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Progress in fabrication and testing of
grazing-incidence metal mirrors
John Pulsifer and Mark Tillack
with help from Patrick Rye and Matt Aralis
HAPL Project Meeting
30-31 October 2007
Washington DC
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Trials and Tribulations in GIMM Testing
Trial
Tribulation
Outcome
No vendor could fabricate
high-quality custom mirrors
We had to learn to make
our own mirrors
3x higher damage obtained in
1%Cu solid solution coatings
High shot count requires high A custom beam homogenizer Stable, smooth irradiations
rep rate and multiple gas fills, had to be designed and tested are now possible
creating unpredictable profiles
High shot count requires a
reliable light source
Our LPX is anything but
reliable; <50% up time
It’s working now; we’ve
acquired over 1.6 x 107 shots
Chamber contamination has
returned to haunt us
Maximum fluence is
compromised; absorption
on specimen a problem
TBD
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1%Cu solid-solution Al alloy coatings have
been fabricated and tested at UCSD
• Small alloy additions can have a large
enhancement to mech. props. (10 X hardness)
• Precipitates are dangerous for a mirror,
so solid solution films were fabricated.
• 5-µm Al-1%Cu sputtered on 4” Si wafers,
polished by CMP to < 1 nm RMS, < 10 nm PV
• Much smaller grains than pure Al, and
smoother finish as coated.
Evidence suggests a 3 X increase in Ys
Nano-indentation
performed on:
•Alumiplate, D-T
•UCSD sputtered 1%Cu
alloy, 5 microns thick
Magnetron coater,
UCSD fabrication lab
E (GPa)
H (MPa)
Ys (MPa)
Alumiplate
96
983
-
1%Cu alloy
151
10,556
-
71
-
20
-
-
65
Pure Al
0.5%-Cu alloy*
*Mechanical Behavior of Contact Aluminum Alloy
David T. Read, Yi-Wen Cheng, J. David McColskey, and Robert R. Keller
Mat. Res. Soc. Symp. Proc. Vol. 695 © 2002 Materials Research Society
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Beam homogenization is used to create a
more uniform intensity distribution
A custom homogenizer design was needed to provide
standoff and low divergence across the shallow target
MLA1
MLA2
relay plane
(homogenized)
1m
focus lens
relayed,
demagnified
target plane
GIMM
Fourier lens
new gas
old gas
homogenized
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Both beam and specimen are more uniform
resulting in more uniform damage
Beam NOT homogenized
Footprint: 0.4 x 4 mm
Beam homogenized
Mirror #111 (D-T Al)
Location #3:
2 to 4 J/cm2
1,596,164 shots
Footprint: 1 x 10 mm
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Contamination on mirrors and chamber
window is evident at high shot counts
• Location #1 is a 129,000 shot test at 4.8 J/cm2 (no visible layer)
• Locations #2 and #3 are both ~1.5 million shots each with fluence
ranging from 4.6 to 2.2 J/cm2
Absorption on surface increases due to contamination.
Fluence on specimen falls as chamber window is being coated
during testing.
Will this happen in a power plant?
• Chamber pressure is < 2 x 10-6 Torr for all tests.
• Electron spectroscopy will be performed to identify the
contaminant so the source can be eliminated.
•Possible suspects:
Translation stage grease
Epoxy on electrical feedthrough
Pump oil from rough pumping
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High cycle data is our top priority,
with a goal of >108 shots
We have acquired 1.65 x 107 shots so far without specimen failure.
Current homogenizer limits fluence to < 5 J/cm2; A better homogenizer can
allow us to collect data in the 1-to-2 x 106 range to fill out the curve.
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The LPX laser seems to be failing
more than the mirrors these days…
Borrowed from Roundel, Nov. 1992.
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Plastic strain accumulation,
extrapolation revisited
Empirical model: Fn = F1 N(s-1)
s = 0.89 , Jee et al* pure Al
Contamination on the specimen
surface could be affecting our
high-cycle data more negatively
than low-cycle data.
Surface preparation and material
composition, however, can also
affect the value of s.
*Laser-induced
damage on single-crystal metal surfaces
Yong Jee, Michael F. Becker, and Rodger M. Walser
J. Opt. Soc. Am. B/Vol. 5, No. 3/March 1988
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Summary
•Alloy mirrors fabricated at UCSD show a 3-fold improvement in damage
fluence that suggests a 3-fold improvement in yield strength over pure Al.
•The homogenizer produces a smooth beam that eliminates doubt about the
local fluence on target and produces uniform damage.
•Contamination (activated by UV light) is a problem for high-cycle data
collection and may be a problem in a fusion power plant.
•1.65 x 107 shots is the longest test performed so far, and the specimen has
not failed. The laser fails more often these days…
•Provided that surface absorption remains constant, plastic strain
accumulation theory may accurately predict long term damage.
•“Effect of laser pulse duration on damage to metal mirrors for laser-IFE”
was presented at the 39th Boulder Damage Symposium on 24 September.
•“Development of damage-resistant metal mirrors for laser-IFE” was
presented at the 22nd Symposium on Fusion Energy on 21 June.
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Future plans
1. Achieve more fluence with a different homogenizer (larger aperture,
AR-coated)
•
Current homogenizer limitations:
 Small aperture (1 cm x 1 cm) limits energy input
 Non AR-coated surfaces cause more energy loss
 Max fluence on target is 4.8 J/cm2
2. Chamber contaminant source to be identified and eliminated.
•
•
Throughput to target is reduced up to 50% after millions of shots due to
window contamination
Absorption at specimen surface is likely changing drastically due to
contamination and negatively affecting high-cycle data.
3. Continue shooting to achieve 108 shots