Minutes of the Special Meeting on Orion RF Gun Parameters:

 Oct. 12, 2000


 Location: Room 144D (Beige Room),

           Admin. and Engin. Bldg., SLAC

 Present:  Eric Colby, Dennis Palmer, James Rosenzweig,

           Robert Siemann, Robert Noble

 Minutes recorded by: Robert Noble




 1. Items Discussed:



    - The meeting's purpose was to begin defining the design

      parameters for the Orion RF Photoinjector. J. Rosenzweig's

      group (UCLA) will be responsible for its design and fabrication.

      The present working plan to have the long-pulse thermionic NLCTA

      gun and the new Orion RF Gun be interchangeable, with only

      one of these attached to the X-band accelerator, was reviewed.

      (See Minutes of the Oct. 9, 2000, Orion Technical Meeting.)

      The S-band, BNL/SLAC/UCLA photoinjector is the present baseline

      source given the wide experience with this device, the desire

      to minimize schedule risk during Orion construction, and the

      requirement that the bunches fit into the X-band accelerator




    - Understanding the beam requirements of the experimenters is

      essential for specifying the source parameters. It was noted

      that a wide range of experiments on beam-plasma interactions,

      particle acceleration and focusing by lasers and plasmas,

      and coherent radiation production will require beam from the

      new RF Photoinjector. However these experiments actually

      imply a range of values that the source must deliver in terms

      of charge per bunch, number of bunches (one or two, with

      variable charge), transverse emittance, bunch length and

      energy spread. One can at best design the photoinjector to

      be optimized about a point in this parameter space and then

      check the design with a trade study to see if it has a wide

      enough range to serve the anticipated experiments. Design

      iterations then follow to bring the predicted performance

      in line with the experimental beam requirements. It was

      proposed that in parallel with the photoinjector design

      effort, the leaders of the Major Research Components

      (W. Mori, C. Joshi, T. Katsouleas, R. Siemann, R. Byer,

      J. Rosenzweig) should review and update the beam parameter

      requirements for the anticipated experiments.


    - The range of transverse and longitudinal beam emittances

      accessible with the source is typically constrained by the

      range in bunch charge desired. This is because, given the

      technical limits on acceleration gradient, solenoidal focusing

      and emittance compensation in the confined injector region,

      space charge forces tend to imply a scale for the beam charge

      density and emittances. One finds that in a given design, bunch

      dimensions and divergences scale like charge to the one-third

      power. Emittance and current then scale as charge to the

      two-thirds power. Also there is the constraint that the bunches

      from the S-band source fit into the transverse aperture and

      longitudinal rf buckets of the downstream X-band accelerator.

      It was conjectured that the limiting bunch charge that might be

      fully transferred into the X-band linac from the photoinjector

      is about 4 nC. Participants agreed that for the first design

      iteration, the photoinjector will be optimized at 0.25 nC of

      charge per bunch with the figure of merit being minimum

      emittance. The trade-study goals are to maintain, at the

      entrance to both the first X-band accelerator section and the

      experimental halls, the rms transverse normalized emittance

      at less than or equal to 5E-6 meter-rad and the rms

      longitudinal emittance at less than or equal to 2E-8 eV-sec

      for bunch populations up to 1 nC.



    - A copper cathode for the photoinjector is the baseline choice

      owing to the robustness of this surface. Its low quantum

      efficiency (less than 6E-5) is a drawback. Short bunch lengths

      and a high charge per bunch imply a short pulse (picosecond),

      ultraviolet laser with 0.5 to 1 mJ energy per pulse. This is

      standard for commercial Ti:Sapphire lasers.


    - The rf power needs of the proposed photoinjector were reviewed.

      J. Rosenzweig indicated that the photoinjector is expected to

      need about 12 MW of peak power for a gradient of 140 MV/m. Two

      input coupling schemes that protect the klystron from reflected

      power transients were discussed, but no decision made. One is

      to install an S-band circulator (roughly $20K). The other is

      to use an inexpensive power splitter with half the klystron

      power going to a load. The klystron would have to supply

      24 MW, which is well within the 65 MW capability of the SLAC

      5045 tube. Reverse power is then limited to only 6 MW in

      this arrangement.



    - It was noted that on October 10, 2000, A. Donaldson and

      R. Cassel of the Power Conversion Dept. briefed D. Palmer,

      D. Walz and R. Noble on the new solid-state modulator design

      for NLC applications. The modulator is completely compatible

      with the pulse transformer for a 5045 klystron and may serve

      as a model for the modulator needed for the Photoinjector's

      klystron. A prototype modulator is being tested at SLAC

      Sector 30. The pulse flat-top of the prototype is about

      2 microsec and is adequate for the 1.6 cell photoinjector

      risetime of 0.7 microsec. The modulator technical description

      and design drawings from R. Cassel are available at the Orion



 2. Action Items:



   -  J. Rosenzweig and D. Palmer will collaborate on HOMDYN, PARMELA,

      and TRANSPORT calculations for the RF Gun design and beam

      characteristics. A trade study of bunch charge and emittances

      around the 0.25 nC design point will follow. The goal for the

      initial design results is November 3, 2000 so these can be

      incorporated into the first draft Orion Technical Design Study

      (TDS) being compiled by R. Noble. The target date for the TDS

      first draft is Nov. 17, 2000.



    - The MRC leaders are requested to review the experimental beam

      requirements from the Feb. 2000 Orion Workshop and provide

      feed-back to the photoinjector design effort. The goal for

      completing this review is also Nov. 3 so results can be

      included in the draft TDS.



    - Towards the end of the meeting, the topic of Orion beam

      simulations was raised. Once baseline designs exist for the

      photoinjector and the low-energy and high energy transfer

      lines, a complete end-to-end, beam simulation of the Orion

      facility is needed. This will provide information about the

      output beam's sensitivity to machine errors and the production

      of particle background in the experimental halls. J. Rosenzweig

      indicated his group could perform these simulations.