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 Los Angeles, California, June 2,
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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Outline

• Review of previous ion mitigation research
  including
• the ion threat to laser optics
• the simple concept to protect them
• the modeling used to evaluate the viability of
  the this concept
• Summary of new findings about
• the threat posed by neutrals
• sputtering products
• the costs of implementing ion mitigation (money
  and power)
• Putting it all together
• Loose ends and uncertainties
• additional modeling

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Ions pose a threat to laser optics in IFE chambers

• Target heating constraints severely limit
  background Xe gas pressure
• earlier designs called for as much as 500 mTorr
• current understanding limits this to between 10
  and 50 mTorr
• Reduced gas pressures will be unable to stop
  harmful target burn and debris ions
• Ion Range (m) Fluence _at_ 30m ( / m2)
• H 50 350m 7.98x1016
• He 80 1000m 5.31x1015
• C 50 150m 6.18x1014
• Au 150 370m 7.48x1012
• Designs call for laser optics at 15 30 m from
  chamber center
• Ions may cause adverse effects necessitating
  frequent optic replacement

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Ions You cant stop them, you can only hope to
deflect them!
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DEFLECTOR was developed to determine all these
ion paths
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Modest fields can be used to deflect most (if not
all) ions
To Optic 99.4 of ions 81.4 of energy
In Gas 0.6 of ions 18.6 of energy
To Wall 0.0 of ions 0.0 of energy
NO FIELD
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Modest fields can be used to deflect most (if not
all) ions
To Optic 1.4e-4 of ions 6.1e-3 of energy
In Gas 8.5e-3 of ions 6.2 of energy
To Wall 99.99 of ions 93.8 of energy
0.1T FIELD
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Neutrals were identified as a threat not
sufficiently modeled

• Equilibrium charges were used and the effects of
  more realistic charge distributions neglected
• It was unknown if a significant fraction of the
  ions would be neutral and unaffected by magnetic
  fields
• To address these questions the neutral threat was
  evaluated in greater detail

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A conservative analysis indicates a minimal
neutral threat

• CHARGE (GSI) was used to determine the
  equilibrium neutral fraction for the lighter burn
  and debris ions (1,2,3H, 3,4He) at start of
  magnetic field (8m from center of chamber)
• When combined with the target output spectra at
  30m (after some stopping has occurred), the
  maximum possible neutral ion fluence to the optic
  is obtained

He Fluence Spectrum at 30m
He Neutral Fraction Distribution
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A conservative analysis indicates a minimal
neutral threat
He Neutral Fluence Spectrum at 30m

• Even in this impossible worst case scenario, the
  light ion fluence at the optic has been reduced
  by a factor of 100,000
• In reality, charge exchange cross sections
  indicate that no ion will be neutral over any
  significant distance (e.g., mean free path for 1
  MeV He ionization is only 45 mm in 10 mTorr Xe)
• The neutral fraction curves for Hydrogen are
  similar to those for Helium

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Heavier ions are unlikely to have significant
neutral fractions
Au Fluence Spectrum at 30m
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Wall impact sputtering products could pose an
optic threat
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
• Entire range of gold ions impact at shallow angles

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A sputtering product calculation example for gold

• DEFLECTOR calculates fluxes and angles for all
  wall impacting ions. These results can be
  coupled with SRIM calculations to predict the
  sputtering threat

• Depending on where impacts occur, all gold
  sputtering products may be stopped by the
  background gas
• Results may differ for aluminum or other beam
  tube materials
• A gas pressure gradient may be sufficient to
  flush the beam tubes of sputtering products

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The costs of implementing ion mitigation will be
reasonable

• The moderate fields required by the concept will
  require only normal copper magnets
• Example
• 0.1 T coils have a cross section of 500 cm2
• Power dissipation is 80 kW/coil ? 10 MW for
  full, 120 coil set
• Each Helmholtz pair requires 2800 kg of copper
  and costs 28K to fabricate
• Total magnet cost of 1.7M

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When summed up, ion mitigation proves an
attractive option

• Conservative analysis shows ion fluences can be
  dramatically reduced or eliminated with modest
  fields
• No exotic materials or technology are required
• Hardware placement is flexible with many workable
  variations in field size, strength, and location
• The cost of implementing this option is
  reasonable

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There are several loose ends that need to be
addressed

• Final optic standoff is not fully decided upon
  (12-30 m)
• Alternate beam-tube geometries should be
  evaluated
• Coil cross section/field strength/cost trade-off
  studies are needed
• Consider ion dump or gas pressure gradient to
  handle sputtering
• Additional magnet shielding and activation
  calculations are needed

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Summary I told you what I told you I was going
to tell you

• The threat posed by neutrals is minimal if
  nonexistent
• sputtering products
• the costs of implementing ion mitigation (money
  and power)
• Putting it all together
• The ion mitigation concept presents an attractive
  concept to protect final optics
• Loose ends and uncertainties
• additional modeling
• experimental validation