Next Meeting
Agenda for Next Meeting
Item Person Responsible Time ---- ------------------ -------- Status of Open Action Items All 5 min Status of Gun Legg 5 min PARMELA Beam at Multislits Piot 20 min Status of Commissioning Preparations Bohn 10 min Scheduling Bohn 10 min New Issues All 5 min Agenda for next week All 5 min
This Week's Attendees
J. Bennett, S. Benson, G. Biallas, C. Bohn, J. Boyce, D. Douglas, F. Dylla, D. Engwall, J. Fugitt, A. Hofler, K. Jordan, G. Krafft, R. Legg, R. Li, L. Merminga, D. Oepts, P. Piot, J. Preble, M. Shinn, T. Siggins, R. Walker, B. Yunn
Closed Action Items
Items of Discussion
F. Dylla indicated that the Navy Program Office was pleased with their review of our FEL program held this week, during which we presented a candid status report on FEL commissioning.
R. Legg summarized the status of gun refurbishment. T. Siggins continues to work on polishing the ball. Per C. Sinclair's recommendation, G. Biallas designed a stainless steel shield for the getter pumps and is having it machined. The shield will protect the pumps and hopefully reduce carbon impurities in the high-voltage region. Considerable discussion ensued about alignment tolerances in the gun. Evidence is that the cathode was tilted about 2 mrad in our last run. A decision whether to straighten the tilt involves trading the benefit of easier drive-laser alignment given the design of the light box against the cost of risking damage to the braze and/or weld on the stalk in the process of straightening it. Alignment tolerances have not yet been measured. Accordingly, a "gun alignment plan" was formulated with the trades in mind:
B. Yunn presented his optimized PARMELA settings for 60 pC and 135 pC bunch charges. For first light, we will adopt his optimized 60 pC settings as the baseline. We will defer the 135 pC settings until such time that we embark on kW-level lasing with energy recovery. Yunn's settings generate a 40 MeV beam, whereas a 38 MeV beam is desired for first light. He needs to rework his numbers accordingly and then pass them to R. Legg for insertion into a special save/restore file. Yunn also showed images at the various viewer locations. All but two of the viewers are in low-dispersion regions, so the beam should be round at these locations. In these cases, if the beam were observed not to be round, then something would be wrong. [Note: After the meeting, R. Li pointed out that the 60 pC settings may be just as useful, if not more useful, than the 135 pC settings for CSR experiments. The emittance growth should scale roughly as (bunch charge)/(bunch length)^(4/3). The bunch charge is slightly less than half, but Yunn's bunch lengths were also roughly halved in the wiggler region compared to the 135 pC case. She will check further.
C. Bohn followed with a strawman plan for configuration control. As hoped and anticipated, profuse discussion ensued. His underlying assumption is that, based on the early February finding of round beam after the cryounit, we should be able to proceed systematically through machine setup. He also considers that, because they are organized activities involving detailed test plans, parametric studies do not constitute "tweaking". Instead, they check for and ensure an optimal setup. The point is not to do parametric studies per se, but rather to obtain the desired electron-beam parameters at the wiggler. Once that is done, parametric studies will cease, and we will proceed directly to lasing.
Bohn's strawman plan, slightly modified per the discussion, is as follows:
Post baseline settings in special save/restore (S/R) files, guarded by FEL CC
Do parametric studies of solenoids, buncher phase, and cryounit with multislit #1
Do parametric studies of cryomodule and quadrupole triplets prior to wiggler with quad/viewer
Verify reproducibility and robustness of cw (1.1 mA) beam quality at wiggler location The basic idea is to input an initial save/restore file with optimal 60 pC machine settings based on PARMELA, check resulting beam properties along the machine, and pursue parametric studies, if necessary, starting in the injector. Possible complications are inherent do-loops if verification of beam parameters prove unsuccessful, and uncertainties in the mechanics of FEL CC's guarding and updating the S/R files. We might install alarms in the master S/R file once we are lasing, but not before. Technical details are also uncertain, but it seems that (roughly speaking), since space-charge is important mostly only in the injector, once the injector setup is accomplished, the rest of the machine will follow easily. Bohn asked people to be thinking about contingencies (test plans) in case there are difficulties during setup. We already have a good start for the injector courtesy of R. Legg. Bohn will solicit advice from A. Hutton concerning configuration control.
New Issues
None.
New Action Items
Old Action Items
Procedures in Work (will update status at next meeting)
Emittance Growth from CSR