uv laser-induced damage to grazing- incidence metal mirrors m. s. tillack, j. pulsifer, k. sequoia...
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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
nsi
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.