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The final laser optic: options, requirements & damage threats Mark S. Tillack ARIES Project Meeting Princeton, NJ 18-20 September 2000 Geometry of the final laser optics (20 m) (30 m) Prometheus-L reactor building layout (SOMBRERO values in red) Mirrors vs. transmissive wedges metal mirror Fused silica wedge • Used in Prometheus-L and Sombrero • Used in DPSSL power plant study • Tighter tolerances on surface finish • Neutron damage concerns: • Low damage threshold larger optics (tends to result in less sensitivity to defects) • – absorption, color centers B-integral effects Why Aluminum is a Good Choice Multi-layer dielectric mirrors are doubtful due to rapid degradation by neutrons Al is a commonly used mirror material • usually protected (Si2O3), but can be used bare • easy to machine, easy to deposit Thin, protective, transparent oxide 100 Reflectivity, % Good reflectance into the UV Normal incidence reflectivity of metals 95 90 Al 85 Ag Au Normal incidence damage threshold ~0.2 J/cm2 @532 nm, 10 ns 80 250 500 750 Wavelength, nm 1000 S-polarized waves exhibit high reflectivity at shallow angles of incidence Al2O3 reflectivity at 532 nm Aluminum reflectivity at 532 nm 1 1 s-polarized 0.75 Reflectivity Reflectivity 0.95 0.9 p-polarized 0.85 0.5 0.25 0.8 0 0.75 0 10 20 30 40 50 60 Angle of incidence 70 80 90 0 10 20 30 40 50 60 angle of incidence 70 80 90 Reflection of s-polarized (TE) waves including thin oxide coating Operation of the fused silica wedges Orth, Payne & Krupke, Nuclear Fusion 36(1) 1996. • • Linear array used in DPSSL study, coupled to slab design of gain medium. 5˚ wedge, angled at 56˚ Brewster's angle Amplifier slab 62 cm • • • Key concern is laser absorption -- 8% after 1 hr. irradiation. Operated at 400˚C for continuous annealing of defects 60 times worse at 248 nm vs. 355 nm 57 cm Threat Spectra Two main concerns: • 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 Nonuniform sputtering by ions Wavefront distortion of <l/3* (~100 nm) (6x108 pulses in 2 FPY: 2.5x106 pulses/atom layer removed Defects and swelling induced by g-rays and neutrons Absorption loss of <1% Wavefront distortion of < l/3 Contamination from condensable materials (aerosol and dust) Absorption loss of <1% >5 J/cm2 threshold Diffraction and Wavefront Distortions Diffraction-limited spot size: do = 4 l f M/pD l = 1/3 mm f = 30 m (distance to lens) do = 200 mm (zoomed) D=1m M <16 • “There is no standard theoretical approach for combining random wavefront distortions of individual optics” (ref: Orth) • Each l/3 of wavefront distortion translates into roughly a doubling of the minimum spot size (ref: Orth) Proposed Design Solutions Threat SOMBRERO Prometheus-L DPSSL study Laser damage mirror size mirror size, coatings continuous anneal X-ray ablation gas jet/shutter* Xe gas, plasma closure (1 Torr Ar) Ion sputtering gas jet/shutter* Xe gas , plasma closure not addressed Radiation damage lifetime limit (unknown) Ne gas continuous anneal Contamination gas jet/shutter,* cleaning system mechanical shutter, plasma closure not addressed *per Bieri Laser damage threshhold of GIMM’s • If damage threshold scales as (1-R), then we should be able to obtain J/cm2 at 85˚. • With cos q=0.0872, the transverse energy is >20 J/cm2 • For a 1.2 MJ driver energy and 60 beams, each beam is ~1 m2 85ϋ 1m stiff, lightweight, actively cooled, neutron transparent substrate 11.5 m 2 Gas protection of beamlines • Beamline volume = 7.7 m3 • Mass @1 Torr = 60 g (7700 Torr-liters) • A credible turbopump speed is 50 m3/s (50 Torr-l/s @1 mTorr) 10~15 m coverage fraction ~1% (60 beams) 1m 1 Torr 25 cm 1 mTorr pulsed or steady gas puff chamber • Possible solution: evaporation/recondensation • Reduce pressure difference (e.g., 10 mTorr --> 100 mTorr) vacuum vessel Neutron and gamma effects • Conductivity decrease due to point defects, transmutations, surface roughening – Estimated in Prometheus at ~0.5% decrease in reflectivity (ref: private conversation) -- need to check this • Differential swelling and creep – – – Swelling values of 0.05-0.1% per dpa in Al (ref. Prometheus) The laser penetration depth is d=l/4pk where k>10, so the required thickness of Al is only ~10 nm. Swelling in Al can be controlled by keeping it thin. The substrate is the real concern. Porous (10-15%) SiC is expected to have very low neutron swelling. • Absorption band at 215 nm in fused silica Final Optics Tasks • Re-assess protection schemes in more detail – In previous studies, issues were identified and potential design solutions proposed, but detailed analysis of phenomena was not performed • Correlate damage mechanisms with beam degradation – Estimate defect and contamination rates from all threat spectra – Analyze result of mirror defects and deformations on beam characteristics • System integration – Flesh out the beam steering and alignment issues – Integrate with target injection and tracking system High conversion efficiency is achievable with wall temperatures under 1000˚C First wall material TFW Tcoolant h ARIES-RS vanadium alloy 700˚C 610˚C 45% ARIES-ST ODS ferritic steel 600˚C 700˚C 45% ARIES-AT SiC/SiC 1000˚C 1100˚C 59% Blanket designs for high efficiency • • • • Use neutrons (80% of power) to maximize outlet temperature Segment radially and optimize routing Use thermal insulation if necessary Optimize conversion cycle ARIES-AT ARIES-ST He-cooled Ferritic Steel 18 10 3.5 SiC ARIES-RS 232 Pb83Li17 18 250