Progress on GIMM Fabrication & Testing

1. Progress on GIMM Fabrication & Testing M. S. Tillack, J. Pulsifer, K. Sequoia High Average Power Laser Program Project Meeting University of Wisconsin – Madison 24–25 September 2003

2. Background (1): GIMM design concept 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 q)

3. Background (2):Key Issues • Shallow angle stability• Damage resistance/lifetimeGoal = 5 J/cm2, 108 shots• Fabrication & optical quality• Contamination resistance• Radiation resistance

4. When last we met... • Defects on thin-film mirrors were plaguing us. • Schafer Al coatings on superpolished SiC showed promise, but pin-point defects and darkening were observed. • Some of these surfaces operated over long periods of time after surface changes occurred. Extended damage studies were planned. • Overcoating the Al to eliminate oxide effects was considered. • Monolithic Al mirrors provided good resistance previously. More testing of polished and diamond-turned Al, as well as Al-coated Al and novel Al microstructures were considered.

5. What we’ve done... • Continued to work with Schafer to improve coatings, and MER to develop substrates (see posters). • Resolved the issue of “darkening”: • Built a new chamber with cryopump. • While waiting for the new chamber, used He and Ne backfill to eliminate pump oil decomposition. • Extended the testing to shot counts up to 100,000. • Tested more GA diamond-turned Al. • Obtained and tested electroplated mirrors. • Started to explore scale-up issues.

6. Summary of Schafer collaboration • Source of pin-point defects identified; defect-free substrates yielded defect-free coatings. • Reactive oxidation used to overcoat Al in-situ. • Stripping and recoating successfully demonstrated. • Scale-up pathway 31550 cm identified. mirror #41, s/n 10157-024 50 nm sputter+1 mm e-beam 500 shots at 5 J/cm2

7. A new vacuum chamber was built mirror #38, s/n 10157-021 100 nm sputter+2.0 mm e-beam 5.0 J/cm2 for 1000 shots • Cryopumped for higher purity • Added flexibility in sample manipulation • Improved diagnostic access

8. In-situ monitoring helps us identify the onset of damage camera • Brightfield beam profiling• Darkfield beam profiling• Surface imaging microscopy in-situ imaging darkfield

9. Testing continues... • 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 mm• Solid polycrystalline metal – polished – diamond-turned• Electroplated and turned Al

10. Thin films are delicate, and damage easily and catastrophically 250 nm e-beam23,000 shots @4 J/cm2 1.5 mm e-beam86,000 shots @4 J/cm2 Nevertheless, we are continuing to explore methods to improve the coating quality and survivability

11. 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, whereas diamond-turning helps stabilize the surface polished sample for comparison

12. Electroplated Al solves problems with coating thickness and weak grains • 50-100 mm Al on Al-6061 substrate • Grain size ~10 mm • Survived 100,000 shots at 3-4 J/cm2 • No discernable change to the surface • The performance, design flexibility and scalability make this our leading concept • Still need to demonstrate Al on SiC • Thick e-beam coatings are another possible option

13. Damage was obtained finally at 11 J/cm2 • Exposed to 78,500 shots at 11 J/cm2 • Apparently melted at “micro-scratches” (which are smaller than diamond lines), probably caused in shipping • Damage resistance should improve if these micro-scratches can be eliminated

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

15. Final optic concept: many advantages to mirror segmentation and multiplexing amp 1 amp 2 • Easier to fabricate • Easier to maintain • Less variation of laser and neutrons over one optic • Beam overlap reduces require-ments on both mirror and laser • Can be tested on Electra & Mercury 1’ x 2’ 1-kJ mirror

16. For Reference: NASA Technology Goals for JWST James Webb Space Telescope (formerly known as NGST) Deployment in 2011 7-m diam. lightweight optic $825M project budget Goal mirror cost of $300k/m2 Different candidates considered (Be is prime candidate) http://ngst.gsfc.nasa.gov Based on a 1996 Optical Telescope Assembly study, the following requirements were placed on JWST's optics: The mirror should be sensitive to 1-5 microns (0.6-30 extended). It should be diffraction limited to 2 microns. It will have to operate at 30-60 K. It should have an areal density of less than 15 kg/m2.

17. Future Plans • Choose electroplated Al on R&H CVD SiC as our prime candidate mirror coating and substrate (for now). • Continue to develop alternate coatings and substrates. • Fabricate and test a small batch of electroplated Al on SiC. • After successful demonstration to 105 shots, place an order large enough to satisfy all testing (x-ray, ion, neutrons, etc.) • Fill out damage curves with long-term exposures. • Scale up (fabricate) mirrors to 500 J (25 W absorbed). • Install optics testing capability at Electra. • Perform large-scale tests. • Perform radiation damage tests (XAPPER, others?)

18. Acknowledgements and Links Schafer Corp. www.schafercorp.com Rohm and Haas www.cvdmaterials.com Alumiplate www.alumiplate.com II-VI www.ii-vi.com Sigma Technologies www.sigmalabs.com MER corporation www.mercorp.com Surface Optics www.surfaceoptics.com

19. Backup

20. X-ray dose to the final optic • Attenuation calculation verified J. Latkowski’s earlier result: we need a fair bit of gas to protect the optic

21. Cooling requirements • Currently: • 20 mW absorbed power • V=5 cc, r=3.2 g/cc, mass ~15 g, Cp~1 J/mol-K, MW=10 g/mole, C=0.1 J/g-K • adiabatic dT/dt=Q/mCp = 0.02/1.5 = 1/75 K/s • Prototype power plant optic • 100 W absorbed power • r=15 kg/m2, L=0.2 m2, mass ~3 kg, Cp~1 J/mol-K, MW=10 g/mole, C=0.1 J/g-K • adiabatic dT/dt=Q/mCp = 100/300 = 1/3 K/s

22. Defect-free surfaces are needed for damage resistance in thin film coatings Fabrication and handling protocols are under development: • Ensure the substrate has no defects • micrographic and scattered light inspection • Clean the substrate adequately before coating • established cleaning protocols • Provide an Al coating that is defect-free • use clean sputter chambers • Ensure that the natural or applied overcoat is defect-free • explore reactive oxidation, natual oxide, overcoating • Ship samples in a clean container • custom containers? • Examine the samples before testing • Perform laser cleaning very carefully • protocol developed, additional optics purchased

23. Logic Behind Coating Development • Al was chosen as the most promising reflector • Coatings are desired because pure Al is not an attractive substrate (mechanical & radiation issues) • Thick coatings generally suffer from damage at grain boundaries and intragrain slip • Thin (amorphous) coatings suffer from differential stress at interface • Environmental overcoats are desirable (but possibly not necessary) • Whatever coating we adopt must be scalable