Grazing-Incidence Metal Mirrors for Laser-IFE M. S. Tillack, J. E. Pulsifer, K. L. Sequoia J. F. Latkowski, R. P. Abbott 21-22 March 2005 US-Japan Workshop on Laser IFE San Diego, CA
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 γ-rays Prometheus reactor layout Mirror requirements: 5 J/cm 2 2 yrs, 3x10 8 shots 1% spatial nonuniformity 20 µm aiming 1% beam balance
We are developing damage-resistant final optics using 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 reflectivity S-polarized Shallow angle of incidence 1 0.8 Al reflectivity at 248 nm 0.6 Reflectivity 0.4 Ag Al 0.2 Cu Au W Hg Mo 0 200 400 600 800 1000 Wavelength, nm
Laser damage is thermomechanical: highcycle fatigue of Al bonded to a substrate Basic stability Differential thermal stress S-N curve for Al alloy High cycle fatigue
Laser testing is performed at the UCSD laser plasma & laser-matter interactions lab 400 mj, 25 ns, 248 nm QuickTime and a MPEG-4 Video decompressor are needed to see this picture. beam diagnostics viewing port dump cube 1/2 waveplate specimen mount cube dump
Several factors are known to influence mirror lifetime 1. Morphology: No surface height features >λ/4 High quality diamond turning Post-polishing 2. Microstructure: No grain structures or precipitates >λ/4 Use thin film deposition 3. Coating: No material interface within 10-20 µm of the surface Thick thin films followed by surface finishing Thin thin film on polished Al alloy (on a separate substrate) 4. Composition: Increased yield strength through alloying
1. Morphology: Pure Al is notoriously difficult to polish or turn When peak-to-valley is >50 nm, we see early failure Quality turning has been done at II-VI and Schafer > 30 µm evaporative coating on LiF Diamond turned to 6 nm rms Passed test at 10 J/cm 2, 10 4 shots Post-polishing is under investigation
2. Microstructure: Pure Al can have large grains, resulting in slip plane transport and grain boundary separation n(i) s(i) n * ( i) s * ( i) * n(i) s(i) n(i) s(i) n(i) s(i) C
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 nearly acceptable LIDT in electroplated Al End of life exposures are needed to confirm this
A power plant temporal pulse is more damaging than our simulation sources σ~t~t 1/2 Compex KrF laser At the same fluence, stress is ~double We need a 10 J/cm 2 optic at 25 ns, plus an additional safety factor
3. Coatings: Three techniques have been attempted to improve performance of thin films Thin films on SiC require a near-perfect interface with the substrate to avoid damage (200 nm coating, 4 J/cm 2, 5000 shots) Strategies for improvement: Strengthen bonding of coating to substrate Thicken coating to prevent heating at interface Use Al alloy substrate to eliminate differential stress (Al on Al)
4. Alloying: We hope that solid solution or nanoprecipitated alloys can be fabricated into high-quality optics Mirrors will be sputtered from: Al + 3%Cu and Al + 3%Zn These were chosen for high yield strength in the annealed state: pure: 20 MPa 2024: 97 MPa 7075: 145 MPa We rejected 1000, 3000 and 5000 because their strength comes from cold working Alloy Main Alloying Typical uses Series Elements 1000 Pure Al Mirrors pure Al 2000 Cu High strength alloy used in the aerospace industry 3000 Mn Low- to medium-strength alloys, used in beverage cans and refrigeration tubing 4000 Si Most mostly welding or brazing filler materials 5000 Mg Structural applications in sheet or plate metals - weldable 6000 Mg and Si Heat treatable and commonly used for extrusions, can be crack sensitive. 7000 Zn High strength aerospace alloys that may have other alloying elements added
X-rays provide thermomechanical loading similar to lasers, with deeper penetration HAPL reference direct drive target emissions (160 MJ case) 50 mj/cm 2 3-4 kev average energy
The XAPPER experiment is used to study damage from x-ray exposures and confirm that damage is purely thermomechanical Source built by PLEX LLC: Provides x-rays from 80-150 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 Z-pinch plasma Ellipsoidal condenser Sample plane
Exposures were performed at higher fluences; lower fluence, high-cycle data is coming soon ~820 mj/cm 2 1000 shots 10,000 shots
Polished aluminum disks have been irradiated with 3 MeV alphas Initial surface roughness of this dummy mirror was 11.7±1.8 nm Mirror was irradiated to 2.8 x 10 17 α/cm 2 in three locations: Angles normal to mirror surface, 60 from normal, 78 from normal Beam current held ~constant, so different angles result in different fluxes Irradiation times were adjusted to yield the same fluence for each spot normal (6 x 6 mm beam) 78 60 Fluence is equivalent to 1-4 days of operation for a mirror at 15 or 30 meters
Spectrophotometry indicates no significant change in reflectivity This is a good result in that we do not see significant damage Based upon these results, we are unable to determine if normal angle irradiation is equivalent to grazing incidence irradiation Surface roughness is unchanged 100 80 Normal Reflectivity (%) 60 40 0º (normal) 20 60º 78º Unexposed 0 200 400 600 800 Wavelength (nm)
Mitigation of the high-energy ion threat 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/cm 2 ) Ion Range (m) Fluence @ 30m (# / m 2 ) H: 50 350m 7.98x10 16 He: 80 1000m 5.31x10 15 C: 50 150m 6.18x10 14 Au: 150 370m 7.48x10 12 DEFLECTOR was developed to determine all these ion paths
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
Future plans Fabrication Thick films, Al on Al, strengthening techniques Laser-induced damage High cycle data, large-scale tests X-ray damage studies High cycle data Ion damage studies Continued ion damage testing at LLNL Neutron damage studies Limited specimen testing at HFIR
Backup Slides
The KrF driver uses multiplexing, implying ~3000 small GIMMlets 64 beams 50 beamlets per beam 13x13 cm beamlet aperture LONG PULSE AMPLIFIER (~ 100's nsec) Last Pulse Multiplexer Array (beam splitters) First Pulse Demultiplexer Array (mirrors) Target FRONT END ( 20 nsec) Only three pulses shown for clarity
Fabrication techniques have been explored to determine 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 Thick film deposition Surface finishing diamond-turning polishing, MRF Advanced Al alloys solid solution hardening nanoprecipitation hardening
The steps to develop a final optic for a Laser IFE power plant (1 of 2) 1. Choose a candidate final optic Al coated SiC GIMM: UV reflectivity, industrial base, radiation resistance Key Issues: Shallow angle stability Laser damage resistance goal = 5 J/cm 2, 10 8 shots Contamination Optical quality Fabrication Radiation resistance 2. Characterize threats to the mirror: LIDT, radiation transport, contaminants 3. Perform research to explore damage mechanisms, lifetime and mitigation Bonding/coating Microstructure Fatigue Ion mitigation Al: 20-500 nm SiC: 10 µm q =10 mj/cm 2
The steps to develop a final optic for a Laser IFE power plant (2 of 2) 4. Verify durability through exposure experiments 10 Hz KrF laser UCSD (LIDT) XAPPER LLNL (x-rays) ion accelerator, LLNL neutron modeling and exposures 5. Develop fabrication techniques and advanced concepts 6. Perform full-scale testing