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

The final optic in a laser-IFE plant sees line-of-sight exposure to target emissions Laser-induced damage x-rays ions contaminants neutrons and  -rays Damage threats: 5 J/cm 2 2 yrs, 3x10 8 shots 1% spatial nonuniformity 20  m aiming 1% beam balance Mirror requirements:

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  ) Al reflectivity at 248 nm

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

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

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

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

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/cm 2, 5000 shots) Coatings between 2-5  m survived 10 5 shots at 5 J/cm 2

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

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

X-rays provide thermomechanical loading similar to lasers, with deeper penetration  mJ/cm 2  3-4 keV average energy HAPL reference direct drive target emissions (160 MJ case)

The XAPPER experiment is used to study damage from x-ray exposures and confirm damage is purely thermomechanical Source built by PLEX LLC: –Provides x-rays from eV –Operation for ~10 7 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

Exposures were performed at higher fluences; more prototypical high-cycle data is coming soon 1000 shots10,000 shots ~0.82 J/cm 2

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 ( J/cm 2 ) DEFLECTOR was developed to determine all these ion paths 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

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

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

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