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M. S. Tillack, J. E. Pulsifer, K. L. Sequoia

This technical paper discusses the development of damage-resistant final optics for laser-driven inertial fusion energy (IFE) plants. The study focuses on grazing-incidence metal mirrors and explores various fabrication techniques and materials to enhance damage resistance against laser-induced damage, x-rays, ions, contaminants, neutrons, and g-rays.

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M. S. Tillack, J. E. Pulsifer, K. L. Sequoia

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  1. Grazing-Incidence Metal Mirrors for Laser-IFE M. S. Tillack, J. E. Pulsifer, K. L. Sequoia J. F. Latkowski, R. P. Abbott 11-13 October 2004 Third IAEA Technical Meeting on “Physics and Technology of Inertial Fusion Energy Targets and Chambers”

  2. Mirror requirements: • 5 J/cm2 • 2 yrs, 3x108 shots • 1% spatial nonuniformity • 20 mm aiming • 1% beam balance The final optic in a laser-IFE plant sees line-of-sight exposure to target emissions Damage threats: • Laser-induced damage • x-rays • ions • contaminants • neutrons and g-rays

  3. We are developing damage-resistant final optics based on grazing-incidence metal mirrors The reference mirror concept consists of a stiff, light-weight, radiation-resistant substrate with a thin metallic coating optimized for high reflectivity (Al for UV, S-polarized, shallow q) Al reflectivity at 248 nm

  4. S-N curve for Al alloy Laser damage is thermomechanical in nature: high-cycle fatigue of Al bonded to a substrate Basic stability Differential thermal stress High cycle fatigue

  5. Several fabrication techniques have been explored to enhance damage resistance • Monolithic Al (>99.999% purity) • Electroplating • Thin film deposition on polished substrates • sputter coating, e-beam evaporation • Al, SiC, C-SiC and Si-coated substrates • Surface finishing • polishing, diamond-turning • magnetorheological finishing • friction stir processing • Advanced Al alloys • solid solution hardening • nanoprecipitation hardening

  6. Laser testing is performed at the UCSD laser plasma and laser-matter interactions laboratory 420 mJ, 25 ns, 248 nm

  7. Pure Al can have large grains, resulting in slip plane transport and grain boundary separation

  8. Three techniques have been attempted to improve performance of thin films • Strengthen bonding of coating to substrate • Thicken coating to prevent heating at interface • Use Al alloy substrate to eliminate differential stress Thin films require a near-perfect interface with the substrate to avoid damage (200 nm coating, 4 J/cm2, 5000 shots) Coatings between 2-5 mm survived 105 shots at 5 J/cm2

  9. Finer-grained electroplated Al withstands higher fluence, but eventually goes unstable • At 18.3 J/cm2 laser fluence: • Grain boundaries still separate • Damage is “gradual” at 18.3 J/cm2 • At 33 J/cm2 laser fluence: • Rapid onset (2 shots) • Severe damage (melting) • probably starts with grains

  10. High shot count data extrapolates to acceptable LIDT; end-of-life exposures are still needed

  11. X-rays provide thermomechanical loading similar to lasers, with deeper penetration HAPL reference direct drive target emissions (160 MJ case) • 20-80 mJ/cm2 • 3-4 keV average energy

  12. Source built by PLEX LLC: Provides x-rays from 80-150 eV Operation for ~107 pulses before minor maintenance X-ray dose can be altered by changing focus, voltage, gas pressure or species Facility is flexible and dedicated to the study of x-ray damage The XAPPER experiment is used to study damage from x-ray exposures and confirm damage is purely thermomechanical

  13. ~0.82 J/cm2 1000 shots 10,000 shots Exposures were performed at higher fluences; more prototypical high-cycle data is coming soon

  14. Mitigation of the threat from high-energy ions is possible using modest magnetic fields Low expected gas pressure (10-50 mTorr) will be unable to stop harmful target burn and debris ions (0.4-1.1 J/cm2) Ion Range (m) Fluence @ 30m (# / m2) H: 50 – 350m 7.98x1016 He: 80 – 1000m 5.31x1015 C: 50 – 150m 6.18x1014 Au: 150 – 370m 7.48x1012 DEFLECTOR was developed to determine all these ion paths

  15. A modest field surrounding the beamlines deflects nearly all of the ions without B, 99.4 % of ions, 81.4 % of energy reach final optic with 0.1 T, 1.4x10-4 % of ions, 6.1x10-3 % of energy reach final optic

  16. Contamination transport from the chamber to the final optic was modeled using Spartan Pa Displacement field after 1st shot Test particle trajectories Pressure @100 ms • Net flow toward chamber center – need to include rad-hydro displacements • Net flow toward optic – need to use pressurized beamlines as initial condition

  17. Future plans • Fabrication • Al on Al, strengthening techniques • Laser-induced damage • High cycle data, large-scale tests • X-rays • Xapper data is expected soon • Ions • Ion damage testing at LLNL to begin soon • Contaminants • Gasdynamic simulations, test mirrors in IFE devices • Neutrons • Modeling, Wait for testing opportunities

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