Free Electron Laser Commissioning Meeting

Thursday, 26 Jun 98
Recorder: C. Bohn

Next Meeting


Date: 2 Jul 98 (stay tuned -- may get cancelled)
Time: "0845"-0945
Place: FEL Facility Break Room

Agenda for Next Meeting


Item Person Responsible Time ---- ------------------ -------- Status of Open Action Items All 5 min Commissioning Schedule Bohn 10 min Status of Lasing Activities Neil 20 min Overview of Injector Setup Procedure Biallas 20 min New Issues All 5 min

This Week's Attendees


G. Biallas, J. Bisognano, C. Bohn, J. Boyce, D. Douglas, F. Dylla, J. Fugitt, L. Harwood, C. Hovater, K. Jordan, G. Krafft, L. Merminga, G. Neil, D. Oepts, T. Siggins, M. Shinn, M. Spata, R. Walker, B. Yunn

Closed Action Items


None.

Items of Discussion


To take full advantage of the availability of people to complete the entire vacuum system for the FEL, we did not run electron beam this week. By week's end, the entire machine was under vacuum. We also heat treated the photocathode this week and subsequently subjected it to high-voltage processing and recesiation. All seemed to go well, but at this writing (1500, 26 Jun 98) the final state of the cathode was unknown. It is a matter of concern in principle because the last two heat treatments contaminated the cathode and resulted in the need to replace the wafer, a process that takes two weeks. Yet, the need for a good heat-treatment procedure cannot be circumvented because the cathode must periodically be rejuvenated.

Regarding matters of commissioning, we spent a good fraction of the week planning how to attempt kilowatt operation this summer. As both F. Dylla and C. Bohn emphasized at this morning's meeting, our foremost goal during the summer will be to achieve the kilowatt, and we will interleave the associated commissioning activities with a nominal effort to support user experiments. On Tuesday, 23 Jun 98, a "kW planning meeting" was held that led to the identification of installation tasks that remained as well as some notional ideas for a commissioning approach. The meeting minutes were sent to the FEL Commissioning Team under separate cover.

Today's commissioning meeting focused on a general discussion of the commissioning approach to achieve a kilowatt quickly. The top-level elements of the approach (from Tuesday's meeting) are:

Once the electron beam is set up, commissioning of the recirculation arc and energy recovery will proceed generally in pulsed mode with 60 pC bunch charge. Initially the process will involve 18.7 MHz beam, after which the frequency will be doubled to the full 37.4 MHz, corresponding in cw mode to 2.2 mA average current. Finally, the bunch charge will be raised until kilowatt lasing is achieved. J. Fugitt pointed out in the discussion that, as presently configured, the high-voltage power supply for the gun will support average current up to 4 mA. To go higher requires modification of the power supply. A 4 mA beam corresponds to a bunch charge of 107 pC. >From what we saw last week, the wiggler coupling efficiency would seem to be sufficiently high to provide at least a kilowatt with this beam, but we will find out in due time. At least for the foreseeable future, an accelerator setup that supports 60 pC bunches would seem to suffice. B. Yunn estimated that the 60 pC settings are good for bunch charges up to about 80-90 pC. D. Douglas pointed out that it might be far easier to achieve a kilowatt by lasing at high efficiency (meaning high energy spread) than by going to the highest bunch charge (meaning strong space charge). Again, we will be exploring questions of machine optimization in due time. The Commissioning Coordinator will work hard to provide sensible configuration control as we progress.

L. Merminga gave a nice presentation on rf aspects of energy recovery as a prelude to her writing the respective procedure. Her thinking is in line with the top-level strategy outlined above, and her analysis shows that there should not be undue problems from beam loading during energy recovery. Douglas pointed out that there will be a phase shift during deceleration in the cryomodule that will especially show up as the beam becomes less relativistic near the cryomodule exit, and Merminga plans to factor that consideration into her analysis. Merminga intimated that to support high cw current in the injector requires the magnetostrictive tuners, especially in cavity #3 of the cryounit (the exit cavity). Tuner tests are scheduled for Day Shift on 7 Jul 98. She also said that her preference would be to attempt energy recovery at first without lasing, partly to ensure people stay focused on recirculation as opposed to lasing. The Commissioning Coordinator will resolve this procedural matter at the proper time.

D. Douglas commented that he has been making progress on the "Lase with Energy Recovery Metaprocedure".

C. Bohn met off-line with R. Lauze and M. Spata to discuss with them the general need for CEBAF operator support especially when lasing is planned (typically on Swing Shifts). After the meeting Lauze was arranging for the Crew Chief stamp to include a space for the Crew Chief to assign an operator for the FEL. This should circumvent any ambiguity about operator support for the FEL at shift changeovers. Lauze and Spata also reminded Bohn that during 2-10 July the main machine will be down and there will be no operator support. Consequently, the "standby" FEL'er will need to be on duty during these dates.

To see the latest two-week schedule and operator assignments, please check the www at http://www.cebaf.gov/accel/fel/documentation/feldoc4/feldoc4-6.html. This address is linked to the FLOG page and so is easily accessible from there. G. Krafft announced that Uwe Happek will be here the first part of next week, and therefore we may be devoting some Swing Shift time to the Happek devices while he is here (during which Krafft is scheduled as "standby" operator).

New Issues


None.

New Action Items


None.

Old Action Items


Test Plans in Work


Bohn reminded everyone to be conscientious about preparing test plans for their respective systems. This is especially needed if we are going to succeed in generating a kilowatt laser beam this summer.

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