Next Meeting
Agenda for Next Meeting
This Week's Attendees
J. Bennett, S. Benson, G. Biallas, C. Bohn, J. Boyce, I. Campisi, D. Douglas, K. Jordan, R. Li, L. Merminga, G. Neil, P. Piot, T. Siggins, B. Yunn
Closed Action Items
New Action Items
Old Action Items
Items of Discussion
T. Siggins briefly reported the status of the gun. It has been processed without cesium to a quiet 440 kV, and the processing is therefore deemed to be complete. The new cathode electrode behaves much differently from the old one. Specifically, field emission stayed quiet up to 370 kV, whereas the onset of field emission with the old ball was much lower, ~200 kV. Apparently the old ball has a persistent field-emission site that degraded with time. The HVPS still needs to be checked thoroughly. As it stands now, the current output seems to fluctuate inexplicably, and the HV stack has yet to be carefully inspected for damage. K. Jordan has the work planned and coordinated, and it will start today.
G. Neil proposed that we plan to begin the next run sometime next week. C. Bohn said that if progress is rapid, then we'll do it. However, hot checks remain (e.g., gun with cesium, the MPS, rf systems, the HVPS), most of which must be done next week. The optimistic plan for hot checkouts is as follows:
Today's meeting focused on discussions of coherent synchrotron radiation (CSR). R. Li gave a very nice presentation of her simulation results to date, including how they contrast with P. Piot's measurements of emittance growth around the first recirculation arc. Li's approach is as follows:
One result of the theoretical and numerical studies is that the CSR-induced longitudinal wakefield dominates the beam dynamics. The transverse wakefield is negligibly small and can be ignored, thereby aiding the run time. Basically what happens is that the longitudinal wakefield induces and energy spread during the arc which spoils the achromaticity and gradually induces an emittance growth as the beam traverses the arc.
Li's simulations nominally predict small emittance growths ( 7% or less) with bunch charges in the range 20-50 pC, which is a puzzle given Piot's measurements of up to 80% growth. To try to resolve the puzzle, Li looked at several aspects of the problem. First, she noted that M_26 is antisymmetric through the arc, but M_16 is not. Consequently, the effect of CSR-induced energy spread on x' must be small, but the effect on x can be substantial. If the phase space at the entrance to the arc is rotated to make the horizontal beam size smaller, then the influence of CSR on the emittance (through x) will be more pronounced. The CSR-induced energy spread is identical, but it's influence is enhanced. Li can "see" 20% emittance growth at 60 pC in her simulations by rotating the phase space in the manner described. She also noted that the measured Twiss parameters change (modestly) as the cryomodule gang phase changes, and so follow- on runs should include this dependence. In addition, fine structure (meaning gross deviations from gaussian density) may influence the emittance growth; it is thought based on analytic calculations that the effect will be small, but it still needs to be checked via simulation.
Implications for the upcoming experimental program are as follows:
Li will continue her simulations per the focus delineated above. Bohn, Li, Douglas, and Piot likewise will continue to evolve and iterate the overall CSR program as new information becomes available.
New Items
None.