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Join John Pulsifer and Mark Tillack at the HAPL Project Meeting in Santa Fe, NM on April 8-9, 2008, as they aim to test alloy mirrors at high shot counts. Learn about the challenges and progress in achieving goals of 5 J/cm2 and 3x108 shots. Discover the impact of homogenization on data quality and mitigation strategies for contamination. Explore findings on damage morphology, absorption issues, and the mirror's reflectivity. Future plans include addressing contamination concerns, improving reflectivity, and exploring alternative alloy compositions. Be part of the discussion on resolving high-cycle testing challenges and advancing laser damage resistance research.
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GIMM testing to millions of shots… and beyond John Pulsifer and Mark Tillack HAPL Project Meeting 8-9 April 2008 Santa Fe, NM
Our current goal is to test alloy mirrors at very high shot count to complete our proof of principle demonstration of laser damage • Challenges of high-cycle testing • Need high quality mirrors • Goal of 5 J/cm2, 3x108 shots • Need a reliable, high quality laser • Need control over the environment • Need time – a lot of time (months) • Status • Al-1%Cu is our current standard due to better performance than alumiplate • Fabricated in the nano3 lab at UCSD by sputter coating on Si • Typical test runs of 107 shots, fluence limited by homogenizer • Contamination has become an issue for high cycle testing
Previous “future plans” andprogress since Oct. 2007 Plans Progress Achieve higher fluence with a different homogenizer (larger aperture, AR-coated) Identify and eliminate chamber contaminant source Continue shooting to achieve 108 shots Peak of 4 J/cm2 now possible with homogenized beam. Homogenization is essential for high quality data. Source term better understood. Near-term solution: Ne in chamber. Has implications for a power plant. Data acquired in the range of 107 New damage morphology discovered. New issue with absorption.
Contamination has become a major issue with longer-term exposures • The problem: • Coatings on windows and on test sample (offsetting) • Not seen previously due to differences in exposure time and vacuum • Appears under hard vacuum (10–7 Torr), even with a cryopump • Not unique – problems exist in space telescopes (e.g. JWST) From top to bottom: 1 million shots @ ≤1.4 x 10-7 Torr 16.1 million shots @ ≤1.1 x 10-6 Torr 1.4 million shots @ ≤1.2 x 10-5 Torr Entrance window Test mirror
Spectroscopic analysis was performed on a W witness plate to determine impurities Electron spectroscopy for chemical analysis • Source term was identified: • Pump oil was found in cryo head • ESCA identified C and O • Near-term solution: • Chamber baking • Ne background gas in chamber • A scroll pump might help • Implications for a power plant, and possible future R&D: • Our chamber is more controlled than a power plant • A power plant will have a continuous source term from targets • This issue needs further study!
Facility improvements enable us to obtain higher quality data • Homogenizer • Addition of AR coatings helped (increased from 3 to 4 J/cm2) • Combined w/ contamination control, we can maintain 4 J/cm2 • We can now perform uniform exposures over 10 mm2 area • Chamber baking • 30 ˚C for 13.5 hrs • Base pressure improved: • to <10–7 from <10–4 Torr • Leak rate improved: • to 7 x 10–7 from 1 x 10–5 Pa-m3/s • LPX: • no problems since • power supply repairs
Latest results of high-cycle testing:6.7 million shots at 4 J/cm2 unhomogenized homogenized
Damage morphology in Al-1%Cu at ~ 6x106 shots consists of triangular features m119, 3 J/cm2; failure @ 5,770,860 shots Laser direction m120, 4 J/cm2; failure @ 6,734,665 shots
Triangular damage features at ~ 6x106 shots Laser direction • This is the 1st known observation of this phenomenon. • Is it debonding? Can it be prevented?
Latest results of high-cycle testing:Damage resistance is lower than expected • The alloy’s better low-cycle damage resistance is not maintained at high shot count • Is it fundamental to metal mirror damage? • Similar degradation observed with DT vs. CMP • Finish seems more important at low cycle, inherent microstructure at high cycle • Al-1%Cu is stronger – why doesn’t it exhibit better high-cycle behavior? • Contamination may have compromised earlier data. • Poor polarization purity may be affecting our results • Could be the result of homogenization • Every point receives peak fluence every shot • If homogenization is the reason, we need to diagnose and eliminate weak spots in the mirrors
Mirror reflectivity is lower than expected • 97% specular reflectivity in Al-1%Cu (and Alumiplate) at 85˚ using the excimer laser (in-situ) with current optics • Part of this may be non-specular scattering • Imperfect polarization may be the cause • Need to revisit this and demonstrate that our mirrors have acceptable absorptivity
Summary and Future Plans • Data have been obtained up to 6.7x106 shots withhomogenized fluence levels up to 4 J/cm2 over 10 mm2 • Contamination appears to be a larger concern than previously understood. More study is recommended. • High cycle data is discouraging. We need to resolve this. • We need to resolve the polarization/reflectivity issue first • We need to understand the source of triangular damage. • We would like to try making/testing a 0.5%Cu alloy • Limitations imposed by the homogenizer limit our database • Take more unhomogenized data to fill in the curves? • Simulate higher fluence data with lower angles (e.g. 80˚)?
