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Ion Mitigation for Laser IFE OpticsRyan Abbott, Jeff Latkowski,
Rob SchmittHAPL Program WorkshopLos Angeles, California, June 2,
2004This 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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OutlineReview of previous ion mitigation research includingthe
ion threat to laser opticsthe simple concept to protect themthe
modeling used to evaluate the viability of the this concept
Summary of new findings aboutthe threat posed by
neutralssputtering productsthe costs of implementing ion mitigation
(money and power)
Putting it all together
Loose ends and uncertaintiesadditional modeling
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Ions pose a threat to laser optics in IFE chambersTarget heating
constraints severely limit background Xe gas pressureearlier
designs called for as much as 500 mTorrcurrent understanding limits
this to between 10 and 50 mTorr
Reduced gas pressures will be unable to stop harmful target burn
and debris ionsIonRange (m)Fluence @ 30m (# / m2)H: 50 350m
7.98x1016He: 80 1000m5.31x1015C:50 150m6.18x1014Au:150
370m7.48x1012Designs 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 In
Gas: 0.6 % of ions18.6 % of energyTo Wall: 0.0 % of ions0.0 % of
energyTo Optic: 99.4 % of ions81.4 % of energyNO FIELD
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Modest fields can be used to deflect most (if not all) ions In
Gas: 8.5e-3 % of ions6.2 % of energyTo Wall: 99.99 % of ions93.8 %
of energyTo Optic: 1.4e-4 % of ions6.1e-3 % of energy0.1T 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 30mHe Neutral Fraction Distribution
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A conservative analysis indicates a minimal neutral threat 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 HeliumHe Neutral
Fluence Spectrum at 30m
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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
threatHydrogenHeliumCarbon
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Sputtering is enhanced for grazing incidence impactsStiff 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
incidenceGold ions illustrate this well
Entire range of gold ions impact at shallow angles
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A sputtering product calculation example for goldDEFLECTOR
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
Gold Ion Energy (MeV)Impacts @ > 88oYield for Iron
(atoms/ion)Average Atom Energy (keV)Range in 10mTorr Xe (m)Number
of Sputtered Atoms0 - 51.84x10111203.00.342.21x10135 -
202.02x10121604.50.413.13x101420 - 353.67x10101505.00.435.39x101235
- 505.92x1061805.00.431.07x109
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The costs of implementing ion mitigation will be reasonableThe
moderate fields required by the concept will require only normal
copper magnets
Example0.1 T coils have a cross section of 500 cm2Power
dissipation is ~80 kW/coil 10 MW for full, 120 coil setEach
Helmholtz pair requires ~2800 kg of copper and costs ~$28K to
fabricateTotal magnet cost of ~$1.7M
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When summed up, ion mitigation proves an attractive
optionConservative 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 addressedFinal
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 youThe
threat posed by neutrals is minimal if nonexistentsputtering
productsthe costs of implementing ion mitigation (money and
power)
Putting it all togetherThe ion mitigation concept presents an
attractive concept to protect final optics
Loose ends and uncertaintiesadditional modelingexperimental
validation