UV laser-induced damage to grazing-incidence metal mirrors

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

4th US-Japan Workshop on Laser-Driven Inertial Fusion Energy

TechnologyOsaka University

March 13-15, 2003

Design concept for a grazing-incidence metal mirror

Issues:• Shallow angle instability• Damage resistance/lifetime

Goal = 5 J/cm2 • Optical quality• Fabrication

The mirror consists of a stiff, radiation-resistant substrate with a thin metallic coating optimized for high reflectivity

Metal reflectors are chosen due to concerns over radiation damage to multi-layer dielectrics

Reflectivity of oxidized Al to s-polarized light

Normal incidence reflectivity of various metals vs. wavelength

248 nmHigh reflectivity at shallow angles gives aluminum a potentially high damage threshold

Outline of the talk:

1. UV damage testing of mirrors in air; comparisons with visible light

2. Damage testing in vacuum

3. Preliminary data on contaminated surfaces

4. Coated vs. solid optics

5. Perturbations to transmitted light

Optics were tested using a 0.4-J KrF laser

420 mJ, 25 ns, 248 nm

1 shot, 40 J/cm2

100m

Single-shot damage of pure Al in air

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

earlier data @532 nm

Cyclic damage in air appears to be correlated with grain boundaries

6744 shots, 10-24 J/cm2

10m

104 shots, 40 J/cm2 @532 nm

Slip lines are not observed at 248 nm as with visible light

Specularly reflected intensity is degradedby induced surface roughness

e.g., at 1 = 80o, = 0.1, e-g = 0.97

• The effect of induced surface roughness on beam quality was investigated using Kirchhoff wave scattering theory.

• Grazing incidence is less affected by Gaussian surface roughness

• To avoid loss of laser beam intensity, < 0.1 ~ 25 nm

Io : reflected intensity from smooth surfaceId : scattered incoherent intensityg : (4 cos1/)2

Isc=I0e−g+Id1

2

IscIinc

0 0.1 0.2 0.3 0.4 0.5

1.0

0.8

0.6

0.4

0.2

0

1 = 80o

70o

60o

Inte

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t y D

e gr a

da t

ion

, e–g

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

• A small, fixed-geometry vacuum cell was built to perform scoping tests

• Base pressure ~20 m

• Damage is monitored visually; In-situ profile monitoring is being evaluated

The morphology of damage in vacuum is clearly different than in air

• 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

500x 1000x10m

Diamond-turning lines are etched

450 shots at 10-20 J/cm2

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

Possible contamination source: hydrocarbon from target or from chamber walls

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

Possible contamination source: aerosol and particulate from evaporated chamber mat’ls

Coated optics are currently being evaluated

• Substrate types– superpolished CVD-SiC

– functionally graded SiC foam

– SiC/SiC composite

• Coatings:– RT evaporation coating (120 nm)

– PVD coating by magnetron sputtering at 150˚C (300–1400 nm)

– others under investigation

Interface thermal stress can be very high

300 nm Coating

300

305

310

315

320

325

330

335

340

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 m • Plane stress analysis– Stress at free surface ~ 0

• Peak stress at inteface– 40 MPa @30 ns

• Yield stress ~10 MPa

Coating quality deteriorates above 300 nm

300 nm coating of Al on SiC

1 m coating of Al on SiC

MER PVD coating - 1st attempt

• Imperfect surface exposed to 5 J/cm2 in air for 1000 shots

• No laser damage could be found anywhere on the surface

CVD SiC substrate coated with 300 nm Al

• Surface exposed to 4-8 J/cm2 in air for several shots

• Immediate damage occurred again due to poor substrate

The transmitted wave is an important diagnostic for surface damage

The requirement on “damage” is ~2% change in spatial profile and not the appearance of visible damage

probe laserprofilermain beamdumptest specimentranslation

Surface map of mirror scan

Surface map

QuickTime™ and aMPEG-4 Video decompressorare needed to see this picture.

QuickTime™ and aMPEG-4 Video decompressorare needed to see this picture.

Measurements were made using an 8-bit camera with 640x480 resolution

We plan to acquire a 12-bit XGA camera for future studies

An old, damaged diamond-turned surface was used to highlight various changes to the transmitted beam

Summary & Conclusions

1. No evidence of a “shallow angle instability” has been observed.

2. Irradiation at 248 nm exhibits much more severe environmental interactions, requiring testing in vacuum.

3. Cleaning by UV light appears to be a very important effect: a. Surfaces must be preconditioned

b. External contaminants may be tolerable

4. For coated optics, damage resistance depends on the fabrication technique - coating studies are now underway.

5. Future damage studies will concentrate on the reflected wavefront rather than visible damage.