Performance of Dielectric Mirrors for Inertial Fusion Application Lance Snead, Keith Leonard, and Jay Jellison Oak Ridge National Laboratory Mohamed Sawan University of Wisconsin, Madison Tom Lehecka Penn State University High Average Power Laser Program Workshop University of Wisconsin, Madison October 22-23, 2008

Background : Dielectric Mirrors The reflectivity (R) of a lossless multilayer stack of N successive quarter wave layers of alternating high (n Hi ) and low (n Li ) refractive index. High refractive index layer Low refractive index layer Substrate Mirrors are composed of alternating layers of a high and low refractive index films deposited on a substrate. The path difference between the thinner high index films and the thicker low index films results in constructive interference of the reflected light. Mirrors is tailored to achieve high reflectivity in a specific wavelength band. Ultra-high reflectivity (>99%), as compared to metal mirrors in the UV range: –Aluminum (80-90%) –Molybdenum (50-60%) –Tungsten (40-50%) –Silver and Gold (<40%)

Geometrical Model Used in 3-D Neutronics Analysis Bio-Shield Turning (M3) GIMM (M1) Beam Duct Focusing (M2) Shield Blanket

Fast Neutron Flux Distribution in Final Optics of HAPL SiC GIMM M2 M3 Flux (n/cm 2 s).02 dpa lifetime.0003 dpa lifetime 1.0 dpa 2 year

Background: Neutron Irradiation of Dielectric Mirrors Differing opinions as to the use of dielectric mirrors in nuclear environments. E.H. Farnum et al. (1995) HfO 2 /SiO 2, ZrO 2 /SiO 2, and TiO 2 /SiO 2 mirrors on SiO 2 substrates. Neutron fluence: n/cm 2, ºC. Excessive damage in HfO 2 /SiO 2 and ZrO 2 /SiO 2 mirrors, including flaking and crazing of films. Orlovskiy (2005) TiO 2 /SiO 2, ZrO 2 /SiO 2 mirrors on KS-4V silica glass. Neutron fluence: up to n/cm 2, 50 ºC. Dielectric mirrors showed no significant damage under irradiation, mirrors were severely damaged upon annealing (crazing.) Observations and opportunity ? Fewer and thinner bi-layers may improve resistance to radiation and thermal effects. Poor performance from SiO 2 substrates may be limiting performance; suggested use of more damage resistant substrates (eg: sapphire.) Damage resistance is sensitive to quality/purity of materials. Explore very high purity.

HAPL Irradiation : Test Samples Test samples consisted of 3 dielectric mirror types along with single-layer films to evaluate film / substrate interactions. Higher damage tolerant sapphire substrates used instead of SiO 2. Films deposited by electron beam with ion-assist; Spectrum Thin Films Inc. GE-124 fused silica bars included in test matrix. SampleQuantityFilm Thickness / Description Sapphire substrates only186 mm diameter x 2 mm thickness Al 2 O 3 single-layer on sapphire18Film thickness (36 nm) SiO 2 single-layer on sapphire18Film thickness (40 nm) HfO 2 single-layer on sapphire18Film thickness (27 nm) Al 2 O 3 / SiO 2 mirror on sapphire1826 Bi-layers, 2036 nm total thickness Al 2 O 3 / HfO 2 mirror on sapphire1814 Bi-layers, 924 nm total thickness HfO 2 / SiO 2 mirror on sapphire1811 Bi-layers, 768 nm total thickness

Pre-Irradiation Mirror Thickness

Neutron Irradiation mirror SiC TM Fused silica 3 samples of each mirror, monolayer and substrate irradiated to 0.001, 0.01 and 0.1 dpa One order higher than Farnum and Vukolov Factor of five higher than HAPL M2 mirror Irradiation temperature °C

Post Irradiation Examination and Testing Visual inspection. Signs of delamination, cracking or flaking. Measurement of relative specular reflectance. Perkin Elmer, Lamda 900 photospectrometer, equipped with 6º relative specular reflectance accessories. Measurements were made on the dielectric mirrors relative to an aluminum mirror standard. Thermal annealing treatment. 300 and 400ºC, 1.5 hrs with 3ºC/min heating/cooling rate Vacuum <1x10 -6 torr. Density of bars by density gradient column.

Visual Inspection of Neutron Irradiated Samples Changes in color are observed with increasing neutron exposure.  Highest dose samples nearly opaque to visible light.  Some annealing of color centers observed following thermal treatments. No visible signs of cracking or delamination. Slight speckling appearing on some annealed samples: no correlation between temperature, dose or material type. Unirradiated controls are all clear to visible light dpa 0.01 dpa 0.1 dpa controls

Visual Inspection of Neutron Irradiated Samples dpa 0.01 dpa 0.1 dpa controls Compared to Vukolov study, current material is quite stable upon post-irradiation thermal annealing. Substrate KS-4V Fused Silica TiO 2 /SiO 2 layers Vukolov 2005

Fused Silica Bar Samples Amorphous SiO2, through gamma or neutron irradiation rapidly densifies. Annealing will recover both dimension and refractive index.

Stress Induced in Dielectric Mirrors sapphire SiO 2 Al 2 O 3 SiO 2 HfO 2 Al 2 O 3 Sapphire was chosen as a substrate for it relatively stable performance under irradiation: relatively small swelling, and shallow temperature dependence Assumption: by closely matching irradiation-induced dimensional change of substrate and layers, induced stress will be minimized, increasing lifetime. However, we don’t know the irradiation performance of these microcrystalline or amorphous materials.

248 Optical Property Changes: HfO 2 / SiO 2 mirrors Gradual shift in or peak reflectivity range to lower wavelengths with dose. Limited effect at 0.01 dpa. Maximum reflectivity measurement may have a systematic error due to use of limiting aperture (due to small sample) on the normal spot size of the spectrometer.

Optical Property Changes HfO 2 / SiO 2 mirrors Annealing Effects Shifting of peak reflectivity range to lower wavelengths occurring with increasing annealing temperature. Shifting observed in both irradiated and control materials. Spectra of 0.1 dpa irradiated mirror annealed at 400ºC suggests considerable damage to mirror. Neutron Irradiation Effects Slight shift in working range, little or no reduction in reflectivity.

Optical Property Changes: Al 2 O 3 / SiO 2 mirrors Doses up to 0.01 dpa resulted in a peak shift to slightly higher wavelengths. Reflectance spectra exhibits limited change with dose. 248

Optical Property Changes Al 2 O 3 / SiO 2 mirrors Annealing Effects Annealing resulted in limited shifting of the peak reflectivity as compared to other mirror types. Differences between 300 and 400ºC annealing diminishes with increasing dose. A significant loss in reflectivity with 400ºC annealing is possible (further evaluation underway.) Irradiation Effects Relatively small amount of change measured of all mirror types.

Optical Property Changes: Al 2 O 3 / HfO 2 mirrors Peak reflectivity of as-received mirrors were off-specifications. No significant change in reflectance spectra to 0.01 dpa. Lower wavelengths shift observed following irradiation to 0.1 dpa. 248

Optical Property Changes: Al 2 O 3 / HfO 2 mirrors Neutron Irradiation Effects No significant change in reflectance curves to 0.01 dpa. Annealing Effects Mirror type appears more stable to thermal anneal than that of other mirror types examined.

Summary Samples exposed up to 0.1 dpa with and without thermal annealing at 300 and 400ºC show no signs of delamination or cracking. Mirrors show no significant degradation in reflectance up to doses of 0.1 dpa, with shifts in the peak reflectance curve of up to 10 nm towards lower wavelengths occurring at higher doses. Of the materials studied, the HfO 2 /SiO 2 mirrors show the most sensitivity to radiation dose and thermal effects despite having the fewest number of film layers. Reflectance spectra of Al 2 O 3 / HfO 2 mirrors appear least sensitive to radiation and combined radiation + annealing. –Stability and matched behavior of constituent materials, low number of film layers?

Implication for HAPL The initial poor performance and resulting dismissal of dielectric mirrors as unstable in reactor environments appears unfortunate and misguided. The combination of a more stable substrate (sapphire,) combined with higher quality materials, and the selection of more behavior matched materials under irradiation appear to have led to more stable materials. Farnum(95) : glass substrate, glass/ceramic layers, failed by 0.01 dpa Vukolov (2005) : fused silica substrate glass/ceramic layers, failed by 0.01 dpa This Work : sapphire substrate, glass/ceramic and ceramic/ceramic, ok to 0.1 dpa Results of this work, while preliminary, are encouraging for use of dielectric in HAPL - HAPL first mirror (assumed dielectric), lifetime dose is 0.03 dpa, appears ok. - HAPL final mirror (assumed grazing incidence metal mirror), has substantially flux. Its 2-year dose is approximately 1 dpa. Suggests possibility of dielectric.

Future Work Optical Testing Spectrophotometer – further evaluate absolute reflectance of films through transmission technique (may not be possible on high dose samples) Ellipsometry – evaluate film thickness changes in single-layer deposited samples. Carry out LIDT of non-irradiated mirrors, if results are acceptable, carry out irradiated testing. Structural Characterization X-ray diffraction analysis. Cross-sectional transmission electron microscopy. Raman microprobe – evaluate interfacial strains at interface. Higher Dose Irradiation Complete irradiation to 1 dpa (2 year assumed limit for GIMM) and 3 dpa. Include representative bulk materials for bulk property measurement.

Silica densified 0.77% 20.02±0.01mm

Label Working range, nm Coating Manufacturer MaterialsLayers PT#590 – 740TiO 2 /SiO 2 13 “Luch”, Podolsk, Russia PZ#590 – 670ZrO 2 /SiO 2 15 LT#550 – 650TiO 2 /SiO 2 17 “LOGF”, Lytkarino, Russia LZ#640 – 740ZrO 2 /SiO 2 23 Substrate: Silica glass KS-4V,  25 x 2 mm

Neutron Irradiation ContainerSamplesFluence, n/cm 2 Duration, hours Flux, n/cm 2 s 1PZ5, PT5, LT5, LZ x PT6, PT7, LT8, LT x The samples were inserted in two hermetic aluminium containers which then were filled with He gas. Irradiation was performed in the water-pool nuclear reactor IR-8. The fast neutron fluences were measured by accompanying iron films of isotope Fe 54 enriched to 99,92%. In result the reflectivity bands of LT8 and LT9 shifted towards short wavelengths, the spectra of other samples remained unchanged.

Heating regimes in Vacuum and Atmosphere Coatings of all Lytkarino samples were damaged during heating. Podolsk samples kept their coatings and optical properties Reflectivity bands of all mirrors shifted towards short wavelengths in heated state ~2 nm / 10  C. Working range of Lytkarino samples remained shifted at 2 nm after getting colder Thermal Tests

Expected IFE Mirror Irradiation Environment Expected doses: Total neutron flux to mirror: ~ 2.2x10 13 n/cm 2 s (first mirror) to 1x10 11 n/cm 2 s (final mirror) Total neutron fluence in IFE in one year, assuming 80 % plant availability: About 7.2 x n/cm 2 per FPY (first mirror), ~0.02 dpa for 30 year lifetime About 4.4 x n/cm 2 per FPY (final mirror), ~ 1 dpa in 2 years Effects on mirrors: Differences in radiation and thermally induced swelling or contraction of the film layers. Changes in surface roughness. Radiation / thermally induced structural changes within a given layer. Radiation / thermally induced mixing or formation of interlayer compounds. Reduction in peak reflectivity and shift towards lower wavelengths.