The RAL Front End Test Stand

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The RAL Front End Test Stand
Alan Letchford CCLRC RAL ISIS Injector
Group Joint Accelerator Workshop 2829 March
2006
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The Front End Test Stand (FETS) is a
collaborative project, between CCLRC and UK
universities, to design and construct a test
stand for demonstrating key technologies for the
front end of next generation high power
accelerators. By working closely with young
university teams it is hoped to pass on the
wealth of knowledge and experience of CCLRC staff
to the next generation of accelerator
engineers/physicists. The FETS team has members
from ASTeC and ISIS as well as the physics
departments of Imperial College London and
Warwick University.
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The FETS team
John Back (Warwick) Aaron Cheng (Imperial) Mike
Clarke-Gayther (ISIS) Dan Faircloth (ISIS) Simon
Jolly (Imperial) Ajit Kurup (Imperial) David Lee
(Imperial) Alan Letchford (ISIS) Ciprian
Plostinar (ASTeC) Jürgen Pozimski
(ASTeC/Imperial) Peter Savage (Imperial) Chris
Thomas (ISIS)
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• Proton beam powers in the MW range are called for
  in many applications eg
• Spallation Source
• Neutrino factory
• Waste transmutation
• A big challenge for these future machines is
  controlling beam loss. Beam loss leads to
  component activation component activation
  hinders hands-on maintenance.
• Absolute loss levels in the new machines (1 10
  MW beam power) must be similar to that on ISIS
  (160 kW beam power). Fractional loss must
  therefore be reduced by orders of magnitude.

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A significant source of beam loss in circular
accelerators (synchrotron/accumulator/compressor)
is trapping.
Some of the beam is not trapped by the RF.
Untrapped beam leads to loss.
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Chopping the beam in the linac can lead to near
100 trapping efficiency.
Chopping the linac beam gives little or no beam
in the gap.
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The linac macro pulse is composed of beam bunches
at the linac RF frequency typically 100s MHz.
To achieve perfect chopping a very high speed (lt2
ns) chopper is required
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• The purpose of the RAL Front End Test Stand is to
  demonstrate high quality chopped beams of H-
  ions.
• 60 mA
• 3 MeV
• up to 2 ms pulse length
• up to 50 pps repetition rate

The RF frequency choice is important and was the
subject of prolonged discussion. Too high and
perfect chopping isnt feasible. Too low and its
not a credible demonstrator for future
machines. A high power source must be available.
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A frequency of 324 MHz has been chosen for
FETS. A suitable high power pulsed klystron is
available off-the-shelf from Toshiba - E3740A,
3 MW peak power.
A klystron has been ordered for delivery in the
3rd quarter of 2006.
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• FETS main components
• High brightness H- ion source.
• Magnetic Low Energy Beam Transport (LEBT).
• High current, high duty factor Radio Frequency
  Quadrupole.
• Very high speed beam chopper.
• Comprehensive diagnostics.

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Dan Faircloth Peter Savage Chris Thomas
Ion Source
FETS will use the world leading ISIS H- Penning
type ion source. This source routinely produces
gt40 mA in 250 ?s pulses at 50 pps. An extensive
ongoing development programme will increase the
duty factor and fully characterise the beam.
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Recent work has succeeded in producing 1.2 ms, 35
mA pulses at 50 pps.
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A series of Oxford university student projects
has resulted in energy spread measurements of
the H- beam.
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A pepper-pot system will measure the beam
emittance as a function of position downstream of
the source. Provides data for the LEBT design and
information about space charge/neutralisation.
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John Back
Magnetic LEBT
A 3 solenoid magnetic LEBT is being designed,
based on the one successfully used on the ISIS
RFQ pre-injector upgrade. An electrostatic LEBT
has been rejected due to the close proximity of
the caesiated ion source.
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Optimisation of the magnetic design is nearing
completion.
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An initial beam optics design has been completed.
Further optimisation will be carried out based on
the results of the pepper-pot measurements.
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Aaron Cheng Ajit Karup Alan Letchford Peter Savage
324 MHz RFQ
A 0.5 m, 324 MHz 4-vane RFQ cold model design is
almost complete. Machining tests have been
successfully completed in Aluminium. The cold
model contains all the significant features of
the final 4 m long design.
Cold model production will start very soon
delayed by machine tool problems at IC. Tests of
the brazing technique were completed successfully.
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Final optimisation of some details such as the
vane end cut-aways is all that remains to be
done.
A 4 rod RFQ design is also being investigated.
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A computer controlled bead pull system is under
development to perform cavity field measurements
on the cold models
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An initial beam dynamics design has been
completed for the FETS 324 MHz RFQ. Negligible
emittance growth and 94 transmission for a 60 mA
beam. Further optimisation is expected.
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Giulia Bellodi Mike Clarke-Gayther Frank Gerigk
Ciprian Plostinar
High speed beam chopper
A novel tandem chopper technique has been
developed at RAL to overcome the conflicting
requirements of fast rise time (lt 2ns) and long
flat-top (up to 100 ?s). A fast chopper
creates a short, clean gap in which a slow
chopper can switch on. The fast pulser is limited
in flat-top but can switch between bunches. The
slow pulser cannot switch between pulses but can
generate the required flat-top.
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A state of the art fast switch developed for RAL
has achieved 1.4 kV with rise and fall times
less than 2 ns.
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A trimmable helical structure with adjustable
delays replaces the more typical meander line.
Adjustable L-C trimmer
Adjust cable lengths to change delay
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Three chopper line optics designs are under
investigation. A short line keeps the emittance
growth low but makes chopping harder and requires
some challenging technology. A long line is
easier but controlling the emittance is more
challenging.
The short ESS solution will require novel,
compact, high gradient quadrupoles and DTL-like
cavities. Hybrid permanent and electromagnetic
quads are being investigated.
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David Lee Jürgen Pozimski (Christoph Gabor)
Diagnostics
Accurate diagnostics are essential for FETS to be
successful. In addition to more usual diagnostic
devices, non-destructive laser stripping
techniques will be employed.
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SNS laser beam profile monitor system.
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IAP Frankfurt laser transverse emittance
measurement system.
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Timescales
A fairly aggressive schedule has been set Ion
source commissioned by beginning 2007. LEBT
commissioned by end 2007. RFQ commissioned by end
2008. MEBT/Chopper commissioned by mid-end
2009. Diagnostics commissioned in parallel. A
tough timescale to meet for our small group and
dependent on appropriate resources.