Mark S. Tillack T. K. Mau Mofreh Zaghloul (S. S. and Bindhu Harilal) Laser Induced Damage Threshold (LIDT) of Grazing Incidence Metal Mirrors Mark S. Tillack T. K. Mau Mofreh Zaghloul (S. S. and Bindhu Harilal) Laser-IFE Program Workshop February 6-7, 2001 Naval Research Laboratory

Statement of Purpose and Deliverables 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. June 30, 2001 Model reflectivity and wavefront changes of smooth surfaces. Aug. 30, 2001 Measure effects of defects and surface contaminants on reflectivity, LIDT and wavefront. Jan. 31, 2002 Model reflectivity and wavefront changes due to defects and Jan. 31, 2002 contamination. Budget: $330k

1. Background

Reference Geometry of the Final Optics (SOMBRERO values in red) (30 m) Prometheus-L reactor building layout

Damage Threats Two main concerns: Final Optic Threat Nominal Goal Damage that increases absorption (<1%) Damage that modifies the wavefront – spot size/position (200mm/20mm) and spatial uniformity (1%) Final Optic Threat Nominal Goal Optical damage by laser >5 J/cm2 threshold (normal to beam) Nonuniform ablation by x-rays Wavefront distortion of <l/3 (~80 nm) Nonuniform sputtering by ions (6x108 pulses in 2 FPY: 2.5x106 pulses/atom layer removed Defects and swelling induced Absorption loss of <1% by g-rays and neutrons Wavefront distortion of < l/3 Contamination from condensable Absorption loss of <1% materials (aerosol and dust) >5 J/cm2 threshold

Mirror Parameters Grazing incidence metal mirror Al at normal incidence (1/3 or 1/4 mm) ~0.2 J/cm2 x10 due to cos q x10 due to increase in reflectivity Transverse energy ~ 20 J/cm2 is possible For 1.2 MJ driver w/ 60 beams @5 J/cm2, each beam would be 0.4 m2

Aluminum is the 1st choice for the GIMM Lifetime of multi-layer dielectric mirrors is questionable due to rapid degradation by neutrons Al is a commonly used mirror material • usually protected (Si2O3, MgF2, CaF2), but can be used “bare” • easy to machine, easy to deposit Good reflectance into the UV Thin, protective, transparent oxide Normal incidence damage threshold ~0.2 J/cm2 @532 nm, 10 ns

GIMM development issues are well established* Experimental verification of laser damage thresholds Wavefront issues: beam smoothness, uniformity, shaping, f/number constraints Experiments with irradiated mirrors Protection against debris and x-rays (shutters, gas jets, etc.) In-situ cleaning techniques Large-scale manufacturing Cooling * from Bieri and Guinan, Fusion Tech. 19 (May 1991) 673.

2. Experiments

• superpolished fused silica (2”, l/10 , $335) Fabrication capabilities are important to enable us to optimize mirror performance Mirror Fabrication: • Diamond turning • Sputter coating Substrates considered: • Bulk Al (cheap) • CVD SiC ($550 for 3-cm disk, l/50, <2Å) • superpolished fused silica (2”, l/10 , $335) • 30-cm Si wafers (free) Rohm & Haas SiC: l/50 flat, <2Å Si wafers (TBD) 1.5 x 15 cm diamond-turned Al flat

SEM photos of damaged Al Surface profile of undamaged Al mirror Surface Analysis Surface Analysis: • WYKO white light interferometer • SEM with energy dispersive x-ray • Auger electron spectroscopy 50x 1 mm SEM photos of damaged Al 1000x Surface profile of undamaged Al mirror 20 mm

The UCSD laser lab is used to test GIMMs Spectra Physics QuantaRay laser: 2J, 10 ns @1064 nm 700, 500, 300 mJ @532, 355, 266 nm Peak power~1014 W/cm2

A ring-down reflectometer is used to obtain accurate measurements of reflectivity specimen

A Shack-Hartmann sensor is used to measure wavefront changes Spherical wave from a pinhole: 144 mm spatial resolution l/50 sensitivity

2. Modeling

Tools for modeling effects of damage on beam characteristics

Fresnel Modeling of Reflectivity Wave propagation in four stratified layers of media is modeled, each with complex n Refraction : n1 sin q1 = nj sin qj j = 2,3,4 Reflection : ri,i+1 = (ni cos qi - ni+1 cos qi+1) / (ni cos qi + ni+1 cos qi+1) Reflectivity for 3 layers: ri = [ri-1,i + ri+1 exp (i2bi)] / [1 + ri-1,i ri+1 exp (i2bi)] metal substrate n4, k4 coating n3, k3 n2, k2 contaminant n1, k1 q1 Incident medium where bi = (2p/lo) di ni cos qi , i = 2,3 and di is the layer thickness. Overall intensity reflectance : R = |r2|2 Tasks: • Examine effects of coating material and contaminant on mirror optical properties, and compare with experiment • Assess importance of transmutation on optical properties of coating and substrate

Example: Effect of Surface Contaminants Surface contaminants (such as carbon) on mirror protective coatings can substantially alter reflectivity, depending on layer thickness and incident angle. Uniform film thickness is assumed. d2=0 q1 = 80o d2=0 q1 = 0o 80o 60o reflectivity reflectivity d2=2 nm q1 = 80o 40o lo = 532 nm Al2O3 coating (10 nm) Al mirror 20o lo = 532 nm Carbon film Al mirror d2=2 nm q1 = 0o q1 = 0o Al2O3 coating thickness, d3/lo Carbon film thickness (nm)

Ray Tracing When surface defect d > l, the effect on beam propagation can be assessed using a ray tracing approach. ZEMAX-EE optics design software will be used: - User-defined surfaces (shape, optical properties) - Complete polarization ray tracing - Nonlinear model of thermal effects on index of refraction and material expansion. Tasks: - Evaluate surface deformation from expected loads. - Quantify allowable surface deformation (shape and size) to meet beam propagation requirements (spot size/location, intensity uniformity, absorption).

Scattering Theory When surface deformation d < l, scattered wave is composed of specular and diffuse components. Two analysis approaches: - Perturbation theory (Raleigh-Rice): d << l - Physical optics (Kirchoff): d < l Surface roughness characterized by surface height distribution, d(r). For Gaussian d(r), overall scattered intensity is in the form: Isc = Io e-g + Id where Io = scattered intensity from flat surface, g = f (s/l, q1, q2), s : rms height, q1, q2 : incident, reflected angle Id = scattered diffuse intensity Tasks: - Characterize surface damage using measured data and/or modeling - Evaluate scattered intensity onto targets in terms of wave front distortion and depolarization, using analytic (and numerical) models.

Final Optic Threats and Planned Research Activities

Final Optics Program Plan FY 2001 | FY 2002 | FY 2003 | FY 2004 | FY2005 RADIATION DAMAGE (neutron and gamma effects) Scoping Tests: Irradiation & PIE (incl. annealing) Extended testing of prime candidates Damage modeling LASER-INDUCED DAMAGE LIDT scoping tests for GIMM, materials development System Integration Laser damage modeling, 3w data from NIF CONTAMINATION THREATS Modeling Test simulated contaminants Mitigation System Integration X-RAY ABLATION Scoping tests (laser-based x-ray source) Mitigation System Integration Modeling ION SPUTTERING Calculate sputtering, gas attenuation Mitigation System Integration

Final Optic Threats and Planned Research Activities