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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