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

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 )

~50 cm85˚

Background (2):Key Issues

• Shallow angle stability

• Damage resistance/lifetime

Goal = 5 J/cm2, 108 shots

• Fabrication & optical quality

• Contamination resistance

• Radiation resistance

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.

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.

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 31550 cm identified.

mirror #41, s/n 10157-024

50 nm sputter+1 m e-beam

500 shots at 5 J/cm2

A new vacuum chamber was built

• Cryopumped for higher purity

• Added flexibility in sample manipulation

• Improved diagnostic access

cubedumpcube1/2 waveplatebeam diagnosticsdumpviewing portspecimenmount

mirror #38, s/n 10157-021

100 nm sputter+2.0 m e-beam

5.0 J/cm2 for 1000 shots

In-situ monitoring helps us identify the onset of damage

• Brightfield beam profiling

• Darkfield beam profiling

• Surface imaging

microscopy in-situ imaging darkfield

probe laserprofilermain beamdumptest specimentranslation

camera

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 m

• Solid polycrystalline metal– polished– diamond-turned

• Electroplated and turned Al

Thin films are delicate, and damage easily and catastrophically

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

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

Nevertheless, we are continuing to explore methods to improve the coating quality and survivability

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

Electroplated Al solves problems with coating thickness and weak grains

• 50-100 m Al on Al-6061 substrate

• Grain size ~10 m

• 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

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

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

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)Multiplexer

Array(beam

splitters)

drawing courtesy of J. Sethian, NRL

Final optic concept: many advantages to mirror segmentation and multiplexing

1’ x 2’ 1-kJ mirror

1. Easier to fabricate

2. Easier to maintain

3. Less variation of laser and neutrons over one optic

4. Beam overlap reduces require-ments on both mirror and laser

5. Can be tested on Electra & Mercury

amp 1

amp 2

Deployment in 20117-m diam. lightweight optic$825M project budgetGoal mirror cost of $300k/m2

Different candidates considered (Be is prime candidate)http://ngst.gsfc.nasa.gov

For Reference: NASA Technology Goals for JWST

James Webb Space Telescope (formerly known as NGST)

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.

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?)

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

Backup

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

Cooling requirements

• Currently:

– 20 mW absorbed power

– V=5 cc, =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

– =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

Defect-free surfaces are needed for damage resistance in thin film coatings

1. Ensure the substrate has no defects• micrographic and scattered light inspection

2. Clean the substrate adequately before coating• established cleaning protocols

3. Provide an Al coating that is defect-free• use clean sputter chambers

4. Ensure that the natural or applied overcoat is defect-free• explore reactive oxidation, natual oxide, overcoating

5. Ship samples in a clean container• custom containers?

6. Examine the samples before testing7. Perform laser cleaning very carefully

• protocol developed, additional optics purchased

Fabrication and handling protocols are under development:

Logic Behind Coating Development

1. Al was chosen as the most promising reflector

2. Coatings are desired because pure Al is not an attractive substrate (mechanical & radiation issues)

3. Thick coatings generally suffer from damage at grain boundaries and intragrain slip

4. Thin (amorphous) coatings suffer from differential stress at interface

5. Environmental overcoats are desirable (but possibly not necessary)

6. Whatever coating we adopt must be scalable