Progress on gimm fabrication testing
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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.

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Progress on GIMM Fabrication & Testing

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Progress on gimm fabrication testing

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

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)

Background 2 key issues

Background (2):Key Issues

• Shallow angle stability• Damage resistance/lifetimeGoal = 5 J/cm2, 108 shots• Fabrication & optical quality• Contamination resistance• Radiation resistance

When last we met

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

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

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

A new vacuum chamber was built

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

In situ monitoring helps us identify the onset of damage

In-situ monitoring helps us identify the onset of damage


•Brightfield beam profiling•Darkfield beam profiling•Surface imaging


in-situ imaging


Testing continues

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

Progress on gimm fabrication testing

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

Diamond turned al exhibits superior damage resistance

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

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

Damage was obtained finally at 11 j cm 2

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

Optic scale up multiplexed beams enable smaller more tolerant final optics

Optic scale-up: multiplexed beams enable smaller, more tolerant final optics


drawing courtesy of J. Sethian, NRL


(~ 100's nsec)

Last Pulse






(beam splitters)



( 20 nsec)

Only three pulses shown for clarity

Final optic concept many advantages to mirror segmentation and multiplexing

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

Progress on gimm fabrication testing

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)

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

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

Acknowledgements and Links


Rohm and






X ray dose to the final optic

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

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

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

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

Logic behind coating development

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

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