Next Meeting
Agenda for Next Meeting
Item Person Responsible Time ---- ------------------ -------- Status of Open Action Items All 5 min Status of Gun/Injector Siggins/Biallas 10 min Status of Commissioning Bohn 20 min Commissioning Schedule Bohn 10 min New Issues All 5 min
This Week's Attendees
J. Bennett, C. Bohn, J. Fugitt, K. Jordan, G. Krafft, L. Merminga, M. Shinn, B. Yunn
Closed Action Items
New Action Items
None.
Old Action Items
Items of Discussion
Today's meeting was sparsely attended and brief, but we accomplished what we needed to do. The status of LSS/ARR documentation is summarized in the above "Old Action Item".
At this writing (0945 Friday, 6 November 1998), we are running the FEL. Translated, that means the gun is presently operational. However, we may need to replace the cathode wafer starting early next week. A detailed summary follows:
At week's beginning, after finishing the rework of the cesiator, we still had achieved neither a good cathode photoresponse nor a gun that behaved quietly at 350 kV. Consequently, we did two things. First, we high-voltage processed the gun to 500 kV in an effort to suppress what appeared to have been two field-emitting sites (a soft statement, given the grossly coarse resolution of the geiger-tube array, though the emission was observed unambiguously to be bimodal). Then we heat-cleaned the cathode at a high temperature (675 C) for a long time (8 hours), an action that was motivated by vacuum data from previous heat cleanings. Finally, we recesiated the cathode yesterday morning. The resultant photoresponse appeared to be about a factor three better than previously with this cathode, but still poor. Basically, the cathode exhibited a photoresponse like this summer's cathodes after they had been well used and were in need of replacement. We then did a high-voltage check with the now-cesiated electrodes and found a quiet gun (no field emission) at 350 kV -- finally achieving at least one success.
Given the state of the gun yesterday afternoon, we decided to try pulling pulsed electron beam off of the cathode, and we found we could do it. We are presently unsure of the calibration of our beam-current-monitoring (BCM) cavity and will need to check it, but taken at face value, it indicates we are achieving 60 pC bunch charge with modest power (a 25% polarizer setting) from the drive laser. Consequently, we embarked on a 32-hour program (through Swing Shift tonight) of commissioning with the fundamental goal of taking pulsed beam through the machine and putting together a punch list of hardware/software problems; one would expect problems given the gun has been down for 12 weeks and considerable work has been done on the machine in the interim. At this writing, we have already put the beam into the straight-ahead dump and were making some steering adjustments in the wiggler region in preparation for pulsed lasing to check the FEL systems. The electron beam looks quite reasonable, again given the machine has sat dormant for so long. Plans are to take the beam all the way around, check the new diagnostics in the recirculation loop, and then go back to the straight-ahead dump for a brief cw run to calibrate the BCM cavity. Presently the conditioning resistor is in place, meaning the running resistor still needs to be put in prior to going cw. The new resistor-change hardware should make it possible to do this in just a few minutes, versus the two hours it used to take.
Presently we cannot know whether the cathode will need changing next week. The photoresponse measurement suggests that we should expect a very short lifetime. We shall find out soon, perhaps by trying to turn on again Monday morning and seeing what is left of the cathode. We will run the machine as long as is sensible to do so before replacing the wafer. Shift assignments for next week are on the two-week scheduling board, should we be able to run.
Another way to view the developments is as follows: The exorcism our gun owner, Tim Siggins, undertook on Halloween night turned out to be a protracted one. Relics of the exorcism are still present and remain to be purged from the FEL vault. We gave Tim the day off today. He has a new twitch, but we don't think he needs the straight jacket yet.
G. Krafft reported that the two new M_56 cavities are ready, and they will be installed in parallel with the next cathode replacement. These cavities, along with the available software, will provide data on path length and phase transfer. An off-line calculation will be needed to translate the data into M_56 values because the software has not yet been upgraded to do this. That should not pose a big problem since the calculation is straightforward.
New Issues
None.
Procedures in Work
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 Users' Guide 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