Bolko Beutner: Resistive Wall and Surface Roughness Wakefield Studies for the Bunch Compressor Vacuum Chamber Room: WBGB 020 Selected questions and discussion: AAn (on p.5 ): Is the field we see behind the bunch really generated by wall roughness only? BB: Yes! RI: Where does the regularity come from? BB: Don't know. Perhaps only the main modes above some cutoff are shown (MP). FLP (on p.6-8): Are these formulae also valid for ellipses/rectangles? BB: No, they are only for a circular beampipe, which is the worst case (highest losses). MP (on p.7 ): What's the difference between capital and small J? BB: The small j are the roots of the Bessel function J. FLP: How do these formulae compare with the ones given by Karl Bane and Roger Jones? BB: I only did a rough comparison, the formulae I use are valid in a larger frequency domain. FLP (on p.9) "Ar" should be Ag (silver). FLP (on p.10/14): The effects from the two contributions do not seem to add up? BB: They are not additive, but have to be combined using the formulae on p.6 MP: The effect on the energy spread is probably negligible (micro-bunching). RI (on p.15): In the top graph at 1 mm, the wake function is almost the same, why then is the energy spread so large? BB: I am not quite sure about the first two points, they need to be checked! AO: Are these wake effects included in the ELEGANT code? BB: I do not know. FLP points out that the beam pipe corrugation can act as a grating emitting light back to the bunch (THz radiation). But the fact that it is a random pattern is to our favor. BB: You can modify the formalism to arbitrary modulation, not just sinusoidal, as long as they are periodic. MP: How do you evaluate the effect from the entire bunch compressor? BB: It is a very simple estimation: I compute the wake function, then assume it is the same all along the bunch compressor and just add up the effect. I further assume maximal dispersion. Clearly an overstimation that should be checked with a tracking code! FLP points out that the wall impedance is quite similar for Al and for Cu. But machining of Al is much easier than Cu! BB: It looks like the dominant effect is due to the surface roughness rather than the wall impedance. TG: Vacuum requirements will also play a role when choosing a material! FLP: Why is our beam pipe rectangular? AO: The beam grows transversally. FLP: Then why not elliptical? MP: That would be too complicated to manufacture.

Terence Garvey: Remarks concerning efforts in XFEL simulations Room: WBGB 020 In the new XFEL organigramm, TG is in charge of coordinating beam dynamics simulation. While the FELSI meeting is a good forum to exchange information, we need a strategy meeting in addition. The purpose of that meeting would be to ensure that the work that needs to be done is done, and is done only once, given our limited manpower. The first of these strategy meetings will take place this Thursday, September 4, at 10:30. The work should be focused towards the CDR due in April 2009. AO: Will there be presentations or just discussions? TG: Primarily discussions on how we organize the work; we have the FELSI meetings to present the actual work.