Next Meeting
Agenda for Next Meeting
Item Person Responsible Time ---- ------------------ -------- Status of Open Action Items All 5 min Commissioning Schedule Bohn 10 min Status of Lasing Activities Bohn/Neil 30 min New Issues All 5 min
This Week's Attendees
S. Benson, C. Bohn, D. Douglas, F. Dylla, J. Fugitt, A. Hofler, K. Jordan, R. Li, L. Merminga, P. Piot, J. Parkinson, J. Preble, M. Shinn, T. Siggins, B. Yunn
Closed Action Items
Items of Discussion
This week saw the achievement of three notable FEL accomplishments: (1) recirculating pulsed beam (60 pC @ 2 Hz and 18.7 MHz, yielding a 1.1 mA average current over a 200 µs macropulse length) all the way around the recirculation loop while recovering energy from the macropulses (which means beam was taken through the entire machine), (2) lasing with this beam while maintaining energy recovery from the macropulses and keeping the beam squarely on the viewer in the energy-recovery dump, and (3) lasing into the straight-ahead dump to achieve 311 W cw. S. Benson authored the following summary of the straight-ahead lasing activities:
Lasing with the high-power (90% reflectivity) mirror set was characterized this week. First, lasing was established and optimized using the steering, cavity alignment, buncher gradient and cryomodule gang phase. It was found that the optimum orbit through the wiggler and the optical cavity alignment was close to that obtained using the setup procedure. The optimum buncher gradient was that which also produced the smallest energy spread at the exit of the injector. The cavity length detuning curve was over 25 microns in length which is similar to the optimized lasing with the 98% reflectivity output coupler. Next a series of curves of power versus cavity length was taken as a function of micropulse repetition rate and charge. Lasing was possible with 4.7 MHz beam (four round trips per gain pass). Lasing was also possible with 6 pC of charge. Consequently, at least a factor-of-four margin in gain is indicated. Spectra were taken versus cavity length and clear evidence for a sideband was found for the longest cavity lengths. Finally, the laser was optimized with cw electron beam and an output power of 311 W was achieved. In pulsed mode with a 1.2% duty cycle, the power measured upstairs in the Optical Control Room was 4.1 W, indicating a power during the macropulse of 340 W. Assuming 15% losses in the optical transport line (a number that still needs to be measured), this corresponds to 400 W out of the laser. Lasing with an intracavity power of 3 kW therefore reduces the power output by around 25%. When the laser was operated at 200 W (where the spectrum is narrow) it could be turned on and off very repeatably. When operated at 300 W the cavity length had to be adjusted to reestablish lasing. CW lasing was quite stable and no degradation in performance was seen over a couple of hours of operation. In fact the laser seems to operate better as the cathode degrades, as long as the bunch charge can still be produced.
Yesterday's day shift was spent toward improving the match of the recirculated electron beam. Yesterday evening we were preparing to do another run of cw lasing straight ahead to try out some ideas for improving the cw laser power and also to measure the loss in the optical transport line. Unfortunately, the gun arced twice, and later a third time, polluting the cathode and severely degrading its quantum efficiency. We still did some lasing at 10 pC bunch charge, but could not carry out our plans for the evening. This morning, the maximum bunch charge that could be extracted was only 5 pC. Interestingly, and this remark is speculative based on C. Bohn's vague recollection in context with last night's sequence of activities, there may be a correlation between gun arcs and opening/closing the gun valve while the gun is at high voltage. Perhaps the action effuses contaminants that lead ultimately to an arc. The gun had to be operated all week at voltages in the range 324-330 kV because field emission was problematic at the nominal 350 kV. It also needed daily recesiation. Nonetheless, we achieved the successes described above.
In view of the latest developments, this meeting was devoted principally to identifying in detail the plan for the next two weeks. The result appears on the FEL scheduling board, which is accessible via the www at http://www.cebaf.gov/accel/fel/documentation/feldoc4/feldoc4-6.html. The basic idea is to use the weekend to try to bring the gun back online, then further improve the match of the recirculated beam, set up the recirculation BLMs and the energy-recovery dump to permit cw recirculation, do cw recirculation with energy recovery, and then lase cw with energy recovery with an average electron current of at least 2.2 mA. If all goes well, and if we can get the necessary Laser Safety System and Laser Safety Operating Procedure fully completed, we may also be able to squeeze in a user experiment in Lab 1 (that being prepared by M. Shinn and Old Dominion University). We will accomplish all we sensibly can prior to the FEL Conference, which will start here in Williamsburg on 16 Aug 98 and end on 22 Aug 98. The FEL will be open (no running) during the Conference.
For shift assignments, please check the two-week scheduling board. As always, this will be a team effort, so please continue to be willing and flexible, and offer a hand where you see the need.
New Issues
None.
New Action Items
None.
Old Action Items
Procedures in Work (UNREVIEWED AT Meeting)
Procedures Needed for Recirculation
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. Laser Turn-On Procedure Benson