NOTE: C. Bohn will be on vacation from 14-24 Jul 98; D. Gilchrist will have phone numbers by which he can be reached during that time. J. Fugitt is on vacation from 8-19 Jul 98. Therefore, G. Neil will serve as FEL Commissioning Coordinator from 14-19 Jul 98. Please treat him accordingly. Fugitt will serve as FEL CC from 20-24 Jul 98. Bohn will resume after that. Please make your vacation plans known to FEL CC as far in advance as possible.
Next Meeting
Agenda for Next Meeting
Item Person Responsible Time ---- ------------------ -------- Status of Open Action Items All 5 min Commissioning Schedule Neil 10 min Status of Lasing or Gun Activities Neil 20 min Overview of Injector Setup Procedure Biallas 20 min New Issues All 5 min
This Week's Attendees
J. Bennett, S. Benson, J. Bisognano, C. Bohn, J. Boyce, D. Douglas, A. Grippo, K. Jordan, R. Li, L. Merminga, G. Neil, P. Piot, M. Shinn, M. Spata, B. Yunn
Closed Action Items
None.
Items of Discussion
Yesterday we lost the photocathode due to arcing and vacuum activity near 350 kV. Recesiation did not help; apparently a field-emitting site came into being that emits after cesiation at the voltage indicated. Consequently, we have been processing the gun at 500 kV since about 1900 yesterday evening and will stop at about noon today, with the intent of burning away this site. This afternoon we will heat-treat the cathode at 675 C for 1-2 hours to try to drive out contamination, and then leave it to cool overnight. Tomorrow morning we will cesiate and check for a photoresponse. If favorable, we will try to resume beam operations and lasing. If unfavorable, we will open the gun and begin the process of replacing the cathode wafer. If the latter occurs, the process will take about ten days, but the time will be well spent toward completing more installation tasks and working on additional detailed procedures for kW lasing. [UPDATE: At 1245 we decided to pull the cathode since it exhibited observable field emission at 440 kV coincident with the location of the drive-laser spot, and since we had some evidence of hydrocarbon contamination during high-voltage processing that would make recovery improbable.]
Prior to losing the cathode we accomplished several tasks this week, including checking the magnetostrictive tuner for one of the cryounit cavities (its current drive circuit needs more work); running the cryomodule cavities at their peak gradients (for 1.5 hours with only one rf trip, indicating we should be able to produce and sustain 47 MeV beam for straight-ahead lasing at 3.2 µm, the wavelength needed for Metals Lab experiments); recalibrating the current-monitor cavity with cw beam; calibrating the radiation detector at the straight-ahead dump with cw beam to monitor power deposition there (it will alarm in the MCC Control Room if 50 kW is exceeded); doing lasing studies with both cw and pulsed beam; replacing the outcoupling mirror in the optical cavity with a 90% reflective mirror to deliver higher power (the original mirror was almost surely 98% reflective, as inferred from the laser pulse profile, and in particular the short fall time at the end of the laser pulse); and subsequent length adjustment of the optical cavity in preparation for lasing with the new mirror.
After going over a summary of the week's activities, and especially those relating to the gun, C. Bohn led a general discussion concerning the philosophy for expeditiously pursuing high-power lasing with energy recovery, coupled with doing CSR experiments and lasing experiments. Bohn advocated staying as close to 60 pC bunch charge as possible, referencing earlier discussions that indicate the associated machine setup will accommodate bunch charges up to about 80-90 pC, above which new machine settings will likely be needed. He also advocated staying with the present optical cavity as long as it is sensible to do so (to be determined once data is available -- there will be a thermally induced FEL power limit with these CaF mirrors, but we are unsure where the limit is). Bohn suggested that it could prove advantageous to pursue running at 75 MHz with 60 pC charge for 4.4 mA current; again, more data will clarify the matter. If so, we will need another MPS machine mode. Generally the injector settings appear to be "good enough", so in the near term, barring something unforeseeable, injector development will be done "on the side" as opposed to being a focus of activity. In the mid-term to long-term, we may need to do more on the injector to accommodate high bunch charge.
Concerning CSR experiments, the emittance growth will scale roughly as bunch charge divided by the 4/3 power of the bunch length. Lasing places a requirement on the peak current, which is to say the 60 pC bunch charge necessarily carries a correspondingly shorter bunch length than does, e.g., a 135 pC bunch charge. So, 60 pC likewise provides for interesting CSR experiments. R. Li reported that she has now simulated CSR through both optical chicanes. Using Yunn's 60 pC bunch at the cryomodule exit as the initial condition for her simulations, she calculates CSR-induced emittance growth of 15% after the first chicane and 43% (total) after the second chicane. She is in process of extending the simulation around the first recirculation arc. There was some discussion about how emittance measurements were progressing, e.g., multimonitor versus quad/viewer. We should compare the results from the two methods; the diagnostics in the back leg will permit a comparison.
We went over in detail the remaining "Subsidiary Tasks for Phase-Space Metaprocedure", and the revised list appears at the end of these minutes. D. Douglas said that today he will post his "Metaprocedure for High-Power Lasing" as a pdf file on his FODOMAT web site.
In view of the gun going south (see UPDATE above), there will be no running with beam for the next 7-10 days. The FEL vault will be in Restricted Access all of next week, at minimum, though of course we will be trying to expedite resumption of commissioning. Fugitt and Neil will need to work out staffing and program plans immediately following that; before departing on 14 Jul, Bohn will offer advice to Neil on this matter. Please stay abreast of plans via the www at http://www.cebaf.gov/accel/fel/documentation/feldoc4/feldoc4-6.html (linked to the FLOG page) and by asking questions.
New Issues
None.
New Action Items
Old Action Items
Procedures in Work
Emittance Growth from CSR
Procedures Needed for Recirculation
RF Stability during Energy Recovery Merminga, 17 Jul 98
Unfinished Subsidiary Tasks for Phase Space Metaprocedure
Task Principal
---- ---------
I. Stabilize Drifts in Drive-Laser RF Phase Walker/Fugitt
II. Calibrate RF Gradients
A. Buncher Yunn
B. Cryounit Cavities (fix control screen) Merminga/Krafft/Yunn
C. Cryomodule Cavities Merminga/Krafft/Yunn
III. Reconcile Methods for Setting RF Phase Merminga/Krafft/Yunn
A. Buncher Cavities
2. Minimum bunch length/time of flight
B. Cryounit Cavities
1. Transient phasing
2. Cresting
3. Time of flight
IX. Complete Diagnostics Procedures and Training Krafft/Piot
A. M_55 #1, #2
B. Happek #1, #2
C. BPMs
1. button calibration factors
2. rotated MEDM spike charts
3. difference orbit data acquisition script
D. Multislit #1, #2
E. Quad/Viewer (at Wiggler, after Wiggler)
F. Multimonitor (at Wiggler)
G. SLM/CSR
H. Zero Phasing
I. Momentum Spread (Injector, Linac Pre-Wiggler, Linac Post-Wiggler)
J. Energy
XIV. Update (as appropriate) "First Light" Procedure Benson