Laser Induced Damage of Grazing Incidence Metal Mirrors Mark S. Tillack T. K. Mau Mofreh Zaghloul High Average Power Laser Program Workshop Nov , 2001 Livermore, CA

Statement of purpose Our research seeks to develop improved understanding of damage mechanisms and to demonstrate acceptable performance of grazing incidence metal mirrors, with an emphasis on the most critical concerns for laser fusion. Through both experimen- tation and modeling we will demonstrate the limitations on the operation of reflective optics for IFE chambers under prototypical environmental conditions. Deliverables: Measure LIDT at grazing incidence with smooth surfaces.Sept. 1, 2001 Model reflectivity and wavefront changes of smooth surfaces.Sept. 1, 2001 Measure effects of defects and surface contaminants on reflectivity, LIDT and wavefront.April 1, 2002 Model reflectivity and wavefront changes due to defects andApril 1, 2002 contamination. Statement of Purpose and Deliverables

Accomplishments for this period of performance Compare damage on % Al with Al-1100done Perform tests at 5 J/cm 2 done Perform sub-threshold irradiation of amorphous Alongoing to explore recrystallization Establish methods for creating contaminated surfacesdone Obtain samples for neutron irradiation multi-layer dielectric mirror done Al mirrordeferred Exercise ZEMAX to assess wavefront degradationdelayed Status Goal

Testing up to 180 J/cm 2 has been completed by focusing the beam

Damage occurs at a higher fluence compared with normal incidence Silicide occlusions in Al 6061 preferentially absorb light, causing explosive ejection and melting Fe impurities appear unaffected Several shots at 80˚, 1 J/cm 2 peak Damage to Al 6061 at grazing angle (review) Fe MgSi 1000x

Damage Regimes for Al-1100 (0.1% Cu, Fe) 1 J/cm 2 4kX 1kX 20 J/cm 2

Damage Mechanisms for Al-1100

Damage Estimates for Melting and Yielding 1. Estimate of energy required to melt: T - T o = (2q”/k) sqrt(  t t/  ) e = q”t/[(1-R) cos  ] T-T o = 640˚C t = 10 ns,  =85˚ e = 143 J/cm 2 2. Estimate of energy required to cause plastic deformation:  y = E   T o /(1– )fully-constrained E = 75 GPa,  = 25x10 –6  y = ksi ( MPa)  T ~ 35˚C e ~ 8-10 J/cm 2

Damage Regimes for % pure Al 130X 180 J/cm shots

4 Distinct Regions Are Observed at 10 4 Shots I II III IV Region I Region IV Nikon EpiPhot

The Zone Adjacent to the Melted Area Exhibits Effects of Stress

Why Does the Surface Survive Above 10 J/cm 2 ? Something about grazing incidence irradiation stabilizes the mechanical instability? Al 2 O 3 capping layer stabilizes the interface? What we plan to do to explain this: Perform additional studies on diamond-turned Al in the range of 10–50 J/cm 2. Implement beam smoothing to help interpret the results

Methods of Controlled Surface Contamination Many material options oxides, metals, carbides, carbon 10 nm – 1  m size Many vendors Nanoscale Materials Inc. (Nantek) Nanophase Technolgies Corp. Reade Advanced Materials Altair Technologies Cheap ~$1/g (1) Nanopowders

Methods of Controlled Surface Contamination Aerosol Ultrasonic atomizers Mean droplet size as low as 2–10  m Misonix Inc. *, Sonics and Materials Condensed vapor Nonvolatile liquid residue Uniform coating using spray atomizer (2) Aerosol and Condensibles * $1300

Specularly reflected intensity is degraded by induced mirror surface roughness (review) e.g., at  1 = 80 o,  = 0.1, e -g = 0.97 The effect of induced surface roughness on beam quality was investigated by Kirchhoff wave scattering theory. For cumulative laser-induced and thermomechanical damages, we assume Gaussian surface height statistics with rms height   1 = 80 o 70 o 60 o Intensity Degradation, e –g  Grazing incidence is less affected by surface roughness To avoid loss of laser beam intensity,  < 0.01 11 22 I sc I o : reflected intensity from smooth surface I d : scattered incoherent intensity g : (4   cos  1 / ) 2 I inc

Laser Scattering from Anisotropic Defects Laser induced mirror damages can be spatially anisotropic and may have multiple scale lengths. The surface height statistics may be non-Gaussian. We plan to use Kirchhoff theory to determine the loss of specularly reflected intensity due to realistic mirror defects, and compare with experimental measurements. The total scattered scalar field is evaluated by the integral expression where S M is the mirror surface, h(x o,y o ) is the measured height distribution, F, A, B, and C are functions of the surface reflectivity, and the incident and reflected angles from which intensity profile distortion can be deduced. Kirchhoff theory in vector form can be used to include induced depolarization effects on mirror reflectivity.

Goals for next period of performance Try to understand/explain why pure Al survives beyond 10 J/cm 2 Perform tests with contaminated surfaces, acquire damage curves Perform tests on Al coated surfaces –Damage limits vs. coating technique and degree of attachment –Sub-threshold irradiation of amorphous Al Plan testing of MER mirrors Ray tracing analysis to determine deformation limits Kirchhoff analysis of correlated defects

Normal incidence reflectivity of several metals