Ion Mitigation for Laser IFE Optics

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Ion Mitigation for Laser IFE Optics
Ryan Abbott, Jeff Latkowski, Rob Schmitt HAPL
Program Workshop Atlanta, Georgia, February 5,
2004
This work was performed under the auspices of the
U.S. Department of Energy by the University of
California, Lawrence Livermore National
Laboratory under contract No. W-7405-ENG-48
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Burn and debris ions are a threat to final optics

• Low chamber pressures result in significant ion
  fluences to final optics that could induce
• sputtering
• roughening, bubbles
• changes in optical properties
• Current design calls for 10 mTorr Xe

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The ion mitigation concept protect optics with
modest B fields
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The DEFLECTOR code predicts ion paths through
chamber
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DEFLECTOR data can be plotted to visualize
multiple effects

• Three dimensional impact and stopping positions
  (wall impacting ions with a 0.050 T field)

• Trend plots (He impact angle characteristics)

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Modest fields are can deflect ions away from
final optics

• A 0.125T field reduces the total energy incident
  upon a final optic at 20 m by a factor of 40,000.
  A 0.150T field can deflect all target burn and
  debris ions.

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Magnets would have modest power requirements
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Ions impacting the beam tube wall may cause
sputtering
Hydrogen
Helium
Carbon
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Sputtering is enhanced for grazing incidence
impacts

• Stiff ions (high mass, high energy) are more
  weakly influenced by the magnetic field
• Ions have initial trajectories parallel to tube
  walls stiff ions are only perturbed a minor
  amount ? strike at grazing incidence.

• Gold ions illustrate this well
• Because the entire energy range of gold ions
  impact at shallow angles, this species may
  present a serious sputtering threat

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A sputtering product calculation example for gold

• DEFLECTOR calculates fluxes and angles for all
  wall impacting ions. This will allow coupling
  with SRIM to predict the sputtering threat

• Depending on where the impact occurs, most or all
  gold sputtering products may be stopped by the
  background gas
• Results may differ for Aluminum or other beam
  tube materials
• A gas puff may be required to flush the beam
  tubes of sputtering products

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DEFLECTOR will determine sputtering yield and
transport
SRIM Sputtering Tables
Sputtering products and propagation in background
gas toward optic
Total Ion / Sputter threat to final optics
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Further work

• Fully characterize sputtering threat
• Account for ion charge state distributions and
  production of neutrals
• Investigate alternate beam tube geometries to
  prevent ion backscatter
• Detailed magnet designs
• Neutronics analyses
• Couple DEFLECTOR RadHeat to characterize
  beam-tube and optic heating

TART Model of Sombrero Chamber