Prometheus-L Reactor Building Layout

Two Main Options for the Final Optic (2) Grazing incidence metal mirror (1) SiO 2 or CaF 2 wedges

Neutrons and  -rays create defects in SiO 2 which result in photon absorption MeV n 0 annealing Si O Normal Site Si O 2- Oxygen Deficient Center (ODC, 246 nm) MeV  -rays SiSi + E’ Center (213 nm) Si O-O- Non-Bridging Oxygen Hole Center (NBOHC, 620 nm) Si O O-O- Normal Site annealing +

SiO 2 samples irradiated at LANSCE for rad

Final Optic Damage Threats Damage that increases absorption (<5%) Damage that modifies the wavefront – – spot size/position (200  m/20  m) and spatial uniformity (1%) Two main concerns: Final Optic ThreatNominal Goal Optical damage by laser>5 J/cm 2 threshold (normal to beam) Sputtering by ionsWavefront distortion of < /3 * (~100 nm) Ablation by x-rays(6x10 8 pulses in 2 FPY: (~25 mJ/cm 2, partly stopped by gas) 2.5x10 6 pulses/allowed atom layer removed) Defects and swelling induced by Absorption loss of <1%  -rays (~3) and neutrons (~18 krad/s)Wavefront distortion of < /3 * Contamination from condensable Absorption loss of <1% materials (aerosol and dust) >5 J/cm 2 threshold * “There is no standard theoretical approach for combining random wavefront distortions of individual optics. Each /3 of wavefront distortion translates into roughly a doubling of the minimum spot size.” (Ref. Orth)

Si 2 O irradiated at 105˚C, thermally annealed at 380˚C

MeV  / n ° irradiation of CaF 2 at room temperature also leads to the formation of color centers 10 MRad  -irradiation ( 60 Co) yields no color centers for virgin sample Absorptions at 335, 410, and 540 nm due to color centers Color center is a “missing fluorine” that captures an electron Annealing removes absorption due to n 0 induced color centers CaF 2 is “softened” by n 0 (  -irradiation induces color center of annealed sample) (Goal ~0.01/cm) Absorption Coefficient (cm -1 ) Annealed at 385˚C 766 kRad n˚ 1 Mrad  -rays ( 60 Co), post-annealing

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

Normal incidence reflectivity of various metals

Shallow angle reflectivity measurements of undamaged surfaces

Surface deformation leads to roughening and loss of laser beam quality  Single Shot Effects on LIDT:  Laser heating generates point defects  Coupling between diffusion and elastic fields lead to permanent deformation  Progressive Damage in Multiple Shots:  Thermoelastic stress cycles shear atomic planes relative to one another (slip by dislocations)  Extrusions & intrusions are formed when dislocations emerge to the surface, or by grain boundary sliding.  F 1 varies from a few to ~ 10 J/cm 2.  LIDT is a strong function of material & number of shots – it degrades up to a factor of 10 after only shots (survival to ~10 8 shots is needed).  Uncertainty in saturation behavior

Laser Damage Experiments to Metal Mirrors Spectra Physics YAG laser: 2J, 10 nm; 800, 500, , 266 nm Peak power density ~10 14 W/cm 2

Several kinds of Al surfaces have been fabricated and characterized Al 1100 showing grain boundaries and tool marks 75 nm Al on superpolished flat: ±2Å roughness, 10Å flatness diamond-turned Al 6061 MgSi occlusions

Computer Simulation Experiment Focused laser-induced surface deformation (Lauzeral, Walgraef & Ghoniem, Phys. Rev. Lett. 79, 14 (1997) 2706) (Walgraef, Ghoniem & Lauzeral, Phys. Rev. B, 56, 23, (1997) 1536) Uniform Laser-induced Surface Deformation Computer Simulation Focused Laser-induced Surface Deformation (vacancy density correlates with deformation) Surface deformation patterns after one laser shot of intensity near LIDT (Solution of continuum equations with defect diffusion in the self-consistent elastic field) (The model correctly predicts number of arms)

Progressive damage in multiple shots is caused by successive dislocation slip & grain boundary sliding Low DensityHigh Density

A reflectometer accurately measures reflectivity 100 ppm accuracy

Silicide occlusions in Al 6061 preferen- tially absorb light, causing explosive ejection and melting at only 1 J/cm 2 ; Fe impurities appear unaffected Several shots in Al 6061 at 80˚, 1 J/cm 2 Damage to aluminum at grazing angles Fe MgSi 1000x 1000 shots in Al 1100 at 85˚, 1 J/cm x Exposure of Al 1100 to 1000 shots at 85˚ exhibited no damage up to 18 J/cm 2

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Damage Regimes for Al-1100

Damage Regimes for % pure Al

Damage to aluminum at grazing angles Exposure of Al 1100 to shots at 85˚ exhibits catastrophic damage at fluence >20 J/cm shots in Al 1100 at 85˚, 20 J/cm % pure Al survives single shot damage up to the melting limit Single pulse in pure Al at 85˚, 180 J/cm 2