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

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

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 IonRange 30m (# / m 2 ) –H: 50 – 350m 7.98x10 16 –He: 80 – 1000m5.31x10 15 –C:50 – 150m6.18x10 14 –Au:150 – 370m7.48x10 12 –Designs call for laser optics at 15 – 30 m from chamber center Ions may cause adverse effects necessitating frequent optic replacement

Ions: You can’t stop them, you can only hope to deflect them! Final Optic Magnetic Field Fusion Chamber Ion Paths Beam Tube Wall Background Gas Helmholtz Coil

DEFLECTOR was developed to determine all these ion paths DEFLECTOR Ion Charge States Stopping in Background Gas B Field Interactions B Field Specs Beam Tube Geometry Target Ion Specta SRIM Stopping Tables Simulation Resolution Impact Energies Impact Positions Total Fluence to Wall and Optic Impact Angles

Modest fields can be used to deflect most (if not all) ions In Gas: 0.6 % of ions 18.6 % of energy To Wall: 0.0 % of ions 0.0 % of energy To Optic: 99.4 % of ions 81.4 % of energy NO FIELD

Modest fields can be used to deflect most (if not all) ions In Gas: 8.5e-3 % of ions 6.2 % of energy To Wall: % of ions 93.8 % of energy To Optic: 1.4e-4 % of ions 6.1e-3 % of energy 0.1T 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 ( 1,2,3 H, 3,4 He) 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

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  m in 10 mTorr Xe) The neutral fraction curves for Hydrogen are similar to those for Helium He 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 threat HydrogenHelium Carbon Gold

Sputtering is enhanced for grazing incidence impacts Gold 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

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: Gold Ion Energy (MeV) > 88 o Yield for Iron (atoms/ion) Average Atom Energy (keV) Range in 10mTorr Xe (m) Number of Sputtered Atoms x x x x x x x x10 9 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

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 cm 2 –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

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

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

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