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 Technology Osaka University March 13-15, 2003

Design concept for a grazing-incidence metal mirror Issues: Shallow angle instability Damage resistance/lifetime Goal = 5 J/cm 2 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 nm High 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/cm  m 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 nm

Cyclic damage in air appears to be correlated with grain boundaries 6744 shots, J/cm 2 10  m 10 4 shots, 40 J/cm nm Slip lines are not observed at 248 nm as with visible light

Specularly reflected intensity is degraded by induced surface roughness e.g., at  1 = 80 o,  = 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 I o : reflected intensity from smooth surface I d : scattered incoherent intensity g : (4   cos  1 / ) 2 11 22 I sc I inc  1 = 80 o 70 o 60 o Intensity Degradation, 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 J/cm 2 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 500x1000x 10  m

Diamond-turning lines are etched 450 shots at J/cm 2

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/cm 2 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/cm 2 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 Plane stress analysis –Stress at free surface ~ 0 Peak stress at inteface –40 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/cm 2 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/cm 2 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

Surface map of mirror scan Surface map 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.