UV Laser-Induced Damage to Grazing Incidence Metal Mirrors

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

Mechanical and Aerospace Engineering Department andCenter 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 )

~50 cm85˚

* Sombrero and Prometheus studies, ca. 1992.

Key issues were identified for a GIMM*

• Shallow angle stability

• Damage resistance/lifetime Goal = 5 J/cm2, 108 shots

• Fabrication & optical quality

• Contamination resistance

• Radiation resistance

* R. L. Bieri and M. W. Guinan, Fusion Technology 19 (1991) 673-678.

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

cubedumpcube1/2 waveplatebeam diagnosticsdumpviewing portspecimenmount

420 mJ, 25 ns, 248 nm

In-situ monitoring helped identify the onset of damage

• Brightfield beam profiling• Darkfield beam profiling• Surface imaging

microscopy in-situ imaging darkfield

probe laserprofilermain beamdumptest specimentranslation

camera

Polycrystalline Al is easy to fabricate into a mirror, but has large grains

• 1-mm 99.999% pure Al, bonded with CA to 3-mm thick Al alloy• Polished with 5, 1, and 0.04 m alumina (Al2O3) 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/cm2

• 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/cm2

• 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 SiC300 nm Coating

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0.E+00 1.E-08 2.E-08 3.E-08 4.E-08 5.E-08 6.E-08Time, s

Temperature, K

SurfaceInterfaceSiC (0.5 um)SiC (1 um)SiC (2.5 um)SiC (5.0 um)

q”=10 mJ/cm2Al: 20-500 nmSiC: 10 • Plane stress analysis• 10 mJ/cm2 absorbed • Peak stress at interface ~40 MPa• Yield stress is 10-20 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

QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.

QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.

Thin films are delicate, damage easily and catastrophically

250 nm e-beam23,000 shots @4 J/cm2

1.5 m e-beam86,000 shots @4 J/cm2

Electroplated Al solves problems with coating thickness and large grains

• 50-100 m Al on Al-6061 substrate• 100,000 shots at 3-4 J/cm2

• 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

DemultiplexerArray

(mirrors)MultiplexerArray(beam

splitters)