Final Optics Update Jeff Latkowski, Ryan Abbott, Brad Bell, and Tom Felter HAPL Program Workshop Rochester Laboratory for Laser Energetics November 8-9, 2005 Work performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.

Overview Ions cannot be allowed to strike the final optic at high fluxes/fluences Laser damage testing to begin for (highly) neutron-irradiated fused silica Neutrons and gammas can cause significant damage, even at the penultimate optic and/or vacuum windows Three types of optics are under consideration: Grazing incidence metal mirror (UCSD) Thin Fresnel lens Multilayer dielectrics

Ions will cause severe problems for either final optic concept Aluminum GIMM will exfoliate at a rate of microns/day Fused silica Fresnel will melt in single shot 0.5 MeV a 3 MeV a 5.2 mA-hr 10.3 mA-hr 15.5 mA-hr 13 mA-hr 9.2 mA-hr 4.4 mA-hr Equivalent to 3 days for IFE final optic at 26 m Implantation at 90º (previous work suggests exfoliation more rapid at grazing angles)

Either final optic will need to rely upon ion deflection to survive Ion flux reduction of 104× is the goal (reduces exfoliation to <l/10 in 1 year) Helmholtz coil pairs would be placed ~13 meters from chamber center: Field strengths of <0.1 T (in the center of the pair) would be needed for optic at 26 meters Normal conductors should suffice and shielding looks doable Without deflection and only 10 mTorr of xenon, majority of ions reach the final optic With deflection, the ion flux is reduced by more than 104×

Basecase ion deflection parameters have been developed for the various options All cases assume final optic stand-off of 26 meters All cases use central field strength of 0.75 kG (0.075 T) For material cost of $10/kg, all coil sets <$1M Electrical cost of 5¢/kW-hr assumed Copper current density limit sets minimum coil cross sections Y. Iwasa (1994) reported Cu resistivity increase of 2.5% after fast neutron fluence of 2 × 1017 n/cm2. Significant (100-1000×) shielding will be needed.

It may be possible to reduce fields with alternate coil and/or beam orientations Gunslit option for ion mitigation (e.g., use 4 x 1 array of beamlets & deflect along the short side) Already may be desirable for GIMMs, where mirror ends up with 11:1 aspect ratio due to grazing angle Alternate coil configurations (e.g., non-Helmholtz) will be considered Deflect ions along short angle

The fused silica final optic story is taking shape Laser damage threshold is sufficient Optical absorption (with neutrons) is reasonable Thin Fresnel lenses can be made with required specs

The fused silica final optic story is taking shape, (Cont’d.) Laser-induced damage in SiO2 has been studied extensively Laser damage threshold is sufficient Optical absorption (with neutrons) is reasonable Thin Fresnel lenses can be made with required specs

In 1995, SiO2 samples were irradiated to dose of 1011 rads (~3 months IFE equivalent) Optical absorption looks acceptable at 2, 3w; thermal and “radiation” annealing studied; make thin (~0.5 mm) Fresnel lens for final optic; 4w absorption is too high due to E’ color center in SiO2; still may be material that can be annealed to work at 248nm. Laser damage threshold is sufficient Optical absorption (with neutrons) is reasonable Thin Fresnel lenses can be made with required specs

Additional annealing runs have been completed Previous neutron-induced absorption confirmed by annealing remaining samples at 380ºC Higher temperatures used for ½ of samples: Additional improvement in 2, 3 transmission With higher temperature, even 4 transmission has improved (E’ color center appears to anneal at 500ºC) l (nm) T (%) 248 56 351 96 532 97 72

Laser damage testing of neutron- irradiated fused silica will occur in 2006 Samples have been annealed and polishing will be completed in November. ES&H approval on testing plans in place. Laser system (2, 3) identified. Laser damage testing of the same samples will begin around first of year. Laser damage threshold is sufficient Optical absorption (with neutrons) is reasonable Thin Fresnel lenses can be made with required specs

Fabrication of fresnel lenses that meet IFE specifications will be demonstrated Laser damage threshold is sufficient Optical absorption (with neutrons) is reasonable Thin Fresnel lenses can be made with required specs Holographic technology has been demonstrated at 1mm thickness. Adapting this to 0.5mm is doable. Lithographic approach (needed for equatorial beams) needs to be demonstrated. 4-6” diameter samples will be fabricated.

Two-beam interference lithography can be used to fabricate the off-axis Fresnel lens segments Laser, 351 nm Launch optics d2 TCC Diverging beam d1 Fringe locking components Collimated beam d3 Exposure set up at LLNL Schematic illustration of the Fresnel lens pattern

Schematic of the lithographic process for the Fresnel lens panel fabrication Photoresist Substrate Substrate Photoresist UV light Develop Photoresist mask 1 (schematic) mask 2 (schematic) Substrate Wet etch or dry etch Schematic of the masks Substrate Schematic of the lithographic process The lithographic approach is more appropriate for fabricating the equatorial plane HAPL final optics

Laser damage testing of Fresnel lenses will be conducted at 2 and/or 3 Laser damage threshold is sufficient Optical absorption (with neutrons) is reasonable Thin Fresnel lenses can be made with required specs Fresnels fabricated in previous step will be damage tested on Mercury

A target chamber will be installed on Mercury during FY2006

First Mercury experiments support HAPL: Damage testing of aluminum GIMMs Goal: To validate GIMM mirrors for 5 J/cm2 @ 4 ns IFE operating conditions Experimental conditions: Angle of Incidence: 85º  0.1º Polarization: S Wavelength: 1w, 2w, or 3w Fluence: up to 10 J/cm2 @ 4 ns Vacuum level required: < 1 mtorr Substrate sizes: 2” round, 4” square Scatter/Incandescence detection camera Beam Dump GIMM To beam diagnostic (Energy, pulsewidth, Nearfield modulation) To darkfield damage detection system

Fused silica Fresnel lenses could be segmented and irradiated Given fused silica’s rapid saturation of defect concentrations, a facility such as ORNL’s HFIR might be able to provide relevant neutron doses. Laser damage threshold is sufficient Optical absorption (with neutrons) is reasonable Thin Fresnel lenses can be made with required specs Fresnels (or a portion of them) will be irradiated and the optical absorption will be verified.

Irradiated SiO2 Fresnels could be tested on Mercury to provide the ultimate admiral’s test The same Fresnels then could be damage tested in Mercury using the same methods utilized in the second step of our optics testing plan. Laser damage threshold is sufficient Optical absorption (with neutrons) is reasonable Thin Fresnel lenses can be made with required specs Success?

DPSSL basecase design uses only a single optic in the target bay TC, 18m ID, 20m OD 20m “Neutron pinhole” Telescope brings beams to focus near center of target bay shielding wall Final optic (Fresnel) focuses and deflects beam towards chamber center

Other designs must consider radiation damage to more than just the final optic Previous work with 3-D SOMBRERO neutronics model showed that neutron scattering was quite important: Penultimate optic dose might only be reduced by ~50x compared to final optic Can use shielded beamtubes (see Sawan work on these) Damage testing needed for dielectric, multilayer mirrors in addition to GIMMs and Fresnels Given importance of scattered neutrons, even vacuum windows at bottom of building may be a concern: They must be quite thick Even low neutron doses (e.g., Mrads) darken SiO2, especially at 4

We would like to begin radiation studies for dielectric materials Given large variety of materials & wavelengths, suggestion made (by Bayramian?) to test individual layer materials as first step: Consider Al2O3, CaF2, SrF2, SiO2, etc. Literature search on any known rad effects and unirradiated properties (index of refraction, optical transmission, CTE, etc.) Irradiate 4-8 materials to moderate dose (Mrads on HFIR?) Complete annealing runs, testing optical properties at each step Downselect to most rad-resistant materials; identify best material combinations Produce multilayers of best candidates at 2-4 Laser damage test virgin multilayers Downselect again and irradiate best multilayers; Repeat damage testing Given work on irradiated fused silica, LLNL ready to perform optical testing of rad materials However, overall HAPL cuts make it unlikely we can start work this year

Synthesis High ion fluxes must be kept off the final optics Ions can be deflected away from the final optics with reasonable magnetic fields and at a reasonable cost We are working towards an integrated, self-consistent design for a fused silica Fresnel lens as the final optic: Damage testing of irradiated fused silica and production of prototypical Fresnel lenses are the next steps Laser damage testing of both final optic candidates will be performed on Mercury this fiscal year Other optics, if used within the target bay, must be radiation tested as well