Minutes of the Special Meeting on Orion RF Gun Parameters:

Oct. 12, 2000

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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

downstream.

- 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

website.

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.

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