencies within the bunch spectrum are not possible. Real Schottky observation in the LHC is an issue of som e importance because Schottky signals carry a wealth of beam informati on: tune and transverse emittance if the intensity is known. This info rmation is constantly available during beam collisions. it may be an i nteresting and cheap complement of the more complex synchrotron light transverse profile measurement. Note that the use of wire scanners is excluded. Real Schottky signals can be observed at frequencies beyond the bunch spectrum on the proviso that the bunches are rigid. Care sho uld be taken that the working frequency of such a monitor is chosen be low the cut-off frequency of the vacuum chamber in order to be isolate d from all kinds of possible signal pollution. A monitor based on this principle has been installed in the Tevatron [1]. It behaves exactly as expected but it came as a bad surprise that the bunches are not rig id! This causes very large signals at the revolution harmonics reducin g the dynamic range considerably. It also destroys the hopes of bunche d beam cooling in the Tevatron [2] as it did in the SPS [3]. As pointe d out before, it may be interesting to try to understand this phenomen on in view of possible Schottky observations in the LHC, possible beam cooling in the Tevatron and also in HERA [4].