High Pressure RF Studies

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High Pressure RF Studies

• Mahzad BastaniNejad
• Muons, Inc.,
• Old Dominion University

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Outline

• HPRF Breakdown studies
• Paschen Curve Measurment
• Metallic Breakdown
• Magnetic field dependence
• Electrode Breakdown Analysis
• Surface Characterization
• Fowler Nordheim behavior
• Plans for beam tests
• Mark II (old test cell)
• Mark III

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Pressurized High Gradient RF Cavities

• Copper plated stainless-steel, with GH2 to 1600
  psi 77 K, Hemispherical 1 Electrodes, 800 MHz
  test cell in MTA

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MuCool Test Area (MTA)
5T Solenoid
Wave guide to coax adapter
Pressure barrier
Mark II Test Cell
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HPRF Test Cell Measurements in MTA
Electrode breakdown region

• Paschen curve verified
• Maximum gradient limited by breakdown of metal.
• Cu and Be have same breakdown limits (50 MV/m),
  Mo(63MV/m), W(75MV/m).
• Results show no B dependence, much different
  metallic breakdown than for vacuum
• cavities. Need beam tests to prove HPRF works.

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Electrode Breakdown Analysis in High Pressure
Cavities Scanning Electron Microscope Images of
Electrodes
Beryllium
Molybdenum
Tungsten
Schematic of the mapped Electrode
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Breakdown Data Analysis
Mo
W
Be
Electrons quantum mechanical tunnelling through a
barrier in presence of electric field
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800 MHz Test Cell (MarkII) is not ideal for Beam
tests

• Designed for gas and material breakdown studies
  not beam studies.
• Difficult to change frequency for any Pressure
• Need to disassemble TC to change shims under
  electrodes to change capacitance.
• Need to synchronize Linac and cavity frequency
  and phase
• Pressure changes frequency (by dielectric of
  hydrogen)
• Walls are thick, so the 400 MeV proton beam is
  degraded.

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Model of Beam in Old Test Cell
Half Test Cell
Beam is degraded with 2 of Stainless Steel
(Rakhno).
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HPRF in an Intense Beam

• The current Pressurized RF Test Cell will be used
  in the first beam test.
• We are studying ideas to mitigate the unwanted
  behavior of the test cell.
• A new test cell design is under study

• Designed for beam testing.
• For testing ionization recombination models (A.
  T.)
• Allow clearing electrodes to be installed to
  prevent the accumulation of charges.
• Another design implementation is to install
  grids to test the idea of gas-cooled RF windows
  for higher accelerating gradients.

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Concept of the Next Test Cavity
800 MHz Pressurized RF Cavity with irises and
clearing electrodes

• Conceptual schematic of a test cell design with
  irises and clearing electrodes
• To mitigate the effects of electron and ion
  charge build up.
• The strong magnetic field will confine the
  protons and electrons from the ionized
• hydrogen energy absorber to the central region
  of the cavity, away from the irises.
• Electrons that do not recombine can be absorbed
  in the clearing electrodes.

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Another concept being developed to allow remote
tuning
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HPRF Studies Summary

• Paschen Curve behavior tested for 800 MHz for
  first time with high density hydrogen
• Dense gas suppresses dark currents and
  multipacting.
• HPRF does not degrade in magnetic fields.
• Very fast conditioning
• New way to study RF breakdown discovered.
• Fowler-Nordheim (quantum mechanical) explanation
• Development and Demonstration of HPRF in
  conditions for muon cooling under way
• We really need the MTA beamline!