Free Electron Laser Commissioning Meeting

Thursday, 12 Jun 98
Recorder: C. Bohn

Next Meeting


Date: 19 Jun 98
Time: "0845"-0945
Place: FEL Facility Break Room

Agenda for Next Meeting


Item Person Responsible Time ---- ------------------ -------- Status of Open Action Items All 5 min Status of Lasing Activities Benson 25 min Overview of Injector Setup Procedure Biallas 15 min Commissioning Schedule Bohn 10 min New Issues All 5 min

This Week's Attendees


J. Bennett, S. Benson, G. Biallas, J. Bisognano, C. Bohn, J. Boyce, D. Douglas, F. Dylla, J. Fugitt, A. Hofler, R. Li, L. Merminga, G. Neil, D. Oepts, P. Piot, J. Preble, B. Yunn

Closed Action Items


Items of Discussion


Enormous progress was made this week on commissioning the FEL accelerator driver. The gradients of the cryounit cavities were calibrated to an accuracy sufficient to show that the entrance cavity was operating at about 20% lower than indicated on our control screens. We raised its gradient and thereby boosted the beam momentum in the injection line from about 9.1 MeV/c to about 9.5 MeV/c as inferred from the dipole-magnet fields, an act that should enhance machine performance by giving a momentum closer to the design value. The Happek bunch-length monitor installed in the wiggler location is up and running. We found a very simple procedure for using it to establish a minimum bunch length, and that is to adjust the gang phase of the cryomodule cavities to maximize the signal from the Happek device. We also measured the beam quality in the wiggler region: transverse emittance, spot sizes, bunch length, and energy spread. All of these meet requirements for first-light lasing, and within the accuracy of the measurements they seem generally to agree with simulations (PARMELA predictions). We ran 1.1 mA cw average current; it took just ten minutes to establish this current, then the machine ran stably at 1.1 mA for a half hour, after which we terminated the high-current run simply to preserve the cathode. In short, almost all aspects of the machine fit within lasing requirements, and therefore we decided to install the wiggler tomorrow (13 Jun 98) and go for lasing next week.

We just started operating the Happek device this week and so can only make a few tentative statements about its quantitative output. It is showing that we get rms bunch lengths down to 0.2 mm, which compares well to the <0.3 mm goal for lasing. The bunch length is seen not to change much as the signal amplitude is varied by way of the cryomodule's gang phase. This is not yet understood, but one possible explanation is a redistribution of bunch charge so that it is, e.g., localized within the bunch but with a significant fraction of the bunch charge otherwise smeared over its "length". Our understanding of the device will grow as we continue to use it. It is a nice instrument.

Two areas that we would have liked to improve are the drifts in the drive-laser phase, which seem to be slow, i.e., hour-long time scale, and peak-to-peak noise in the current. Concerning the former, we may be able to use a detector to look at coherent synchrotron radiation at one of the light ports, the amplitude of which will change as the drive-laser rf phase changes, but we will learn more about its consequences in the process of trying to lase. Concerning the latter, the current jitter is of order 5% to the best we can determine after working on drive-laser stability, whereas we wanted 2% peak-to-peak. Given the measured beam parameters, however, the laser gain is estimated to be about 60%, at which we should be able to tolerate the measured current fluctuations.

The program for next week is to lase, first at 10 W (order of magnitude) using the already-installed high-reflectivity CaF mirrors, then at 100 W (order of magnitude) after replacing one of the mirrors with a higher-transmission mirror for more outcoupled power. During the first run, we will narrow the electron-beam phase space for lasing since lasing would be easier. This is an interim step for the higher-power lasing of the second run. Owl Shifts will be devoted to continued electron-beam diagnostics. Plans for the week after next will be determined based on progress next week. Staffing is a bit complicated; please see the FEL schedule linked to the www FLOG page for details. All affected individuals are already notified.

Status of the top-level MUST-priority prerequisites to lasing is:

  1. Stable, easily recoverable 1.1 mA, 38 MeV beam with sufficiently good beam quality.
    Status: Achieved, as mentioned above.
  2. Working diagnostics:
    Status: Happek #2 installed and working, as mentioned above. Have measurements of transverse and longitudinal beam quality, as mentioned above. Generally the diagnostics are not yet user-friendly, however.
  3. Adequate injector setup.
    Status: Achieved, given good beam at the wiggler location. Would like to improve drifts in drive-laser rf phase and jitter in electron-beam current.
  4. Detailed procedures and test plans well communicated to MCC staff.
    Status: Have well organized binder of procedures, including that of first light, but they will need to be refined as we work through lasing. Ultimately we will want a condensed handbook for operators.
  5. Working optical transport, including Laser Safety System for the Optical Control Room.
    Status: Optical Control Room still needs two workstations, LSS alarms, LSS certification, cabling pulled for Control Room Monitor, and signed Laser Safety Operating Procedure. All are scheduled to be complete Monday, 15 Jun 98.
After apprising everyone of the status of the machine and plans for next week, C. Bohn went over the remaining subsidiary tasks that support the Phase Space Setup Metaprocedure. They are listed at the end of these minutes. S. Benson advised that a procedure for electron-beam stability measurements is not needed since the machine has thus far been seen to provide stable beam, and the lasing process will itself be an accurate indicator of beam stability. Accordingly, this task is hereby jettisoned from "Test Plans in Work". However, a new entry appears there, namely, the need for a procedure to assess steering through the cryounit cavities using the newly installed correctors (that are accessible to G. Biallas, but not to operators). B. Yunn is to write the test plan.

P. Piot presented an overview of results from the various beam-quality measurements, to include the status of respective user interfaces. He has been working very hard, and the strength, thoroughness, and impact of his contributions are self-evident. A top-level summary of the results is as discussed above. Generally speaking, the first M_55 cavity still needs some debugging (in progress), an electron-beam orbit still needs to be found with which Multislit #2 can be used, the quad/viewer transverse-emittance measurement after the second optical chicane is still being worked, and the procedures for using the various diagnostics are being refined to make them user friendly.

R. Lauze advised that things are going very well toward the hire of six new operators, with one of them reporting in very soon. He gave details, but the bottom line is that sometime near the end of summer we should start to see newly trained and productive people helping run the FEL as well as CEBAF, and that should ease the staffing difficulties.

New Issues


S. Benson described constraints on use of the three wiggler viewers from the standpoint of protecting the wiggler from radiation that would be generated by their use. Bottom line is that each viewer should be limited to 40 minutes of use per week at 4.7 MHz on the thumbwheel. R. Lauze pointed out that this usage can easily be tracked and monitored by software, with warnings to operators. Benson will generate a requirements document for the Software Group (see "New Action Items" below).

New Action Items


Old Action Items


Test Plans in Work



Procedures in Work


Emittance Growth from CSR



Procedures Needed for Recirculation


Thread Beam around Machine, Top-Level Douglas, finalize 30 June 98 RF Stability during Energy Recovery Merminga, 30 June 98

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

V.      Finalize "Injector Setup" Procedure                     Yunn/Biallas

VIII.   Establish Expected Beam Quality                         Yunn
        F.      Emittance at 2G02                               (with Li)
        G.      Momentum Spread at 2G02                         (with Li)

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

X.      Develop "Injector Rematch" Procedure                    Biallas/(Unk. Physicist)

XIV.    Update (as appropriate) "First Light" Procedure         Benson