Free-Electron Laser Commissioning Meeting

Thursday, 5 Mar 99
Recorder: C. Bohn
(minutes distributed at 1045, 5 Mar 99)

Next Meeting


None formally scheduled -- will let you know.

Agenda for Next Meeting


TBD

This Week's Attendees


J. Bennett, C. Bohn, J. Boyce, D. Douglas, R. Evans, K. Jordan, L. Merminga, G. Neil, M. Shinn, R. Walker

Closed Action Items


New Action Items


Old Action Items


None.

Items of Discussion


C. Bohn opened with a review of shift assignments and philosophy. As always, the FEL commissioning team should stay abreast of the 2-week schedule on the FEL white board and/or the Web posting thereof. Basically, the plan (assuming a gun) is to run the "traditional" commissioning pattern through next week (i.e., high-power runs during Day Shift, lasing studies during Swing Shift, and diagnostics during Owl Shift). By the end of next week we should have reached the machine's high-power capability. Consequently, after next week we will go into two-shift operation (machine on hot standby during Owls), and will continue that way until it again becomes prudent to do otherwise (e.g., after the sapphire mirrors are installed and we go into 3 µm lasing). So, the week after next will be CSR experiments during Day Shift (Piot, Li, others as appropriate), lasing studies during Swing Shift (Neil, Biallas, others as appropriate), and (to repeat) hot standby during Owl Shift.

After it went down early Wednesday morning, the gun was heat cleaned (8 hours at 675 C), high-voltage processed to 516 kV (field-emission-quiet at 512 kV), and heat cleaned again (1/2 hour at 675 C). This morning T. Siggins and J. Gubeli are making a cathode and doing a cathode scan. Hopefully we can bring the machine back up early this afternoon -- we'll see. If so, we will do a high-power-lasing run today, which will possibly continue into Swing Shift. The downstream reflecting mirror in the optical cavity was replaced with a silicon mirror with the hope of raising the thermal limit of the cavity, and now the FEL systems are ready for another try. Monday will not be a maintenance day. Rather, we will proceed right into the program described above.

As a consequence of events that led to the gun going down, A. Grippo has prepared a software interlock that will limit the highest gun voltage accessible to an operator. He is installing the interlock this morning. As we commission, we will undoubtedly find more "gotcha's", and we will correct them as they arise (and hopefully before they arise!).

Known issues that potentially affect the high-power capability of the present setup include:

It is surely of interest that on Tuesday the machine was lasing cw (at roughly 500 W) with a recirculated average current of nearly 4 mA; we are gradually approaching the design value of 5 mA. Of course, in establishing a "good" 5 µm high- power setup, we need to minimize scraping. The radiation survey following the run showed two relatively hot spots (not all that hot -- ~0.5 mrad/hr versus much less everywhere else around the machine). One was at the "K" in "KENNY" taped on the cryomodule, and the other was at the flange just after the wiggler (the site of the trombone). As a result, yesterday we moved two of the diagnostic BLMs, putting one at each of these two locations, and we will monitor them during high-current runs. We also know that the present setup dumps ~100 W beam power into the cryomodule at cw currents in the range 3-4 mA, and of course we want to reduce this number.

Bottom line: Experience to date is such that, given a good cathode, it is now "easy" to restore cw lasing at 400-500 W (~280-350 W upstairs in the Optical Control Room) by loading BURT Allsave #224, and then lase at that level for several hours. For example, on Monday of this week we ran 400 W, 2.5 mA cw for six hours while monitoring various diagnostics, such as vacuum, IR detectors, etc. We continue trying to do better. Meantime, the machine is capable of supporting some very interesting tests in User Lab 1, and (again, given a gun) we plan to be running at least two tests during the coming week.

New Issues


None other than those mentioned above.