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

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

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

Ions: You cant stop them, you can only hope to deflect them!

DEFLECTOR was developed to determine all these ion paths

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

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

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

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

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

Heavier ions are unlikely to have significant neutral fractions Au Fluence Spectrum at 30m

Wall impact sputtering products could pose an optic threatHydrogenHeliumCarbon

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

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

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

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

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

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