Coherent signals from proton bunches beyond the stationary bunch spectrum

Abstract

The beams in the LHC will be permanently excited by ground motion. Special feedback will reduce the emittance blow-up due to this excitation to acceptable levels. However, coherent excitation will remain at a level much higher than the incoherent Schottky noise. This phenomenon may be exploited advantageously for continuous tune measurements, but real Schottky observations at frequencies within the bunch spectrum are not possible. Real Schottky observation in the LHC is an issue of some importance because Schottky signals carry a wealth of beam information: tune and transverse emittance if the intensity is known. This information is constantly available during beam collisions. It may be an interesting 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 should be taken that the working frequency of such a monitor is chosen below the cut-off frequency of the vacuum chamber in order to be isolated 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 rigid! This causes very large signals at the revolution harmonics reducing the dynamic range considerably. It also destroys the hopes of bunched beam cooling in the Tevatron [2] as it did in the SPS [3]. As pointed out before, it may be interesting to try to understand this phenomenon in view of possible Schottky observations in the LHC, possible beam cooling in the Tevatron and also in HERA [4].