Free Electron Laser Commissioning/Diagnostic Meeting

Tuesday, March 19, 1996
Recorder: G. Krafft

Next Meeting

Date: Tuesday, March 26, 1996
Time: 2:30 - 4:00 **** Please note (and complain about, if need be) time
Place: MCC Room 101

Attendees

K. Jordan, J. Denard (2nd Half), C. Bohn, G. Neil, D. Douglas, R. Legg, D. Kehne, and G. Krafft

Items of Discussion

The main body of the meeting consisted in systematically going through the accelerator in order to determine if the diagnostic complement was sufficient in the current drawing. As a start, the commissioning was divided into the steps listed below. We also tried to identify uncertain items that should be investigated off-line in the action items below. The first cut at the steps were:

I-I Beam from gun
I-II Buncher/solenoid centering
I-III Buncher setup
I-IV Unit centering
I-V Unit setup (gross)
II-VI 10 MeV Thread
I-VII Unit setup (fine)
I-VIII Beam on dump
I-IX Matching

At this point one would hope to have a well-characterized beam (known alpha, beta, epsilon, bunch length, and energy spread) entering the main recirculation path. The commissioning steps beyond the injector (M for main) are

M-I ``Linac'' thread
M-II RF Phasing
M-III RF Amplitudes
M-IV Chicane 1
M-V Wiggler
M-VI Chicane 2
M-VII Pi bend 1
M-VIII Back leg thread
M-IX Pi bend 2
M-X Recirculation
M-XI Beam to 10 MeV dump

Associated with these steps the following comments were made:

I-I The pulse structures for initial setup, ``viewer limited'' mode, matching mode, etc., should be agreed upon and established.
I-II Buncher centering is needed to minimize emittance growth, if the solenoid is not centered at the same time, we should move the solenoid to the buncher center. Whether there will be a problem will be determined early in Engwall's measurements. No Action
Having a phase modulation capability, as in the nuclear physics injector will be useful. Having a modulator on the laser reference would make this step, and RF phase verifications easy. Deffered Action
I-III The buncher will be phased first by transient phasing, and its amplitude set by RF power measurements. In order for this to procede, it is needed that the RF power measurement (say forward - reflected power) be calibrated, and the external Q of the cavity must be known before hand. Neil volunteered to make sure such information is available.
In this region, scraping is liable to be an issue. Jordan assured that Geiger tubes will be available if needed. Again, this should get clarified during Engwall's experiment. No Action
I-V Again, transient phasing would provide the first estimates of the phase settings. No Action
I-VI Are stray fields an issue? Legg will investigate.
I-VII Precise final phasing will be done by energy maximization.
There was a question about how precisely the unit gradients must be set. Bohn agreed to talk to Liu about getting that resolved. Like- wise, how precisely must the off-crest phase be set?
A separate but related question is, how precisely must the overall energy out of the injector be set. Douglas to investigate.
I-VIII It was agreed that the test stand matching was not a problem. However in the FEL proper, it was pretty clear that there was no good way to verify the match. If the spot was bad downstream of the full module, there would be no way the show that the injector was/was not the culprit. After much discussion it was agreed that a quadrupole should be added to the first 10 MeV dump line, so that the dispersion at the viewer could be suppressed and a measurement could be done. Such a solution will be somewhat cumbersome compared to the system in the nuclear physics injector, but no good proposal on the main beamline seems possible given the space. Douglas has an action to check the said optics and Bohn has an action to get the drawing up- dated.

Two additional issues, not really covered above, were where and how will we measure the injector current, and how are we going to verify that the beam stays on the dump. Legg will investigate dump instrumentation and report back next week.

The final issue associated with the injector, namely, slit measurements of emittance in the injector, that seem necessary to quantify our emittance credibly (at least to the outside world), needs work, especially in regards to specifying the properties and design of the slits. At some point we need to get someone working on this seriously. Action defferred.

M-II RF phasing can be accomplished in the first instance by transient phasing. A more precise measurement follow from using the BPM at the first chicane bend when the beam gets there. Finally, best precision is achieved at high dispersion points in the lattice: the SLM port in the recirculation case, or in the beam dump in ``first light experiments''. Because of concerns expressed about possible damage to the wiggler as these processes are going on, and partly to ensure that emittance measurements are possible both before and after the first pi bend, it was proposed that the layout include a wiggler bypass and the 42 MeV dump be moved downstream of the first magnet of the first pi bend. The layout should include some optics to allow the dispersion to be controlled. Diagnostics in the bypass were discussed and a tentative plan was proposed. Douglas has the action to finalize the bypass optics and Bohn will make sure the drawing gets updated properly.
M-IV The chicane will be verified to be ok primarily through the fact that beam traverses the chicane properly and through the fact that the bunch length is short after the chicane (a Happek device will be installed there). On the first issue Douglas expressed great con- cern that the chicane magnets be done right, especially that they by cross calibrated to the high degree needed. We would like to avoid problems like those that have plagued the doglegs. On the second issue, a possible disconnect in the present design was un- covered. With the high M56 in the chicane it is necessary to run the module off crest to a level that was thought possibly to have RF implications. G. Neil will find out for next time.
M-V It was thought that the wiggler looked OK. The match will be verified by the OTR diagnostics and the steering can be done with BPMs and OTRs. An issue arose as to whether the layout of the wiggler and chicane could be more optimally done. Reducing M56 in the chicane might be possible, elliminating some portion of the RF problem mentioned above. Douglas, Neil, and Liu were drafted to resolve this issue before the layout is frozen.
M-VI Legg suggested that it would be a convenient to have an OTR diagnostic where the BPM was in the second chicane. The reason is that one could check the energy spread generated by the wiggler, and have a sensitive check that everything is just fine during operations. It was agreed to incorporate this change.
M-VIII According to Douglas, the back leg optics had completely changed. No comments were made other than to assure that we thought an emittance measurement can be made. Douglas has an Action to specify diagnostics locations on the back leg.
M-X Given the short recirculation time, given that recirculation is done more or less 180 degrees off crest, and given that there isn't much room, Krafft has an action to try to resolve how the path length measurement might be done. Because of the large wiggler generated energy spread, it is not likely that energy measurements on the second 10 MeV dump will be very usefull.
M-XI Not much thought has been given to getting the recovered beam on the dump. This is food for a future action item.

After completing this discussion, it was pointed out that we also have to consider longerterm operational issues. For example, if the primary way to control the radiation wavelength is through beam energy, and requests for energy changes are to be frequent, we should have enough capability to make and verify energy changes as expeditiously as possible. Same for pulse changes. How are we going to optimize the power out when a user complains. It was requested that others add to the list.

Action Items (assigned)

Action Items (pending)

Agenda for Next Meeting 

                Item                              Person Responsible     Time
                ----                              ------------------     ----
    *	Review agenda				         Krafft	         5 min.
    *	Corrections to minutes from last meeting	  All		 5 min.
    *   New drawing                                       Bohn          20 min.
    *   Optics issues                                   Douglas         20 min.
    *	Buncher gradient calibration	                  Neil   	10 min.
    *   M56/RF phasing issues                             Neil          10 min.
    *   Dump instrumentation                              Legg          15 min.
    *   Path length                                      Krafft         10 min.
    *	Agenda for next meeting		                  All		 5 min.

Acknowledgement

A templet devised by J. Karn was used to produce these minutes. Comments back (to G. Krafft) are appreciated.