Mirror damage studies – progress report 4th High Average Power Laser Program Workshop San Diego, CA April 4-5, 2002 M. S. Tillack, T. K. Mau, K. Vecchio, T. Perez-Prado (UCSD), J. Blanchard (UWisc), M. Wolford (NRL), W. Kowbel (MER)
Goals from last period of performance Try to understand/explain why pure Al survives beyond 10 J/cm 2 – Analysis performed by J. Blanchard, tests planned at Nike – SBS constructed for YAG (will be tested soon), KrF laser acquired Perform tests with contaminated surfaces, acquire damage curves – Aerosol generator & ablation tested; laser testing deferred until we have smooth beams Perform tests on Al coated surfaces –Damage limits vs. coating technique and degree of attachment – Testing deferred until KrF and SBS are working –Sub-threshold irradiation of amorphous Al – EBSD performed on diamond-turned and sputter-coated substrate Plan testing of MER mirrors – 1” and 4” mirrors have been fabricated at MER Ray tracing analysis to determine deformation limits, Kirchhoff analysis of correlated defects – Gross deformations modeled, local defects to be modeled next – Kirchoff analysis underway, not yet completed
Previous data for % Al Estimate of energy required to cause plastic deformation: 2 y = E T o /(1– )fully-constrained, ratchetting limit E = 75 GPa, =0.33, = 25x10 –6 y = ksi ( MPa) T ~ ˚C e ~ J/cm 2
Base case is quasi-steady stresses induced by uniform (instantaneous) surface heating: Consider impact of: 1.Non uniform heating over surface 2.Volumetric heating 3.Elastic waves Similar analysis will be applied to: –Chamber wall materials –rf ablation of liver tumors Laser-Induced Stress Models
1. Gaussian Heating on the Surface Aluminum; t=10 ns; Gaussian half-width <50 m for thermal effect Gaussian half-width <100 m for stress effect Tau Temperature Ratio
Aluminum; t=10 ns; Penetration depth>0.1 m for effect Actual penetration depth is <10 nm 2. Volumetric Heating
Typical Stress Distribution Aluminum; t=10 ns; dimensionless time ~ 4000, so wave stress is inconsequential Comparison 3. Elastic Waves - Inertial Effects
A 600 mJ Excimer Laser has been Acquired Multigas (KrF, ArF, XeCl, etc.) Unstable resonator option ~20 ns pulse width Unpolarized 248 nm optics purchased
Nike beamline prepared for damage threshold measurements Current set up: 6 J beam energy (60 J beam line) Linearly polarized (at the front end) 4 ns pulse length Square beam size ~15.4 cm x ~15.4 cm f ~3m lens Polarized Beam 7 Breadboard Adjustment for beam diameter Calorimeter Al Mirror Experimental Schematic:
Aerosol is generated with a small nebulizer $69.95 Bi-modal size distribution centered around 1 and 10 m Different contaminants can be aerosolized
Contamination by laser ablation is another technique under investigation Adherent coating of particles with 1-10 m diameter form inside our vacuum chamber 10 m Coated window with cratering at locations of laser- induced damage
Electron BackScatter Diffraction analysis of changes in grain structure Oxford Instruments EBSD EBSD is used on our SEM to provide information about a sample’s microtexture. The local grain orientation is measured and the orientation distribution is displayed as a map. Other measurements can then be derived such as misorientation maps, grain size maps, and texture maps.
Grains in diamond-turned and coated mirrors
Fabrication of a Fusion-Relevant GIMM has Been Performed at MER Large, stiff, lightweight, neutron damage resistant, low activation mirrors are being developed at MER C-C is a good substrate material, but not very polishable This Phase-I project will demonstrate the fabrication technology for a 4” fusion-relevant metal mirror using CVD SiC on a C-C substrate with an optical coating on top Testing includes – Mirror characterization (at MER) – Laser damage testing (at UCSD and NRL)
Schematic of Hybrid Composite/Foam Mirror E-Beam Al (2 m) CVD-SiC (100 m) SiC Foam (3 mm) Composite Face (1 mm) SiC Foam (3 mm)
Microroughness Manufacturer's data with 10 m filtering on the same type of wafer were 0.5 A rms Conclusion: CVD-SiC has about twice higher rms over Si highly polished wafer Si wafer CVD SiC
Interferogram at the Rib Section 4” hex substrate Interferometry spot size is 1”, taken over the rib section Photo shows no print-through, as found on commercial SiC mirrors
The ZEMAX optical design software was used to analyze beam propagation between focusing mirror and target. Gross deformation ( ) in the form of a simple curvature (r c ) due to thermal or gravity load, or fabrication defect were modeled = a m 2 /2r c [ surface sag] Changes in beam spot size on the target and intensity profiles were computed as the defect size is varied. Prometheus-L final optics systems as a reference: Wavelength = 248 nm (KrF) Focusing mirror focal length = 30 m GIMM to target distance = 20 m Mirror radius a m = 0.3 m Grazing incidence angle = 80 o Target half-diameter = 3 mm Beam spot size a sp = 0.64 mm Wall Target GIMM Focusing Mirror Laser Beam Ray Tracing Analysis of Gross Mirror Deformation Limits
Spot Size and Illumination Constraints Limit Allowable Gross Mirror Deformation The dominant effect of gross deformation is enlargement (and elongation) of beam spot size, leading to intensity reduction and beam overlap. Secondary effect is non-uniform illumination: I / I ~ 2% for = 0.46 m Limiting mirror surface sag for grazing incidence is : < 0.2 m, (for a mirror of 0.3 m radius) with the criteria: I / I < 1%, and a sp / a sp < 10%. y-scan =0.92 m 0.46 m 0m0m = 0.92 m = 0.46 m = 0 m -2mm +2mm Relative Illumination
Goals for Next Period of Performance Complete the analysis of nonuniform heat flux with more realistic intensity profiles Finish installation of KrF laser and SBS cell Test GA and MER mirrors at both UCSD aand NRL Complete analysis of localized deformation, perform Kirchhoff analysis of correlated defects