The final laser optic: options, requirements & damage threats

1. The final laser optic:options, requirements & damage threats Mark S. Tillack ARIES Project Meeting Princeton, NJ 18-20 September 2000

2. Geometry of the final laser optics (20 m) (SOMBRERO values in red) (30 m) Prometheus-L reactor building layout

3. Mirrors vs. transmissive wedges metal mirror Fused silica wedge • Used in Prometheus-L and Sombrero • Tighter tolerances on surface finish • Low damage threshold larger optics (tends to result in less sensitivity to defects) • Used in DPSSL power plant study • Neutron damage concerns: • absorption, color centers • B-integral effects

4. Why Aluminum is a Good Choice Multi-layer dielectric mirrors are doubtful due to rapid degradation by neutronsAl is a commonly used mirror material • usually protected (Si2O3), but can be used bare • easy to machine, easy to depositGood reflectance into the UVThin, protective, transparent oxideNormal incidence damage threshold~0.2 J/cm2 @532 nm, 10 ns

5. S-polarized waves exhibit high reflectivity at shallow angles of incidence

6. Reflection of s-polarized (TE) waves including thin oxide coating

7. 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˚ Amplifier slab • 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

8. 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* (~100 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 Threat Spectra Two main concerns: • Damage that increases absorption (<1%) • Damage that modifies the wavefront – • spot size/position (200mm/20mm) and spatial uniformity (1%)

9. 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 = 1 m 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)

10. Proposed Design Solutions ThreatSOMBREROPrometheus-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 Ne gas continuous anneal (unknown) Contamination gas jet/shutter,* mechanical shutter, not addressed cleaning system plasma closure *per Bieri

11. Laser damage threshhold of GIMM’s • If damage threshold scales as (1-R), then we should be able to obtain 2 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

12. 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) • Possible solution: evaporation/recondensation • Reduce pressure difference (e.g., 10 mTorr --> 100 mTorr)

13. 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

14. 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

15. High conversion efficiency is achievable with wall temperatures under 1000˚C First wall material TFW Tcoolanth 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%

16. 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 ARIES-RS