Free Electron Laser Commissioning/Diagnostic Meeting

Tuesday, April 30, 1996
Recorder: G. Krafft

Next Meeting


Date: May 7, 1996
Time: 3:00 - 4:00
Place: Test Lab Conference Room

Attendees


K. Jordan, J. Denard, C. Bohn, D. Douglas, L. Merminga, G. Neil, S. Benson, D. Kehne, and G. Krafft

Items of Discussion


There were no changes to the minutes. Douglas began the discussion by outlining some updates to the optics of the main machine that will have to be made because fringe fields were not done correctly in the 180 deg bends. There was some sort of double counting of the effects of dipole fringe fields. The bottom line is that the optics in the reverse bends will have to be changed somewhat. He is working to get these issues resolved.

Action Items


A-01 Two updates to the machine were proposed and discussed. The first addition was to make the the dipole that merges the injector beam onto the linac have reversible polarity. This would allow the injector tuneup dump to be located in a more convenient location, and perhaps increase the emittance or energy spread measurment precision. It was generally agreed that this was a good idea. Benson is to coordinate this request with Bialas, with the commissioning group's blessing. It was noted however, that this region is not entirely designed from an optics point of view, and will probably be changed a bit when that final design is done.
The second proposed change was to beef up the dipole on the downstream end of the cryomodule that directs the beam into the 10 MeV dump in recirculation mode. The idea is to be able to crest the 42 MeV beam in that dump. There is a clear advantage to crest early after the cryomodule, but given the chicane is nearby, it was not so clear that this proposal buys us much. It seems that the original plan with a quad just downstream of the chicane in the bypass is a better solution (I verified that this was Legg's opinion too!), because of the potentially higher dispersion. I believe that the general feeling from the meeting was that if the addition was easy, go ahead and get it, but if not, we are covered with what we got. Krafft will contact Harwood on this one. From three weeks ago, the magnet choice for the ITS was made. Thanks to Kehne and Harwood for getting this done.
A-03 No documentation confirmation as of 4/9/96.
A-17 A large portion of the meeting was spent discussing, and adding to J. Denard's requirements table for the BPMs. The additions have to do with the performance requirements for the SEE BPMs that will be added in the wiggler region, mainly to help define the beam orbit giving the maximum gain, and allowing us to rereprodicibly achieve that orbit. These BPMs will be the primary data source for a slow lock that will hold the beam in a constant orbit (to order 20 micrometers) in the wiggler. Several related but not decidable issues were brought up as a result of this discussion.
The first issue was what the update rate for the BPMs should be. It was asserted that an update rate of at least 3 Hz is needed to facilite commissioning, and in response, it was asserted that the 3 Hz update rate was not really an issue of the BPMs but and issue of permissible beam loss in tuneup mode. No resolution of the issue was forthcoming, and it was agreed that tentatively 1 Hz will stay in the requirements, but an off-line discussion involving Douglas, Jordan, Krafft, Bohn, and Neil will try to find a mutually acceptable tuneup mode pulse structure.
Once that's done, and Lia is happy about the beam loading, we will update the BPM requirements as needed. However, all other requirements as stated on the document were approved by those present. Denard will distribute his almost final version this week. A question arose whether the having more SEEs near the wiggler, perhaps for example to more precisely measure the incident angle of the beam in the wiggler, might facilitate laser commissioning. There was no clear resolution of this question, and it is added to the pending list. The decission to buy at least 3 SEEs ((2) for wiggler and 1 spare) was generally agreed to.
The final issue regarding the BPMs discussed had to do with the beamline hardware. Denard expressed reservations about heating of the electrodes by beam loss, and proposed to recess the electrodes in order to reduce potential loss. He wished to hear that the impedance change would be OK, and he was assured that given our budget, that would probably be fine. However, he was criticized for his general conception that beam loss could be shielded by recessing the electrodes. In view of a reviewer suggestion that one could more or less expect that at some time the beam would hit the BPM electrodes, after much discussion it was agreed that the BPMs should be designed robustly enough that the full beam power in tuneup mode should not cause problems. (This is of order 200 W beam power) At least one of the meeting participant were dubious about shadowing the electrodes from the beam. Denard was not sure at this point that this could be done, but will report back later.
Some new items were added to the pending list as a result of the review. The only one that ellicited serious comment was the beam scraping plan. Albertine had apparently mentioned this problem again to Bohn in a conversation. Lia mentioned that she had worked on scraping at SLAC. After other discussions on some of the other items (see below) it was agreed that sometime in the next two weeks Lia would present a plan outlining her contributions to these issues over the summer. Krafft and Bohn volunteered to start thinking about What If studies. G. Neil volunteered to handle the suggested FEL commissioning review. He requested an example of an ``acceptable'' plan, and Krafft voluteered to try to find an old commissioning plan for the main machine. Finally, Douglas expressed reservations about the magnet specification process, especially given the relative newness of some of his designs. He was worried that good information would not be used to specify magnets, only engineering judgement. After much discussion, it was agreed that Douglas, Bohn, Harwood, Bialas, Jordan, and Krafft would meet on Monday to try to find some mutually acceptable solution.

First Cut List of Commissioning Tasks


C-1     RF gradients verified
C-2     Operational RF phases determined
C-3     Drifts (of phases if like main machine) quantified
C-4     Gun current verified/operational procedure complete
C-5     Beam jitters determined
C-6     Cryounit Focussing/Convergence determined
C-7     High power CW beam is delivered for ``awhile''
C-8     Cathode lifetime studied
C-9     Engwall/Kehne studies on gun transverse/longitudinal emittance complete
C-10    PARMELA ``verified''

After the move to the FEL building the following tasks must be completed to prepare the beam for injection into the linac.

C-11    Transverse phase space at injection verified
C-12    E-beam matched at injection point
C-13    Longitudinal phase space verified
C-14    Longitudinal tilt checked at injection
C-15    Above determined at high power

At this point C. Bohn emphasized again that some kind of scheduling with the SRF group will be necessary in order to run efficiently during the test stand commissioning, as cryo-commissioning and beam running are best done not in parallel. This item has been added to the pending list.

Action Items (assigned)


A-01	Update Drawing                         Bohn               4/16/96
A-03    Stray fields in test stand             Legg              complete
A-04    Unit gradient precision                Bohn, Liu          4/16/96
A-05    Injector energy precision              Douglas            4/09/96
A-06    10 MeV dump optics                     Douglas            5/14/96
A-07    Bypass optics                          Douglas            4/30/96
A-08    M56/RF phasing and beam load issues    Neil, Douglas, Liu
A-09    Dump Instrumentation                   Legg               5/23/96
A-10    Back leg diagnostic placement          Douglas            5/30/96
A-11   	Multifrequency CSR Measurements        Krafft             4/30/96
A-12    How should the path length be checked? Krafft, Jordan     4/30/96
A-13    Impedance Layout for Wiggler           Yunn, Neil        complete
A-14    Reduce M56 to under 30 cm              Douglas           complete
A-15    Commissioning Procedure                Legg, Krafft       5/16/96
A-16    Average Current Measurement            Legg
A-17    4-Channel BPM on the IRFEL             Bohn, Denard       4/30/96
A-18    Trombones                              Douglas, Krafft    4/09/96
A-19    Experiment Plans, 10 MeV line, etc.    Kehne              4/09/96
A-20	Flip viewer			       Kehne		  5/07/96		

Action Items (pending)


P-01    Determine Beam Pulse structure for setup
P-02    Laser phase modulator
P-03    Slit emittance measurement
P-04    Is an energy compression scheme needed?
P-05    Devise fine-tuning procedure for buncher gradient
P-06    Web Documentation
P-07    Diagnostics "Freeze"
P-08    How do we change energy? What is the energy range?
P-09    Do we need bunching after the module
P-10    Will the SLM monitor work to center the large beam
P-11    Calculate from Hall C data how much scatter the BPMs can take
P-12    FEL/OTR interaction question
P-13    Do we have sufficient diagnostics for emittance degradation measurements
P-14    How is the gun/laser operated?
P-15    Discussion on robustness of controls??????
P-16    Injector diagnostics in the main machine
P-17    Gang of power supply for backleg quads
        (must include emittance meas. capability)
P-18    Technote by Yunn summarizing machine impedance
P-19    Consistent linac/bunching phase sets
P-20    Lock plans (who in particular)
P-21    Save/restore plans
P-22    FSD plans
P-23    Transverse Effects in CSR
P-24    FEL Commissioning review
P-25    RF control sensitivities/Microphonics Diff for FEL/Pathlength effect
P-26    What if studies
P-27    Plan to measure beam loss
P-28    Scrapers: Where and How?
P-29    FEL/linac interaction
P-30    Tracking including realistic wiggler fields
P-31    What drives the vacuum spec.
P-32    Are two sextupole families enough to tune M56
P-33    Is path length feedback needed?
P-34    More than two SEEs in wiggler
P-35    Coordinated ITS beam running/SRF commissioning plan
P-36    Beam power robust BPMs

Agenda for Next Meeting


          Item                              Person Responsible     Time
          ----                              ------------------     ----
    *   Review agenda/corrections to minutes             Krafft          5 min.
    *   New drawing                                       Bohn           5 min.
    *   New issues
          Wiggler instrumentation and planning            Neil
          Dipole gang power scheme                       Krafft
          Requirements and reviews for diagnostics       Jordan
          Flip viewer
          Shielded viewers as bolt-in upgrades of regular viewers
          Overpower dipole after cryomodule; a microdump?
          Asses need for faraday cup
          Special OTR viewers in optical chicane
          Powering of back-leg magnets
    *   BPM requirements                                 Denard         10 min.
    *   Task List Discussions                             All           30 min.
    *   Unit Gradient Precision                         Bohn, Liu       10 min.
    *   Agenda for next meeting                           All            5 min.