Transcript No Slide Title

Laser-IFE Final Optics Mini-Workshop
1. UCSD program on laser plasma and laser-material interactions
2. Damage-resistant optics research for IFE (including DP work)
UC San Diego
8 November 2000
Fusion Activities at UCSD
Center for Energy Research
Forman Williams, director
Charlie Baker, Assoc. Director
Combustion Division
Forman Williams
Kal Seshadri
Bob Cattolica
University of California, San Diego
Center for Energy Research
Fusion Division
VLT
Charlie Baker
Fusion Council
Charlie Baker, chairman
Fusion Programs
PISCES
ARIES
Confinement
Experiments
Plasma turbulence studies
IFE
Technology
Plasma
Processing
Energy
Technology
Theory
Part I. UCSD program on laser plasma
and laser-material interactions
UC San Diego
Laser Plasma and Inertial Fusion
Contributors:
PI’s:
M. Tillack, F. Najmabadi
staff:
M. Zaghloul, T. K. Mau, S. S. and Bindhu Harilal
students: D. Blair, M. Cherry
Program elements
•
•
•
•
Damage-resistant optics for IFE
–
–
Laser damage threshold of grazing-incidence metal mirrors
Environmental effects on laser optics
Beam propagation and breakdown physics
–
–
–
Breakdown physics at low gas pressure
Wavefront distortions beyond breakdown
Atmospheric effects
Damage resistant chamber materials
–
–
Thermomechanical modeling
Experiment planning
•
•
Z-machine proposal
X-ray source studies
Chamber physics
–
–
Modeling
Experiment planning
UC San Diego
Laser Plasma and Inertial Fusion
Historical perspective
summer 1996:
4/97-4/99:
7/99:
12/99:
summer 2000:
1/01:
initial contact with LLNL
IUT with LLNL to explore IFE simulation experiments
start of grant with OFES, lab space obtained,
optics damage activity initiated
YAG laser installed
staff increased, collaboration with GA started
DP work planned on optics and chamber physics
Main laser parameters
Spectra Physics Quantaray Pro 290
2J, 10 ns @1064 nm
700 mJ @532 nm
500 mJ @355 nm
300 mJ @266 nm
Peak power~1014 W/cm2
Nanolase dpssl:
10 mJ, <1 ns, 5 mW, @532 nm
Experiment Layout
photo
detectors
(visible
and UV)
ablation chamber
Al mirror
beam dump
flipper
thermopile
x-ray
detector
nanolaser
sampler
vacuum
chamber
camera
holographic
beam sampler
TE
Nd:YAG laser
TM
flipper
photodiode beam
(200 ps)
dump CCD camera or
variable attenuator
SH sensor
(waveplate and
beam cube)
500 MHz
scope
Shack-Hartmann Sensor
Incident Beam
Micro-lens
array
CCD camera
L
phase
E = | E| exp (i (x,y,z) )
Image acquisition,
Analysis software
The wavefront slope ( ) is determined by the displacement
of the centroids:

––––––
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 xiyj
Ring-Down Reflectometer
beam block
polarizing cube
1/4 w aveplate
partially-reflective
spherical output coupler
photodiode
Jolin, Sanders and Turner, Boulder Damage Conference 1988.
Fabrication and surface analysis is
provided by GA
Mirror Fabrication:
• Diamond turning
• Sputter coating
Surface Analysis:
• WYKO white light interferometer
• SEM with energy dispersive x-ray analysis
• Auger electron spectroscopy
Part II. Overview of damage-resistant
optics research for IFE
Geometry of the final laser optics
(20 m)
(30 m)
Prometheus-L reactor building layout
(SOMBRERO
values in red)
Threat Spectra
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* (~100 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
Mirrors and transmissive wedges are considered
Transverse energy 10-20 J/cm2 possible
85û
• Al at normal incidence ~0.2 J/cm2
• x10 due to cos 
• x10 due to increase in reflectivity
40 cm
stiff, lightweight, actively cooled, neutron transparent substrate
For 1.2 MJ driver w/ 60 beams @5 J/cm2,
each beam would be 0.4 m2
4.6 m
Grazing incidence metal mirror
Fused silica or CaF2 wedges
Mirrors vs. transmissive wedges
metal mirror
Fused silica wedge
•
Used in Prometheus-L and Sombrero
•
Used in DPSSL power plant study
•
Tighter tolerances on surface finish
•
Neutron damage
•
Low damage threshold
larger optics
•
–
Increased absorption
B-integral effects
Operation of the fused silica wedges
Orth, Payne & Krupke, Nuclear Fusion 36(1) 1996.
•
•
Linear array used in DPSSL study,
coupled to slab design of gain medium.
5˚ wedge, angled at 56˚
Brewster's angle
Amplifier slab
62 cm
•
•
•
Key concern is laser absorption -- 8% after 1 hr.
irradiation.
Operated at 400˚C for continuous annealing of
defects
60 times worse at 248 nm vs. 355 nm
57 cm
Why Aluminum is a Good Choice for the GIMM
Lifetime of multi-layer dielectric mirrors is
questionable due to rapid degradation by neutrons
Al is a commonly used mirror material
• usually protected (Si2O3, MgF2, CaF2),
Normal incidence reflectivity of metals
but can be used “bare”
100
• easy to machine, easy to deposit
Good reflectance into the UV
Thin, protective, transparent oxide
Reflectivity, %
95
90
Al
85
Ag
Au
Normal incidence damage threshold
~0.2 J/cm2 @532 nm, 10 ns
80
250
500
Wavelength, nm
750
1000
S-polarized waves exhibit high
reflectivity at shallow angles of incidence
Aluminum reflectivity at 532 nm
1
s -polarized
Reflectivity
0.95
0.9
p-polarize d
0.85
0.8
0.75
0
10
20
30
40
50
60
Angle of incidence
70
80
90
GIMM development issues*
• Experimental verification of laser damage thresholds
• Experiments with irradiated mirrors
• Protection against debris and x-rays (shutters, gas jets, etc.)
• In-situ cleaning techniques
• Large-scale manufacturing
• Cooling
• Wavefront issues: beam smoothness, uniformity, shaping,
f/number constraints
* from Bieri and Guinan, Fusion Tech. 19 (May 1991) 673.
Mirror defects and damage types
Dimensional Defects
Gross deform ations, >l
Compositional Defects
Surface morphology ,  <l
Gross surface
contamination
Local cont amination
CONCERNS
•
•
•
•
Fabrication quality
Neutron swelling
Thermal swelling
Gravity loads
• Laser-induced
damage
• Thermomechanical
damage
• Transmutations
• Bulk redeposition
• Aerosol, dust &
debris
MODELL ING TOOLS
Optical design software
Potential scattering theory
(perturbation analysis)
Fresnel equation solver
Potential scattering theory
(perturbation analysis)
Fresnel Modeling of Reflectivity
metal substrate
coating
n4, k4
n3, k3
n2, k2
contaminant
1
n1, k1
Incident
medium
• Wave propagation in four layers of stratified
media is modeled. (Born & Wolf)
• Each medium is homogeneous and characterized
by complex refractive index: n = n ( 1 + i k )
n = ( e/m )1/2; k = attenuation index
• TE (S) polarization is assumed.
• Refraction : n1 sin 1 = nj sin j
j = 2,3,4
( Snell’s Law )
• Reflection : ri,i+1 = (ni cos i - ni+1 cos i+1) / (ni cos i + ni+1 cos i+1) ( Fresnel )
• Reflectivity is computed by repetitive usage of the 3-layer formula:
ri = [ri-1,i + ri+1 exp (i2bi)] / [1 + ri-1,i ri+1 exp (i2bi)]
where bi = (2p/lo) di ni cos i , i = 2,3 and di is the layer thickness.
Auger electron spectroscopy surface analysis
Reflection of s-polarized (TE) waves
including thin oxide coating
1
1
0.75
0.75
Reflectivity
Reflectivity
Al 2O3 reflectivity at 532 nm
0.5
0.25
0.5
85û
60û
0.25
0û
l=532 nm
0
0
10
20
30
40
50
60
angle of incidence
70
80
90
0
0
0.2
0.4
0.6
0.8
Al 2O3 Coating thickness (h/ l)
1
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
1 = 80o
d2=0
1 = 0o
1
80o
60o
40o
lo = 532 nm
Al2O3 coating (10 nm)
Al mirror
20o
reflectivity
0.8
0.6
lo = 532 nm
Carbon film
Al mirror
0.2
1 =
d2=2 nm
1 = 80o
0.4
0o
d2=2 nm
1 = 0o
0
0
Carbon film thickness (nm)
0.05
0.1
0.15
0.2
0.25
Al2O3 coating thickness, d3/lo
0.3
Surface profile of undamaged Al-1100 mirror
SEM photos of damaged Al
50x
250x
1000x
4000x
Near-term experimental plans
• Install Shack-Hartmann sensor and ringdown reflectometer
• Examine source of grazing-incidence profile distortions
• Test Al-1100 mirrors to damage fluence at 80-85˚ incidence;
compare damage threshold and surface morphology with
normal incidence exposure
• Produce and test ultrapure Al and sputter-coated SiC mirrors
• Perform tests in vacuum (chamber available after Jan. 2001)
Diffraction and Wavefront Distortions
Diffraction-limited spot size:
do = 4 l f M/pD
l = 1/3 mm
f = 30 m (distance to lens)
do = 200 mm (zoomed)
D=1m
M <16
• “There is no standard theoretical approach for combining random
wavefront distortions of individual optics” (ref: Orth)
• Each l/3 of wavefront distortion translates into roughly a doubling of
the minimum spot size (ref: Orth)
Neutron and gamma effects
• Conductivity decrease due to point defects, transmutations, surface
roughening
–
Estimated in Prometheus at ~0.5% decrease in reflectivity (ref: private
conversation)
• Differential swelling and creep
–
–
–
Swelling values of 0.05-0.1% per dpa in Al (ref. Prometheus)
The laser penetration depth is d=l/4pk where k>10, so the required thickness
of Al is only ~10 nm. Swelling in Al can be controlled by keeping it thin. The
substrate is the real concern.
Porous (10-15%) SiC is expected to have very low neutron swelling.
• Absorption band at 215 nm in fused silica