mirror damage studies – progress report 4th high average power laser program workshop san diego,...
TRANSCRIPT
Mirror damage studies – progress report
4th High Average Power Laser Program WorkshopSan Diego, CAApril 4-5, 2002
M. S. Tillack, T. K. Mau, K. Vecchio, T. Perez-Prado (UCSD), J. Blanchard (UWisc), M. Wolford (NRL), W. Kowbel (MER)
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
Previous data for 99.999% Al
Estimate of energy required to cause plastic deformation:2y = E To/(1–) fully-constrained, ratchetting limitE = 75 GPa, =0.33, = 25x10–6 y = 13-24 ksi (150-200 MPa) T ~ 71-107˚Ce ~ 16-24 J/cm2
• Base case is quasi-steady stresses induced by uniform (instantaneous) surface heating:
• Consider impact of:1. Non uniform heating over surface2. Volumetric heating3. Elastic waves
• Similar analysis will be applied to:– Chamber wall materials– rf ablation of liver tumors
Laser-Induced Stress Models
πκ t
k
qTsurface
2=
θ −==
1surface
r
TE
€
q = q0 e−(r / λ )2
Tgaussian
Tuniform
=tan−1 τ( )
τ
τ = 4κ t /λ2
1. Gaussian Heating on the Surface
0
0.2
0.4
0.6
0.8
1
.0001 .001 .01 .1 1. 10.
Time
Stress Ratio
00.2
0.40.60.8
11.2
.001 .01 .1 1. 10. 100.Tau
Ratio
Aluminum; t=10 ns;
Gaussian half-width <50 m for thermal effect
Gaussian half-width <100 m for stress effect
Tau
Tem
per
atu
re R
atio
Aluminum; t=10 ns;
Penetration depth>0.1 m for effect
Actual penetration depth is <10 nm 0
0.2
0.4
0.6
0.8
1
0 5 10 15 20
Z
delta
€
′ ′ ′ Q = q0γe−γ x
z = γ κt
2. Volumetric Heating
€
Tvolumetric
Tsurface
=1−π
2z1− erfc(z)[ ]
Typical Stress Distribution
-1
-0.75
-0.5
-0.25
0
0 5 10 15
X
Stress
-1.6
-1.2
-0.8
-0.4
0
0 2 4 6
Time
Peak Stress
Wave
Surface
Aluminum; t=10 ns; dimensionless time ~ 4000, so wave stress is inconsequential
Comparison
3. Elastic Waves - Inertial Effects
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
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
Polarized Beam 7
Breadboard Adjustment for beam diameter
Calorimeter
Al Mirror
Experimental Schematic:
Aerosol is generated with a small nebulizer
$69.95Bi-modal size distribution centered around 1 and 10 m
Different contaminants can be aerosolized
Contamination by laser ablation is another technique under investigation
Adherent coating of particles with 1-10 m diameter form inside our vacuum chamber
10 m
Coated window with cratering at locations of laser-induced damage
Electron BackScatter Diffraction analysis of changes in grain structure
Oxford Instruments EBSD
• EBSD is used on our SEM to provide information about a sample’s 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.
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)
Schematic of Hybrid Composite/Foam Mirror
E-Beam Al (2 m)
CVD-SiC (100 m)
SiC Foam (3 mm)
Composite Face (1 mm)
SiC Foam (3 mm)
Microroughness
• Manufacturer's data with 10 m 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
Si wafer CVD SiC
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
• The ZEMAX optical design software was used to analyze beam propagation between focusing mirror and target.
• Gross deformation ( ) in the form of a simple curvature (rc) due to thermal or gravity load, or fabrication defect were modeled
= am2/2rc [surface 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 = 248 nm (KrF)Focusing mirror focal length = 30 mGIMM to target distance = 20 mMirror radius am = 0.3 mGrazing incidence angle = 80o
Target half-diameter = 3 mmBeam spot size asp = 0.64 mm
WallTarget
GIMM
Focusing Mirror
LaserBeam
Ray Tracing Analysis of Gross Mirror Deformation Limits
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:I / I ~ 2% for = 0.46 m
• Limiting mirror surface sag for grazing incidence is :
< 0.2 m, (for a mirror of 0.3 m radius)
with the criteria: I / I < 1%, and asp / asp < 10%.
y-scan
=0.92m
0.46m
0m = 0.92m
= 0.46m
= 0 m
-2mm
+2mm
+2mm
Rel
ativ
e Il
lum
inat
ion
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