A comparison of optical trains based on a GIMM

1
A comparison of optical trains based on a GIMM
or a Dielectric Mirror final optic
HAPL San Diego, 9th Aug. 2006
Malcolm W. McGeoch
PLEX LLC 280 Albany St. Cambridge MA
02139 617-621-6300
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• Baseline HAPL final optical
  parameters
• 2.5MJ at 5Hz, 40 illumination beams each 62.5kJ
• 2 Jcm-2 in optical distribution ducts.
• Duct aspect ratio 61, each beam 3x18 beamlets
• (area of one beam 3x18x(0.24)2 3.1m2)
• 4. Focal length 39m (GIMM) or 42m (all-Dielectric
  case)
• Vertical slits in blanket, total 0.6 of 4p
• (slit size 1.35m high x 0.22m wide for GIMM case)

24cm x 24cm beamlet from de-multiplex array
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The final optics must function to attenuate the
neutron flux
target
3x1024 /beamline /FPY
n
laser building
lt 1x1019 /beamline /FPY lt 1mrem/hr/day at one
day after shutdown
(M. Sawan, this meeting)
Attenuation factor of 106 !
4
We can encase the optical path with 1.5m - 3m of
concrete and attenuate using the angles that are
needed in any case for distribution around the
sphere
target
n
3x1024 neutrons/beamline /FPY
lt 1x1019 /beamline /FPY
Concrete ferritic steel H2O coolant
5
40 port arrangement 3 tiers, 8 longitudes each
hemisphere
View from above North Pole
Elevation
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Uniformity for 9-element basket N 3,4,5 with
g 0.8, 0.9, 1.0
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Elevation sketch of beams for all-dielectric
final optics
(3J cm-2)
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Plan sketch of beams in one quadrant for
all-dielectric final optics
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Radiation loads in all-dielectric design
Mirror location total n/FPY/cm2
n gt0.1MeV/cm2 ngt1MeV/cm2 gamma/cm2 M1 32.5m
from target 2.3e20 2.3e20 2.3e20
8e19 M2 9.5m from M1 6e17 4.5e17 3.5e17
3e17 M3 15.5m from M2 1e16 5e15 3e15
1e16 M4 1.6m from M3 2e14 3e13 1.5e13
2e14 M4 6m from M3 2e13 2e12 1e12
2e13 Estimates courtesy of M. Sawan.
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Radiation resistance of multilayers compared to
requirements
Category layer mixing reflectivity
damage resistance Dose 1e19cm-2 OK OK
(vis) ? (4 of FPY) 1e20cm-2 OK
anneal? ? (44 of FPY) 1e21cm-2 4nm?
anneal? ? (4 FPY)
I. I Orlovskiy and K. Yu. Vukolov, Thermal and
neutron tests of multilayered dielectric mirrors
Fus. Eng. Des. 74, 865-869 (2005)
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Layer mixing? (248nm mirror layers are about 35nm
thick)
Mixing of layers was thought () to be a
showstopper for dielectric mirrors, but data on
irradiated multilayers for X-ray optics and
superconductors eases this concern
() R. L. Bieri and M. W. Guinan, Grazing
incidence metal mirrors as the final elements in
a laser driver for inertial confinement fusion,
Fusion Technology 19 673-678 (1991).
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Exposure of XUV mirrors to 1.1e19cm-2 (1-2MeV
neutrons)
low index Si or B4C, or C
high index Mo or W
N bilayers
d spacing
S. P. Regan et al. An evaluation of multilayer
mirrors for the soft X ray and extreme
ultraviolet wavelength range that were irradiated
with neutrons Rev. Sci. Instrum. 68(1), 757-760
(1997)
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XUV mirrors tested by Regan et al. (exposure temp
270-300C) Composition d (nm) substrate
N Mo/Si 8.78nm Zerodur 50 W/B4C 2.28nm Si
wafer 100 W/C 2.53nm Si wafer 100 Mo/Si 18.
7nm Si wafer 25 Results Mo/Si mirrors had
slight shift in peak L and slight
reflectivity decrease, exactly consistent with
270-300C known thermal effects. W mirrors had
opposite shift in peak L and slight increase in
reflectivity, again consistent with known thermal
effects. No effects attributable to fast neutron
exposure of 1.1e19cm-2 (10-2 dpa/atom)
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Layered superconductors have been tested for
fusion reactor use
2nm AlN layers
9 - 26nm NbN layers (not to scale)
20 bilayers
R. Herzog et al. Radiation effects in
superconducting NbN / AlN multilayer films J.
Appl. Phys. 68, 6327-6330 (1990).
Results Jcgt108Am-2 at 4.2K and 20T, before and
after irradiation No degradation of 2nm AlN
layers at 1x1019 neutrons/cm2 (gt0.1MeV)
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Lack of layer mixing is consistent with
expectations
XUV reflectivity and enhanced Jc are both
sensitive to the roughness of the surfaces of
layers, and the above results both indicate less
than about 1nm of induced roughness after
irradiation to 10-2 dpa. Bieri and Guinan ()
estimated mixing of roughly 3nm/(dpa)1/2,
where 1dpa 5x1020 (14MeV) n/cm2 for most
dielectrics For low doses (much less than1
dpa), this diffusion approach does not apply (for
example, at 10-2 dpa, 99 of layer remains
undisturbed). A more critical test will occur at
gt 0.1dpa (5x1019 neutrons cm-2) (20 of
FPY)
() R. L. Bieri and M. W. Guinan, Grazing
incidence metal mirrors as the final elements in
a laser driver for inertial confinement fusion,
Fusion Technology 19 673-678 (1991).
16
Dielectric mirrors are being evaluated for the
final optic
30 layers 1.9µm short absorption
path Neutron-stable sapphire substrate 500C
anneal cycle removes neutron-induced absorption
17
Neutron irradiation produces optical absorption
in dielectrics
18
248nm neutron-induced absorption is annealed out
at 500C
Sample 0.6cm SiO2, 6.6e20 neutrons cm-2. --gt
post anneal loss in 2µm dielectric path is
negligible
Data from J. Latkowski, HAPL meeting, Rochester,
Nov. 2005
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Planned neutron irradiation of mirrors
All the reported multilayer exposures have been
to about 1019/cm2 The key near term need is for
exposure to 1020/cm2 (44 FPY) A variety of
KrF mirrors and GIMM quality aluminum mirrors
will be exposed to up to 1021 neutrons cm-2 at
ORNL (gt1FPY). Some mirrors will be irradiated at
high temperature (250C and 500C) (K. J. Leonard
et al. 14th laser IFE progm. wkshp, ORNL, March
2006)
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GIMM design options (slide courtesy M. Sawan)

• Two options considered for GIMM materials and
  thicknesses
• Both options have 50 microns thick Al coating
• Option 1 Lightweight SiC substrate
• The substrate consists of two SiC face plates
  surrounding a SiC foam with 12.5 density factor
• The foam is actively cooled with slow-flowing He
  gas
• Total thickness is 1/2"
• Total areal density is 12 kg/m2
• Option 2 Lightweight AlBeMet substrate
• The substrate consists of two AlBeMet162 (62
  wt.Be) face plates surrounding a AlBeMet foam(or
  honeycomb) with 12.5 density factor
• The foam is actively cooled with slow-flowing He
  gas
• Total thickness is 1"
• Total areal density is 16 kg/m2

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Radiation loads in SiC GIMM design
5Jcm-2
Mirror location total n/FPY/cm2
n gt0.1MeV/cm2 ngt1MeV/cm2 gamma/cm2 GIMM(M1)2
4m from target 4.2e20 4.0e20 3.6e20
1.4e20 M2 14.9m from M1 1.0e18 9e17 8e17
4e17 M3 1.6m from M2 2e16 6e15 2.8e15
1.3e16 M3 6m from M2 2e15 4e14 2e14
1.3e15 Estimates courtesy of M. Sawan.
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Unknowns remain for both the GIMM and dielectric
approaches After neutron exposure Laser-induce
d damage has not been measured in either
case Reflectivity at 248nm (KrF) has not been
measured in either case Substrate optical
quality has not been measured (except to 1019
cm-2) Other considerations The GIMM requires
polarized light gt less random illumination The
GIMM area is 14m2 vs dielectric area 1.9m2 gt
cost is high The neutron maze is much more
effective in the dielectric case