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.

                         

Grains in diamond-turned and coated mirrors

111

101

001

                         

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