Transcript Magnetic Fusion Power Plants - University of California

Laser Induced Damage Threshold (LIDT)
of Grazing Incidence Metal Mirrors
Mark S. Tillack
T. K. Mau
Mofreh Zaghloul
(S. S. and Bindhu Harilal)
Laser-IFE Program Workshop
February 6-7, 2001
Naval Research Laboratory
Statement of Purpose and Deliverables
Statement of purpose
Our research seeks to develop improved understanding of damage mechanisms
and to demonstrate acceptable performance of grazing incidence metal mirrors, with
an emphasis on the most critical concerns for laser fusion. Through both experimentation and modeling we will demonstrate the limitations on the operation of reflective
optics for IFE chambers under prototypical environmental conditions.
Deliverables:
Measure LIDT at grazing incidence with smooth surfaces.
June 30, 2001
Model reflectivity and wavefront changes of smooth surfaces. Aug. 30, 2001
Measure effects of defects and surface contaminants on
reflectivity, LIDT and wavefront.
Model reflectivity and wavefront changes due to defects and
contamination.
Budget: $330k
Jan. 31, 2002
Jan. 31, 2002
1. Background
Reference Geometry of the Final Optics
(20 m)
(30 m)
Prometheus-L reactor building layout
(SOMBRERO
values in red)
Damage Threats
Two main concerns:
• Damage that increases absorption (<1%)
• Damage that modifies the wavefront –
–
spot size/position (200mm/20mm) and spatial uniformity (1%)
Final Optic Threat
Nominal Goal
Optical damage by laser
>5 J/cm2 threshold (normal to beam)
Nonuniform ablation by x-rays
Nonuniform sputtering by ions
Wavefront distortion of <l/3 (~80 nm)
(6x108 pulses in 2 FPY:
2.5x106 pulses/atom layer removed
Defects and swelling induced
by g-rays and neutrons
Absorption loss of <1%
Wavefront distortion of < l/3
Contamination from condensable
materials (aerosol and dust)
Absorption loss of <1%
>5 J/cm2 threshold
Mirror Parameters
85Þ
40 cm
stiff, lightweight, actively cooled, neutron transparent substrate
4.6 m
Grazing incidence metal mirror
Al at normal incidence (1/3 or 1/4 mm) ~0.2 J/cm2
x10 due to cos q
x10 due to increase in reflectivity
Transverse energy ~ 20 J/cm2 is possible
For 1.2 MJ driver w/ 60 beams @5 J/cm2, each beam would be 0.4 m2
Aluminum is the 1st choice for the GIMM
Lifetime of multi-layer dielectric mirrors is
questionable due to rapid degradation by neutrons
Normal incidence reflectivity of metals
100
Al is a commonly used mirror material
• usually protected (Si2O3, MgF2, CaF2),
but can be used “bare”
• easy to machine, easy to deposit
Reflectivity, %
95
90
Al
85
Ag
Au
Aluminum reflectivity at 532 nm
80
1
250
s -polarized
750
1000
Wavelength, nm
0.95
Reflectivity
500
0.9
• Good reflectance into the UV
p-polarize d
0.85
• Thin, protective, transparent oxide
0.8
0.75
0
10
20
30
40
50
60
Angle of incidence
70
80
90
• Normal incidence damage threshold
~0.2 J/cm2 @532 nm, 10 ns
GIMM development issues are well established*
• Experimental verification of laser damage thresholds
• Wavefront issues: beam smoothness, uniformity, shaping,
f/number constraints
• Experiments with irradiated mirrors
• Protection against debris and x-rays (shutters, gas jets, etc.)
• In-situ cleaning techniques
• Large-scale manufacturing
• Cooling
* from Bieri and Guinan, Fusion Tech. 19 (May 1991) 673.
2. Experiments
Fabrication capabilities are important to enable
us to optimize mirror performance
Mirror Fabrication:
• Diamond turning
• Sputter coating
Substrates considered:
• Bulk Al (cheap)
• CVD SiC ($550 for 3-cm disk, l/50, <2Å)
• superpolished fused silica (2”, l/10 , $335)
• 30-cm Si wafers (free)
Rohm & Haas SiC:
l/50 flat, <2Å
1.5 x 15 cm diamond-turned Al flat
Si wafers
(TBD)
Surface Analysis
Surface Analysis:
50x
• WYKO white light interferometer
• SEM with energy dispersive x-ray
• Auger electron spectroscopy
1 mm
SEM photos of damaged Al
1000x
20 mm
Surface profile of undamaged Al mirror
The UCSD laser lab is used to test GIMMs
Spectra Physics QuantaRay laser:
2J, 10 ns @1064 nm
700, 500, 300 mJ @532, 355, 266 nm
Peak power~1014 W/cm2
A ring-down reflectometer is used to obtain
accurate measurements of reflectivity
beam block
polarizing cube
specimen
1/4 w aveplate
partially-reflective
spherical output coupler
photodiode
A Shack-Hartmann sensor is used to
measure wavefront changes
Incident Beam
Micro-lens
array
CCD camera
L
phase
E = |E| exp (i (x,y,z) )
Image acquisition,
Analysis software
The wavefront slope ( q) is determined by the displacement
of the centroids:

q––––––
L
The wavefront is reconstructed by fitting the measured values to
a basis function, e.g., Hermite or Zernike polynomials
 ao + a 1o x + a 01 y + a 11 xy + ... + a ij xi yj
Spherical wave from a pinhole:
144 mm spatial resolution
l/50 sensitivity
2. Modeling
Tools for modeling effects of damage
on beam characteristics
Dimensi ona l Defects
Gr oss def o r amt i os,n >l
Comp ositio n al Defects
S urface m o r p h o l,o g<l
y
Gr oss s urface
co n am
t i nat i o n
L ocal c o nam
t i nat i o n
CONCERNS
• Fa brica tion
q u ali t y
• Laser-i n d u c e d
d ama g e
• Ne u ron
t swell i n g
• T herm o mec h a ncai l
d ama g e
• T herm al swel ling
• Tran smu t ati o ns
• Aeroso l ,dust &
d e bris
• B u l kre dep o ist i o n
• Gra v ty
i l oad s
MODELIN G TO OLS
Op tcal
i desi g nso f ware
t
Wa ve scat et r i n tg heo r y
Fresnel equat i o so
n l ver
Wa ve scat et r i n tg heo r y
Fresnel Modeling of Reflectivity
metal substrate
n4, k4
n3, k3
coating
n2, k2
contaminant
q1
n1, k1
Incident
medium
• Wave propagation in four stratified layers of
media is modeled, each with complex n
• Refraction : n1 sin q1 = nj sin qj j = 2,3,4
• Reflection : ri,i+1 = (ni cos qi - ni+1 cos qi+1) /
(ni cos qi + ni+1 cos qi+1)
• Reflectivity for 3 layers:
ri = [ri-1,i + ri+1 exp (i2bi)] / [1 + ri-1,i ri+1 exp (i2bi)]
where bi = (2p/lo) di ni cos qi , i = 2,3 and di is the layer thickness.
• Overall intensity reflectance : R = |r2|2
Tasks:
• Examine effects of coating material and contaminant on mirror
optical properties, and compare with experiment
• Assess importance of transmutation on optical properties of
coating and substrate
Example: Effect of Surface Contaminants
• Surface contaminants (such as carbon) on mirror protective
coatings can substantially alter reflectivity, depending on
layer thickness and incident angle.
• Uniform film thickness is assumed.
d2=0
q1 = 80o
d2=0
q1 = 0o
1
80o
60o
40o
lo = 532 nm
Al2O3 coating (10 nm)
Al mirror
20o
reflectivity
0.8
0.6
d2=2 nm
q1 = 80o
0.4
lo = 532 nm
Carbon film
Al mirror
0.2
q1 = 0o
d2=2 nm
q1 = 0o
0
0
Carbon film thickness (nm)
0.05
0.1
0.15
0.2
0.25
Al2O3 coating thickness, d3/lo
0.3
Ray Tracing
• When surface defect  > l, the effect on beam propagation can be assessed
using a ray tracing approach.
• ZEMAX-EE optics design software will be used:
- User-defined surfaces (shape, optical properties)
- Complete polarization ray tracing
- Nonlinear model of thermal effects on index
of refraction and material expansion.
Tasks:
- Evaluate surface deformation from expected loads.
- Quantify allowable surface deformation (shape and size) to meet beam
propagation requirements (spot size/location, intensity uniformity,
absorption).
Scattering Theory
• When surface deformation  < l, scattered wave is composed of specular
and diffuse components.
• Two analysis approaches:
- Perturbation theory (Raleigh-Rice):
- Physical optics (Kirchoff):
 << l
<l
• Surface roughness characterized by surface height distribution, (r).
For Gaussian (r), overall scattered intensity is in the form:
Isc = Io e-g + Id
where Io = scattered intensity from flat surface,
g = f (s/l, q1, q2), s : rms height, q1, q2 : incident, reflected angle
Id = scattered diffuse intensity
Tasks:
- Characterize surface damage using measured data and/or modeling
- Evaluate scattered intensity onto targets in terms of wave front
distortion and depolarization, using analytic (and numerical) models.
Final Optic Threats and Planned Research Activities
Final Optic Threat
R equi r ement
Evaluation
D efects and s wllie ng Abs orption los s 1%
< 60Co, g/nÞ irradiation
(g-rays adnneutrons)
(Al, SiO2, C aF2)
Wavefront distortion
PIE
<0.1 mm
Modeli ng
Optical d ama ge by
>5J/c m2 threshold
Te st Al GIMM
la ser (LI DT)
(normal to be am)
Te st LI DT of
irradiate doptic s
Conta mination
Abs orption los s 1%
< Evaluate loss e snda
da m ge
a u
de to thin
>5 J/c m2 dam age
fil m s
thres hold
Ablation by x-ray s
Sputtering by ionic
de bris
Mitigation
Ann ealing
Ada ptive p
otics
Optimiz e urfac
s es
R econditi on surface s
C alculate eff ect of
ga s loc
b king
Evaluate feasibilit y
of fast shutter
<10–4 monola yer per Me aure
s rate for Al, Evaluate w avefront
shot
SiO2 and CFa2 optic s distortion and pump
po w e
r for ga spuff
Model v ery s m all
ablation rates
-4
<10 monola yer per C alculate sputtering Analyze fe aibility
s
shot
with exi sting models of mag. d eflection
and data b ase
Evaluate ga s uff
p
Final Optics Program Plan
RADIATION DAMAGE (neutron and gamma effects)
Scoping Tests: Irradiation & PIE (incl. annealing)
Extended testing of prime candidates
Damage modeling
LASER-INDUCED DAMAGE
LIDT scoping tests for GIMM, materials development
System Integration
Laser damage modeling, 3w data from NIF
CONTAMINATION THREATS
Modeling
Test simulated contaminants
Mitigation
System Integration
Mitigation
System Integration
Mitigation
System Integration
X-RAY ABLATION
Scoping tests (laser-based x-ray source)
Modeling
ION SPUTTERING
Calculate sputtering, gas attenuation
FY 2001
|
FY 2002
|
FY 2003
|
FY 2004
|
FY2005
Final Optic Threats and Planned Research Activities
Fin a lOp t ci Threa t
Defects a n dwels l i n g
i n d ced
u b g-ra
y ys and
neu t r os n
Co nam
t i ant i o n oo pn ct i
fro mconden sab e
l ma er
t ials (aer oso land d u s) t
Ev a l u a nt i o
Co an dg/ n Þ
i rra dia io
t ns
Al, SOi 2, CaF2
Wa vefr o n tis dt rto i o of
n
Ota
b i nab s otir op nand
<0 . 1mm
ref lectance s pect ra
Ealv uat eswel l i nogfAl
m ri r o sr
Devel o pmo d es tl o
ex t a
r p oate
l t oI FE
2
>5 Jcm
/ t h es
r h o (l n
d oma
r l Test d
amage t h
resh o l ad t
t ob eam)
g razi n gin c ience
d f or Al
Tes tdam aegt h es
r hld
oof
i rra dia ed
t o p cs
ti
Ab s rp
o t i o n sl oof <1
s %
E val uate lo ses and d ama ge
due t ot h i fni l m
s.
2
>5 J/cm d a m
age t h
res h o l d
Ab a
l t i o n xb-ra
y ys
<10–4 m o n ayer
o l p er sh o t
S p ueri
t tn g bi oy nc i deb rsi
<10-4 m o n ayer
o l p er sh o t
Op tcal
i d
amage b yl aser
Req u remen
i
t
Ab s rp
o t i o n sl oof <1
s %
60
Mi t i gtiao n
t i t oa n ea
nl
oHea
f to p cs
defect sat d fi feren tT
E val uate feasi b i l i ot yf
ad a tipv e ocs
pti
Pre pare a n d pshoAl
li
m ri r osr usi n g ario
v us
met h osd
S uace
r f reco n d i t i o n i n g
Calc uate
l effec to fgas p u f f
b l cok i n g
Ealv uat efeas i bli it yof
fast sh u er
tt
Kr o rXe gas p uf:f
C
alcu a
l te wav efr o n t
d si t o r t i an
o nd p u power
mp
E xtrap o ltea at
d awi t h
m o els
d t ov ery smal l
ab a
l t i o rate
n
Meas uerrate f or Al, SiO2
an dCaF2 o p cs
ti
Calc uate
l s p uert it n wi
g t h Anal y ze feas i biti yl o f
ex i sti n gm o els
d an d ata
d
magnet ic def lect i o n
base
C
alcu a
l te b lck
oin g by
gas ( i n
cl u d i nwav
g efr o n t
an d p u m pissi nueg
s)