Laser Induced Stresses

1
Mirror damage studies progress report
M. S. Tillack, T. K. Mau, K. Vecchio, T.
Perez-Prado (UCSD), J. Blanchard (UWisc), M.
Wolford (NRL), W. Kowbel (MER)
4th High Average Power Laser Program Workshop San
Diego, CA April 4-5, 2002
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Goals from last period of performance
Try to understand/explain why pure Al survives
beyond 10 J/cm2 Analysis performed by J.
Blanchard, tests planned at Nike SBS
constructed for YAG (will be tested soon), KrF
laser acquired Perform tests with contaminated
surfaces, acquire damage curves Aerosol
generator ablation tested laser testing
deferred until we have smooth beams Perform
tests on Al coated surfaces Damage limits vs.
coating technique and degree of attachment
Testing deferred until KrF and SBS are
working Sub-threshold irradiation of amorphous
Al EBSD performed on diamond-turned and
sputter-coated substrate Plan testing of MER
mirrors 1 and 4 mirrors have been
fabricated at MER Ray tracing analysis to
determine deformation limits, Kirchhoff analysis
of correlated defects Gross deformations
modeled, local defects to be modeled next
Kirchoff analysis underway, not yet completed
3
Previous data for 99.999 Al
Estimate of energy required to cause plastic
deformation 2sy E a DTo/(1n) fully-constraine
d, ratchetting limit E 75 GPa, n 0.33, a
25x106 sy 13-24 ksi (150-200 MPa) DT
71-107C e 16-24 J/cm2
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Laser-Induced Stress Models

• Base case is quasi-steady stresses induced by
  uniform (instantaneous) surface heating
• Consider impact of
• Non uniform heating over surface
• Volumetric heating
• Elastic waves
• Similar analysis will be applied to
• Chamber wall materials
• rf ablation of liver tumors

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1. Gaussian Heating on the Surface
Aluminum t10 ns Gaussian half-width lt50 mm
for thermal effect Gaussian half-width lt100 mm
for stress effect
Temperature Ratio
Tau
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2. Volumetric Heating
Aluminum t10 ns Penetration depthgt0.1 mm for
effect Actual penetration depth is lt10 nm
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3. Elastic Waves - Inertial Effects
Comparison
Typical Stress Distribution
Aluminum t10 ns dimensionless time 4000, so
wave stress is inconsequential
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A 600 mJ Excimer Laser has been Acquired
Multigas (KrF, ArF, XeCl, etc.) Unstable
resonator option 20 ns pulse width
Unpolarized 248 nm optics purchased
9
Nike beamline prepared for damage threshold
measurements

• Current set up
• 6 J beam energy (60 J beam line)
• Linearly polarized (at the front end)
• 4 ns pulse length
• Square beam size 15.4 cm x 15.4 cm

f 3m lens
Experimental Schematic
Polarized Beam 7
Calorimeter
Al Mirror
Breadboard Adjustment for beam diameter
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Aerosol is generated with a small nebulizer
69.95
Bi-modal size distribution centered around 1 and
10 mmDifferent contaminants can be aerosolized

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Contamination by laser ablation is another
technique under investigation
10 mm
Coated window with cratering at locations of
laser-induced damage
Adherent coating of particles with 1-10 mm
diameter form inside our vacuum chamber
12
Electron BackScatter Diffraction analysis of
changes in grain structure

• EBSD is used on our SEM to provide information
  about a samples microtexture.
• The local grain orientation is measured and the
  orientation distribution is displayed as a map.
• Other measurements can then be derived such as
  misorientation maps, grain size maps, and texture
  maps.

Oxford Instruments EBSD
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Grains in diamond-turned and coated mirrors
111
001
101
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Fabrication of a Fusion-Relevant GIMM has Been
Performed at MER
Large, stiff, lightweight, neutron damage
resistant, low activation mirrors are being
developed at MER C-C is a good substrate
material, but not very polishable This
Phase-I project will demonstrate the fabrication
technology for a 4 fusion-relevant metal mirror
using CVD SiC on a C-C substrate with an optical
coating on top Testing includes Mirror
characterization (at MER) Laser damage
testing (at UCSD and NRL)
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Schematic of Hybrid Composite/Foam Mirror
E-Beam Al (2 mm)
CVD-SiC (100 mm)
SiC Foam (3 mm)
Composite Face (1 mm)
SiC Foam (3 mm)
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Microroughness
CVD SiC
Si wafer
Manufacturer's data with 10 mm filtering on
the same type of wafer were 0.5 A rms
Conclusion CVD-SiC has about twice higher rms
over Si highly polished wafer
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Interferogram at the Rib Section
4 hex substrate Interferometry spot size
is 1, taken over the rib section Photo shows
no print-through, as found on commercial SiC
mirrors
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Ray Tracing Analysis of Gross Mirror Deformation
Limits
The ZEMAX optical design software was used to
analyze beam propagation between focusing mirror
and target. Gross deformation (d gt l) in the
form of a simple curvature (rc) due to thermal or
gravity load, or fabrication defect were
modeled d am2/2rc dsurface sag
Changes in beam spot size on the target and
intensity profiles were computed as the defect
size is varied. Prometheus-L final
optics systems as a reference Wavelength l
248 nm (KrF) Focusing mirror focal length 30
m GIMM to target distance 20 m Mirror
radius am 0.3 m Grazing incidence angle
80o Target half-diameter 3 mm Beam spot
size asp 0.64 mm
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Spot Size and Illumination Constraints Limit
Allowable Gross Mirror Deformation
The dominant effect of gross deformation is
enlargement (and elongation) of beam spot size,
leading to intensity reduction and beam
overlap. Secondary effect is non-uniform
illumination DI / I 2 for d 0.46
mm Limiting mirror surface sag for grazing
incidence is d lt 0.2 mm, (for a
mirror of 0.3 m radius) with the criteria DI
/ I lt 1, and Dasp / asp lt 10.
d 0 mm
d 0.46mm
d 0.92mm
2mm
d 0.92mm
0mm
Relative Illumination
0.46mm
y-scan
-2mm
2mm
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Goals for Next Period of Performance
Complete the analysis of nonuniform heat flux
with more realistic intensity profiles Finish
installation of KrF laser and SBS cell Test GA
and MER mirrors at both UCSD aand NRL Complete
analysis of localized deformation, perform
Kirchhoff analysis of correlated defects