Grazingincidence mirror damage

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Grazing-incidence mirror damage
M. S. Tillack, J. Pulsifer, T. K. Mau UC San
Diego W. Kowbel MER Corp.
High Average Power Laser Program Workshop Naval
Research Laboratory Washington, DC December 5-6,
2002
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Outline

• Final optics integrated program plan
• UV damage testing of mirrors in air
• Damage testing in vacuum
• Contaminated surface tests
• Initial results with coated optics (in air)
• Optical modeling update

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Final Optics Phase I Goals
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Final Optics 3-yr Program Plan
Key program elements I. Quantification of
damage threats II. Development of mitigation
techniques III. Fabrication and system
integration
I. Quantification of Damage Threats (lead
lab) A. LASER DAMAGE 1. Demonstrate stability
of mirror at grazing angles UCSD 2. Perform
damage tests in a controlled environment UCSD 3.
Test coated optics for adhesion and damage
threshold UCSD 4. Perform large scale (4x4)
optics testing NRL 5. Establish a database of
contamination resistance UCSD
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Final Optics 3-yr Program Plan
I. Quantification of Damage Threats (lead lab)
B. RADIATION DAMAGE 6. X-ray damage testing
(XAPPER) LLNL 7. Neutron exposures ORNL 8.
Post-exposure optical testing LLNL 9. MDS
simulations of radiation damage LLNL 10. Ion
exposures ORNL 11. Ion threat modeling UCSD C.
MULTIPLE THREATS 12. Synergistic damage threat
assessment LLNL 13. Multiple damage effects
demonstration tbd 14. Provide optical modeling
to support experiments UCSD
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Final Optics 3-yr Program Plan
II. Mitigation Techniques (lead lab) 1. Ion
mitigation assessment LLNL 2. Contaminant
mitigation assessment UCSD 3. Mitigation
demonstration LLNL/UCSD
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Final Optics 3-yr Program Plan
III. Fabrication System Integration (lead
lab) 1. Screening of candidate fabrication
techniques tbd 2. Fabrication of Al-coated SiC
substrates MER 3. Fabrication of environmental
coatings PVD 4. Fabrication of large-scale
optics MER 5. Thermomechanical design UCLA 6.
Analysis and mitigation of deformations UCLA 7.
Mechanics and failure analysis UCLA 8. Surface
optimization (overcoats, preconditioning,
...) UCLA 9. Integration with target injection
system UCSD
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A Compex 201 excimer laser and UV optics were
installed at UCSD
420 mJ, 1020 ns, 248 nm
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Damage of 99.999 pure Al in air
100mm
previous data, 532 nm
248 nm, 1 shot, 40 J/cm2

• UV light is more damaging than visible light
• Higher photon energy
• Interaction with smaller surface features
• Single-shot damage appears well below the melting
  point

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Damage appears to be correlated with grain
boundaries
100mm
50x
6400 shots, 6-15 J/cm2
10mm
pin-point damage sites glow white hot under
irradiation
500x
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Slip lines are not seen with KrF light
Mirror 22, 6744 shots, 10-24 J/cm2
10mm
previous data, 532 nm
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Surface preconditioning

• Preconditioning was found to be important for
  removing surface contaminants and defects
• During the first laser shot, we typically observe
  bright yellow emission, which disappears after
  3-5 shots
• Applying a few low fluence, low rep-rate shots
  before long-term exposure is important to reach
  high damage threshold
• e.g., in a test of two identical surfaces, one
  exposed to 10-24 J/cm2 without preconditioning
  was destroyed after 3 seconds at 10 Hz
• All mirrors are now tested with preconditioning

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The appearance of chemical reactions led us to
begin testing in vacuum

• A small, fixed-geometry vacuum cell was built to
  perform initial scoping tests
• Base pressure is 20 microns

• Initial results show that the background gas
  matters, suggesting we should fabricate a larger
  multi-purpose vacuum chamber to continue testing

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The morphology of damage in vacuum appears
different than in air
500x
1000x
10mm

• Small surface features lead to characteristic
  blue flourescence after 450 shots at 10-20 J/cm2
• Fluence level where defects appear is not much
  higher than in air, although catastrophic
  destruction was not observed
• Damage is not visible to the naked eye in
  post-test inspection

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Damage exhibits itself as grain boundary
extrusion shallow pitting
450 shots at 10-20 J/cm2
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WYKO shows depth of pits is 100200 nm
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Testing at low fluence was performed to establish
lower bound on damage
3500 shots in vacuum at 4-6 J/cm2
10mm
The surface survived without damage visible to
the naked eye, but clearly the surface quality of
our mirrors must be improved if we hope to
achieve 5 J/cm2 over 108 shots
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An oil-contaminated surface was cleaned in 5-10
shots w/o evidence of damage

• Initial shots caused explosive combustion of oil
• After 5-10 shots at 6-15 J/cm2 the oil was
  completely cleaned from the beam footprint

• Subsequent testing to 100 shots showed no
  evidence of damage

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A mineral-contaminated surface exhibited similar
behavior

• Initial shots exhibited benign (yellow) emission
  of light
• After 5 shots at 6-15 J/cm2 the contaminant was
  cleaned from the beam footprint
• Subsequent testing to 100 shots showed no
  evidence of damage

Laser footprint
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Coated optics from MER

• Mirror substrates are composed of functionally
  graded SiC foam. Subsequently a SiC slurry is
  applied, followed by CVI-SiC and a first layer of
  CVD SiC.
• After initial polishing, one of two Al coatings
  is applied
• 120 nm RT evaporation coating
• 300 nm PVD coating applied by magnetron
  sputtering at 150C

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MER PVD coating

• Imperfect surface exposed to 5 J/cm2 in air
  (sample didnt fit into vacuum) for 1000 shots
• No laser damage could be found anywhere on the
  surface

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MER evaporation coating

• Surface exposed to 5 J/cm2 in air for 6 shots
• Immediate and growing damage observed

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Optical modeling status

• Previous results reported on dimensional defects
  (dgtl and dltl) and gross contamination
• Kirchhoff scattering by non-Gaussian roughness
  under investigation
• Local compositional defects divided into external
  vs. internal
• Methods are under review to determine if a
  cost-effective approach exists for studying local
  compositional defects

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Summary Conclusions

• No evidence of a shallow angle instability has
  been observed
• Irradiation at 248 nm exhibits much more severe
  environmental interactions, requiring us to test
  in vacuum

• Cleaning by KrF light appears to be a very
  important effect.
• Surfaces should be preconditioned
• External contaminants may be tolerable!
• For coated optics, damage resistance depends on
  the fabrication technique
• Some types of defects appear tolerable
• Our top priority is to obtain higher initial
  quality surfaces for further testing