Next Meeting
Agenda for Next Meeting
Item Principal Time ---- ------------ -------- Status of Open Actions Bohn 5 min Status of Gun Siggins 5 min Summary of New Hardware Walker 10 min Summary of New Software Grippo 10 min Summary of Updates to Metaprocedure Douglas 10 min Summary of Updates to Injector Setup Procedures Biallas 10 min Commissioning Schedule Bohn 5 min New Issues All 5 min
This Week's Attendees
J. Bennett, S. Benson, C. Bohn, J. Boyce, R. Campisi, D. Douglas, J. Fugitt, K. Jordan, R. Li, G. Neil, D. Oepts, J. Preble, C. Rode, M. Shinn, R. Walker, B. Yunn
Closed Action Items
New Action Items
Old Action Items
Items of Discussion
C. Bohn began by going over his best guess of the two-week schedule. He has scheduled activities to relieve stress on the installation/maintenance activities while providing for restoration of the "Dec 98" machine by 29 Jan 99. The vault will be open all next week to permit installation/maintenance, gun processing will proceed early the following week, and injector/accelerator/FEL restoration will proceed late the following week. The schedule is only a projection; it will be modified, if need be, as turn-on gets closer.
R. Campisi summarized data and plans for circumventing the waveguide IR detector faults that surfaced in December, which are very likely caused by higher-order modes. Bottom line: A copper screen of ~1 mm mesh size removes all measurable rf response at 19.5 GHz and transmits ~60% of the IR. Screens are being installed over all eight of the detectors in the cryomodule, and three gate valves are also being installed to permit testing alternative solutions and measurement methods with rapid turn-around. The trip points will need to be reset during the time-frame of system checks (likely early the week after next), and srf people need to be on hand to monitor detector response as the beam current is raised (likely late that week). J. Preble will coordinate the necessary srf staffing.
R. Li summarized the latest results from her coherent-synchrotron-radiation simulations. She finds that the emittance growth through the first recirculation arc is sensitive to the energy spread at the wiggler location (which corresponds to the starting point of her runs). For an assumed gaussian longitudinal bunch profile with rms parameters previously measured at the wiggler, she predicts a 34% CSR-induced emittance growth from wiggler to exit of the first arc. She has benchmarked her code against analytic calculations pertaining to the longitudinal CSR force, and is presently doing likewise for the radial CSR force (the latter being more difficult analytically than the former). She will also do runs to check for sensitivity of emittance growth to longitudinal bunch profile to provide guidance on the steps needed for a careful empirical study.
M. Shinn advised that the drive laser is now putting out 33% more power than previously (better than 6 W SHG). She thinks the lamps in our reserve supply are degrading for reasons unknown (traceable to changes in the constituency of their gas?), and she will be contacting the vendor for more information.
S. Benson followed with a discussion of fundamental limitations on pushing the FEL power to higher levels. The essential concern is ultimately reaching an rf instability in the energy-recovery mode that cannot appear in the straight-ahead mode. We have found empirically that the power output and the gain are extremely and nonlinearly sensitive to the optical-cavity length in the vicinity of saturation (which occurs at ~311 W in the straight-ahead mode), a property that will adversely drive the rf system. This means it is possible to excite an rf instability whereby the FEL rapidly switches on and off, i.e., with a frequency ~10 kHz. By contrast, as one backs the optical-cavity length away from saturation (to, e.g., ~210 W in the straight-ahead mode), both the gain and power output become much less sensitive to cavity length and the rf instability will not appear. The bottom line from Benson's analysis is that with ~5 mA beam we should be able to get close to 1 kW (nominally 900 W) with the 5 µm optics presently installed, with some hope of pushing to the full kW, without getting too close to saturation and exciting the rf instability.
After the meeting, Benson sent Bohn the following addendum: "The prediction of 1 kW with 5 mA assumes that there are no mirror limitations. The laser is less sensitive to mirror distortion if we operate in the low-slope region where the small signal gain is high so we may get lucky, especially if we have one silicon mirror. If we are close to 1 kW, and the mirrors are not limiting the performance, we can try to increase the power by:
J. Boyce made a few remarks concerning the FEL ARR. Basically he pointed out that we cannot formally close out the ARR until we are ready to do routine user experiments, for which we are presently unfunded. Presently we are exclusively doing commissioning tasks, ultimately to include test experiments, for which we are following established EH&S policies and procedures. Today Boyce is sending our DOE site office a memo to that effect, namely that lack of operating funds prevents us from reaching the state of being able to support routine user experiments.
New Issues
None.