Brief summary of coffee discussion on crab cavity, 21.09.2006

Participating: Rama Calaga, Fritz Caspers, Rogelio Tomas, Joachim Tuckmantel, Frank Zimmermann

Fritz pointed out that the phase noise of the beam is many orders of magnitude larger than the noise of the cavity. In his opinion, it does not make any sense to ask for cavity phase stability much better than the beam.

We can profit from the fact that only the relative noise of two cavities on either side of the IP matters. With an approximate distance of 100 m, the integration of low-frequency noise should give only a small relative timing jitter. Two cavities on either side should be powered from the same transmitter to cancel the noise between sides.

Fritz drew some analogies to stochastic cooling, and concluded that the calculated tolerances are too tight. In any case they are an order of magnitude tighter than the best measurement apparatus available could provide (the state of the art in measurements is a 20-fs phase jitter).

The theoretical formulae for emittance growth should be calibrated against simulations or experiments. Frank will ask Kazuhito whether he can run such simulations. In particular, it seems off that even with a perfect feedback large emittance growth persists.

Another effect to be looked at is the tolerance to transverse beam offsets and beam loading (does it cancel between the two sides of the IP?).