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

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

Mirrors vs. transmissive wedges Used in Prometheus-L and Sombrero Tighter tolerances on surface finish Low damage threshold larger optics (tends to result in less sensitivity to defects) Fused silica wedge metal mirror Used in DPSSL power plant study Neutron damage concerns: – 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 (Si 2 O 3 ), 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/cm nm, 10 ns

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

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

Operation of the fused silica wedges Linear array used in DPSSL study, coupled to slab design of gain medium. 5˚ wedge, angled at 56˚ Orth, Payne & Krupke, Nuclear Fusion 36(1) 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 Amplifier slab

Threat Spectra Final Optic ThreatNominal Goal Optical damage by laser>5 J/cm 2 threshold (normal to beam) Nonuniform ablation by x-raysWavefront distortion of < /3 * (~100 nm) Nonuniform sputtering by ions (6x10 8 pulses in 2 FPY: 2.5x10 6 pulses/atom layer removed Defects and swelling induced Absorption loss of <1% by  -rays and neutronsWavefront distortion of < /3 Contamination from condensable Absorption loss of <1% materials (aerosol and dust) >5 J/cm 2 threshold Damage that increases absorption (<1%) Damage that modifies the wavefront – – spot size/position (200  m/20  m) and spatial uniformity (1%) Two main concerns:

Diffraction and Wavefront Distortions Diffraction-limited spot size: d o = 4 f M/  D = 1/3  m f = 30 m (distance to lens) d o = 200  m (zoomed) D = 1 m M <16 “There is no standard theoretical approach for combining random wavefront distortions of individual optics” (ref: Orth) Each /3 of wavefront distortion translates into roughly a doubling of the minimum spot size (ref: Orth)

Proposed Design Solutions Threat SOMBRERO Prometheus-LDPSSL study Laser damagemirror sizemirror size, coatingscontinuous anneal X-ray ablationgas jet/shutter*Xe gas, plasma closure(1 Torr Ar) Ion sputteringgas jet/shutter* Xe gas, plasma closurenot addressed Radiation damagelifetime limit Ne gascontinuous anneal (unknown) Contaminationgas jet/shutter,* mechanical shutter,not addressed cleaning systemplasma closure *per Bieri

Laser damage threshhold of GIMM’s If damage threshold scales as (1-R), then we should be able to obtain 2 J/cm 2 at 85˚. With cos  =0.0872, the transverse energy is >20 J/cm 2 For a 1.2 MJ driver energy and 60 beams, each beam is ~1 m 2

Gas protection of beamlines Beamline volume = 7.7 m 3 Torr = 60 g (7700 Torr-liters) A credible turbopump speed is 50 m 3 /s (50 mTorr) Possible solution: evaporation/recondensation Reduce pressure difference (e.g., 10 mTorr --> 100 mTorr)

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 % per dpa in Al (ref. Prometheus) – The laser penetration depth is d= /4  where  >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 materialT FW T coolant  ARIES-RSvanadium alloy700˚C610˚C45% ARIES-STODS ferritic steel600˚C700˚C45% ARIES-ATSiC/SiC1000˚C1100˚C59%

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-RS ARIES-ST ARIES-AT