Performance of Dielectric Mirrors for

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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
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Background Dielectric Mirrors

• 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 (gt99), as compared to
  metal mirrors in the UV range
• Aluminum (80-90)
• Molybdenum (50-60)
• Tungsten (40-50)
• Silver and Gold (lt40)

The reflectivity (R) of a lossless multilayer
stack of N successive quarter wave layers of
alternating high (nHi) and low (nLi) refractive
index.
High refractive index layer
Low refractive index layer
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Geometrical Model Used in 3-D Neutronics Analysis
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Fast Neutron Flux Distribution in Final Optics of
HAPL
.02 dpa lifetime
.0003 dpa lifetime
1.0 dpa 2 year
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Background Neutron Irradiation of Dielectric
Mirrors

• Differing opinions as to the use of dielectric
  mirrors in nuclear environments.
• E.H. Farnum et al. (1995)
• HfO2/SiO2, ZrO2/SiO2, and TiO2/SiO2 mirrors on
  SiO2 substrates.
• Neutron fluence 1019 n/cm2, 270-300ºC.
• Excessive damage in HfO2/SiO2 and ZrO2/SiO2
  mirrors, including flaking and crazing of films.
• Orlovskiy (2005)
• TiO2/SiO2, ZrO2/SiO2 mirrors on KS-4V silica
  glass.
• Neutron fluence up to 1019 n/cm2, 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 SiO2 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.

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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 SiO2.
• Films deposited by electron beam with ion-assist
  Spectrum Thin Films Inc.
• GE-124 fused silica bars included in test matrix.

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Pre-Irradiation Mirror Thickness
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Neutron Irradiation
Fused silica
mirror
SiC TM

• 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 175-200C

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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 lt1x10-6 torr.
• Density of bars by density gradient column.

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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.

controls
0.001 dpa
0.01 dpa
0.1 dpa
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Visual Inspection of Neutron Irradiated Samples
Compared to Vukolov study, current material is
quite stable upon post-irradiation thermal
annealing.
controls
0.001 dpa
0.01 dpa
Vukolov 2005
0.1 dpa
Substrate KS-4V Fused Silica TiO2/SiO2 layers
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Fused Silica Bar Samples
Amorphous SiO2, through gamma or neutron
irradiation rapidly densifies. Annealing will
recover both dimension and refractive index.
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Stress Induced in Dielectric Mirrors
SiO2
SiO2
HfO2
Al2O3
HfO2
Al2O3
sapphire
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
dont know the irradiation performance of these
microcrystalline or amorphous materials.
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Optical Property Changes HfO2 / SiO2 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.

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Optical Property Changes HfO2 / SiO2 mirrors

• Neutron Irradiation Effects
• Slight shift in working range, little or no
  reduction in reflectivity.

• 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.

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Optical Property Changes Al2O3 / SiO2 mirrors
248

• Doses up to 0.01 dpa resulted in a peak shift to
  slightly higher wavelengths.
• Reflectance spectra exhibits limited change with
  dose.

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Optical Property Changes Al2O3 / SiO2 mirrors

• Irradiation Effects
• Relatively small amount of change measured of
  all mirror types.

• 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.)

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Optical Property Changes Al2O3 / HfO2 mirrors
248

• 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.

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Optical Property Changes Al2O3 / HfO2 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.

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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 HfO2/SiO2 mirrors
  show the most sensitivity to radiation dose and
  thermal effects despite having the fewest number
  of film layers.
• Reflectance spectra of Al2O3 / HfO2 mirrors
  appear least sensitive to radiation and combined
  radiation annealing.
• Stability and matched behavior of constituent
  materials, low number of film layers?

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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.
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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.

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Silica densified 0.77 20.020.01mm
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Substrate Silica glass KS-4V, ?25 x 2 mm
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Neutron Irradiation
In result the reflectivity bands of LT8 and LT9
shifted towards short wavelengths, the spectra of
other samples remained unchanged.
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 Fe54 enriched to 99,92.
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Thermal Tests
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
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Expected IFE Mirror Irradiation Environment

• Expected doses
• Total neutron flux to mirror
• 2.2x1013 n/cm2s (first mirror) to 1x1011n/cm2s
  (final mirror)
• Total neutron fluence in IFE in one year,
  assuming 80 plant availability
• About 7.2 x 1017n/cm2 per FPY (first mirror),
  0.02 dpa for 30 year lifetime
• About 4.4 x 1020 n/cm2 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.