The ISIS RFQ

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The ISIS RFQ

• Alan Letchford
• ISIS Injector Group

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The ISIS Pre-Injector upgrade

• The Radio Frequency Quadrupole is an upgrade for
  the Pre-Injector of the ISIS 70 MeV Injector
  Linac.
• The 202.5 MHz Drift Tube Linac injects 20 mA
  peak, 200 uA average of H- into the ISIS
  synchrotron for acceleration to 800 MeV.
• 4 accelerating tanks
• 50 m length
• 7 MW of peak RF
• 100 quadrupole focussing magnets

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The 70 MeV Drift Tube Linac
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The 70 MeV Drift Tube Linac
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The old Pre-Injector
-665 kV DC platform and accelerating column. 10
stage Cockroft-Walton multiplier stack. Storage
capacitor and current limiting resistor.
Ion source and power supplies inside
platform. Bouncer compensates for droop.
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The Low Energy Drift Space

• Quadrupole triplets for focussing and matching.
• Steering magnets.
• Emittance and beam current diagnostics.
• RF buncher cavity.
• Vacuum pumping.
• Beam-stop.

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What is an RFQ accelerator?
The principles of operation of the RFQ where
first presented by two scientists from ITEP
Moscow in the former Soviet Union (now Russia)
I.M. Kapchinsky V.A. Teplyakov Linear ion
acceleration with spatially homogeneous strong
focussing Prib. Tekh. Eksp. No.2 pp19-22 (1970)
First practical RFQ was realised by LANL (then
LASL) in 1980. Much development occurred around
this time in the USA driven by SDI (Star Wars)
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The Quadrupole Lens
Magnetic Quadrupole Lens
Electric Quadrupole Lens
Focusing in one plane, de-focusing in the other
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Radio Frequency Quadrupole
Horizontal Vertical
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RFQ with acceleration
Vertical electrodes
Horizontal electrodes
Axial acceleration field
Quadrupole focusing field
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ISIS RFQ structure
Electrode detail
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The ISIS Radio Frequency Quadrupole

• 665 keV, 202.5 MHz, 4-rod, H- RFQ
• A collaboration between RAL ISIS and IAP
  Frankfurt J.W.Goete University
• 90 kV inter-electrode voltage, 200 kW peak RF
  Power
• Up to 50 mA H-
• Input energy 35 keV, 3 solenoid matching
• All systems commissioned and soak-tested in
  dedicated test facility. 2000 hours of operation.

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The ISIS Radio Frequency Quadrupole
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The ISIS Radio Frequency Quadrupole
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The ISIS Radio Frequency Quadrupole
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The RFQ Test Facility
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The RFQ Test Facility
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First Beam.
Achieved 95 transmission for gt30 mA beams within
hours
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Beam Emittance665 keV, 30 mA, 50 Hz beam
Calculated
Measured
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Beam Energy
Atomic gas scattering energy analyser High mass
gas (Xe) attenuates beam to a few particles per
pulse
High mass gas minimises energy change through
scattering Cs137 calibration source gives high
absolute accuracy
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Beam Energy
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Bunch Length
Fast Faraday cup target. 50O tapered coaxial
line Water-cooled inner
High quality heliax output cable 2 GHz digital
oscilloscope
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Bunch Length
High beam power and small spot size 500 W/cm2
or more Minimal water cooling ...
Predictable results
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Bunch Length
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ISIS RFQ installation
Many systems installed prior to
shutdown Shutdown started 13th April 2004