UV Laser-Induced Damage to Grazing Incidence Metal Mirrors M. S. Tillack, J. E. Pulsifer, K. Sequoia Mechanical and Aerospace Engineering Department and Center for Energy Research 3rd International Conference on Inertial Fusion Science and Applications Monterey, CA 9 September 2003

Design concept for a power plant GIMM * The reference mirror concept consists of stiff, light-weight, radiation-resistant substrates with a thin metallic coating optimized for high reflectivity (Al for UV, S-pol, shallow  ) * Sombrero and Prometheus studies, ca

Key issues were identified for a GIMM * Shallow angle stability Damage resistance/lifetime Goal = 5 J/cm 2, 10 8 shots Fabrication & optical quality Contamination resistance Radiation resistance * R. L. Bieri and M. W. Guinan, Fusion Technology 19 (1991) S-N curve for Al alloy

We tested several Al fabrication options Thin films on superpolished substrates –CVD SiC, 2-3Å roughness, 2-3 nm flatness over 3 cm –magnetron sputtering up to 250 nm –e-beam evaporation up to 2  m Solid polycrystalline metal –polished –diamond-turned Electroplated and turned Al

Testing was performed with 25-ns, 248-nm pulses in a controlled environment 420 mJ, 25 ns, 248 nm

In-situ monitoring helped identify the onset of damage Brightfield beam profiling Darkfield beam profiling Surface imaging microscopyin-situ imagingdarkfield camera

Polycrystalline Al is easy to fabricate into a mirror, but has large grains 1-mm % pure Al, bonded with CA to 3-mm thick Al alloy Polished with 5, 1, and 0.04  m alumina (Al 2 O 3 ) suspension, or Diamond-turned on precision lathe (at GA target fab facility) ~ 25 nm avg. roughness

Polished Al damages due to plastic deformation mechanisms 500 X Exposed for 100 shots in vacuum at 2–5 J/cm 2 Grain boundaries separate Slip lines extrude within grains 500 X

Diamond-turned Al exhibits superior damage resistance Exposed for 50,000 shots in He at 3–4 J/cm 2 No obvious damage Minimal (if any) grain boundary separation Polishing appears to introduce impurities and pre-stress the grain boundaries

Thin film deposition is limited by coating thickness and surface defects Thermal stress, constraints on thickness Added complexity of substrate and film requirements 1  m coating of Al on SiC Plane stress analysis 10 mJ/cm 2 absorbed Peak stress at interface ~40 MPa Yield stress is MPa

Good coatings were obtained using superpolished CVD SiC substrates Superpolished CVD SiC: 2-3 Å smooth, 2-3 nm flat Thin film deposition of Al by magnetron sputtering and/or e-beam evaporation Up to 2  m Al has been successfully deposited by e-beam

Thin films are delicate, damage easily and catastrophically 250 nm e-beam 23,000 J/cm  m e-beam 86,000 J/cm 2

Electroplated Al solves problems with coating thickness and large grains  m Al on Al-6061 substrate 100,000 shots at 3-4 J/cm 2 No discernable change to the surface

Summary Survival above 100,000 shots has yet to be demonstrated in thin film coatings – damage occurs due to imperfections and high interfacial stresses. Thicker coatings appear to be more robust, but detrimental effects of grain structures must be avoided. Thick (>50  m) electroplated Al on SiC provided the best damage response, due to thickness of coating and small grains. Scale-up and further testing are planned.

Acknowledgements Thanks to the following for their advice and technical support: Jim Kaae et. al, General Atomics microfabrication facility Ed Hsieh et. al, Schafer Corp. Lee Burns, Rohm & Haas Co. Advanced Materials Witold Kowbel, MER Corp. Larry Stelmack, PVD Products, Inc. John Sethian and the members of the High Average Power Laser Program This work was funded by US DOE/DP NNSA

Optic scale-up: multiplexed beams enable smaller, more tolerant final optics Target FRONT END ( 20 nsec) LONG PULSE AMPLIFIER (~ 100's nsec) Only three pulses shown for clarity Last Pulse First Pulse Demultiplexer Array (mirrors) Multiplexer Array (beam splitters)