Transcript PPT - Fusion Energy Research Program

Laser Driver-Chamber Interface Research
Contributors:
Mark Tillack
Tak Kuen Mau
Farrokh Najmabadi
Mofreh Zaghloul
Dustin Blair*
(4/25/01)
Stephen Payne
Nasr Ghoniem
Zhiqiang Wang*
Qiyang Hu*
Neil Morley
Sandy Quan*
James Williams*
IFE Chamber Technology Program Review
April 26-27, 2001
*graduate students
Los Angeles, CA
Title of Work: Laser Driver-Chamber Interface Studies
Principal Investigators: M. Tillack (UCSD), S. Payne (LLNL), N. Ghoniem (UCLA)
Overall Objective: Demonstrate acceptable performance and lifetime of transmissive and
reflective final optics and the ability to deliver driver energy on target within specifications.
Technical Approach: (1) Establish damage criteria through detailed analysis;
(2a) Develop mesoscopic models of surface deformation of metals and solve continuum
equations for generation/diffusion/interaction/deformation; (2b) Fabricate, characterize and
test metal mirrors; (3) Perform irradiation tests on Si2O and CaF2; (4) Study beam
propagation physics experimentally and analytically.
Accomplishments:
• Established GIMM damage criteria through analysis of wave propagation
• Developed new algorithms and analyzed surface deformation instability
• (Built new laboratory and) Operated an Al GIMM beyond the normal-incidence LIDT
• Determined transmissive optic darkening (g and n+ g) and effectiveness of annealing
• Analyzed and tested liquid fracture from a GILMM
Relevance to OFES/Fusion: “Final optics” is one of the top 4 IFE power plant issues
described in the FESAC-IFE review panel (1996), see Table 1:
http://aries.ucsd.edu/PUBLIC/INFO/FESAC_IFE96.html
Funding (past 3 years):
FY00 $510K
FY01 $370K
This program emphasizes scientific exploration
applied to solving the key issues for IFE
Primary research objectives:
1. Understand the effects of surface defects on beam
propagation and establish criteria for surface degradation.
2. Understand the mechanisms of laser damage in metals and
find material and mechanical design solutions to maximize
the LIDT and lifetime of GIMM’s in the fusion environment.
3. Understand the formation of color centers in transmissive
optic materials and demonstrate an acceptable solution to
reduce the effects (e.g., by annealing).
4. Understand the physics of gas ionization by lasers at low
pressure and environmental effects on beam propagation.
Establish criteria on chamber media.
Outline
• Background

Design options, research goals, issues and requirements
• Grazing incidence metal mirror research

Laser-induced damage modeling

Damage experiments

Modeling of beam degradation
• Neutron damage to transmissive optics
• Laser propagation in IFE chambers
Geometry of the Driver-Chamber Interface
85ϋ
40 cm
stiff, lightweight, actively cooled, neutron transparent substrate
(20 m)
4.6 m
Grazing incidence mirrors
(30 m)
(SOMBRERO
values in red)
Prometheus-L reactor building layout
Si2O or CaF2 wedges
Final Optic 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)
Sputtering by ions
Ablation by x-rays
(~25 mJ/cm2, partly stopped by gas)
Wavefront distortion of <l/3 * (~100 nm)
(6x108 pulses in 2 FPY:
2.5x106 pulses/allowed atom layer removed)
Defects and swelling induced by
g-rays (~3) and neutrons (~18 krad/s)
Absorption loss of <1%
Wavefront distortion of < l/3 *
Contamination from condensable
materials (aerosol and dust)
Absorption loss of <1%
>5 J/cm2 threshold
* “There is no standard theoretical approach for combining random wavefront distortions of individual optics.
Each l/3 of wavefront distortion translates into roughly a doubling of the minimum spot size.” (Ref. Orth)
Outline
• Background
• Grazing incidence metal mirror research
 Laser-induced damage modeling

Damage experiments

Modeling of beam degradation
• Neutron damage to transmissive optics
• Laser propagation in IFE chambers
Surface deformation leads to roughening
and loss of laser beam quality
Single Shot Effects on LIDT:
Laser heating generates point defects
Coupling between diffusion and elastic
fields lead to permanent deformation
 Progressive Damage in Multiple Shots:
 F1 varies from a few to ~ 10 J/cm2.
Thermoelastic stress cycles shear atomic
planes relative to one another (slip by
dislocations)
 LIDT is a strong function of material
& number of shots – it degrades up to
a factor of 10 after only 10000 shots
(survival to ~108 shots is needed).
Extrusions & intrusions are formed when
dislocations emerge to the surface, or by
grain boundary sliding.
 Uncertainty in saturation behavior
http://puma.seas.ucla.edu/web_pages
Surface deformation patterns after one
laser shot of intensity near LIDT
(Solution of continuum equations with defect diffusion in the self-consistent elastic field)
Focused Laser-induced Surface
Deformation (vacancy density
correlates with deformation)
Computer Simulation
Experiment
Uniform Laser-induced
Surface Deformation
Computer Simulation
(The model correctly predicts number of arms)
Focused laser-induced surface deformation (Lauzeral, Walgraef
& Ghoniem, Phys. Rev. Lett. 79, 14 (1997) 2706)
(Walgraef, Ghoniem & Lauzeral, Phys.
Rev. B, 56, 23, (1997) 1536)
Progressive damage in multiple shots is caused by
successive dislocation slip & grain boundary sliding
Low Density
High Density
Outline
• Background
• Grazing incidence metal mirror research

Laser-induced damage modeling


Damage experiments
Modeling of beam degradation
• Neutron damage to transmissive optics
• Laser propagation in IFE chambers
GIMM development issues*
• Experimental verification of laser damage thresholds
• Protection against debris and x-rays (shutters, gas jets, etc.)
• Wavefront issues: beam smoothness, uniformity, shaping,
f/number constraints
• Experiments with irradiated mirrors
• In-situ cleaning techniques
• Large-scale manufacturing
• Cooling
* from Bieri and Guinan, Fusion Tech. 19 (May 1991) 673.
Aluminum is the 1st choice for the GIMM
• Lifetime of multi-layer dielectric mirrors is
Normal incidence reflectivity of metals
1
questionable due to rapid degradation by neutrons
0.8
• Al maintains good reflectivity into the UV
0.6
Reflectivity
• Al is a commonly used mirror material
– easy to machine, easy to deposit
• Thin (~10 nm), protective, transparent oxide
0.4
0.2
Ag
Al
Cu
W
Au
Hg
Mo
0
200
Aluminum reflectivity at 532 nm
1
Reflectivity
0.95
•
0.9
•
•
•
p-polarized
0.85
0.8
0.75
10
20
30
600
800
1000
Wavelength, nm
s-polarized
0
400
40
50
60
Angle of incidence
70
80
90
Normal incidence damage threshold ~0.2 J/cm2
Grazing incidence raises s-reflectivity to >99%
Larger footprint reduces fluence by cos(q)
Combined effects hopefully raise the damage
threshold to >5 J/cm2
Engineered surfaces: we have fabricated and
characterized several kinds of Al surfaces
75 nm Al on superpolished flat:
±2Å roughness, 10Å flatness
diamond-turned Al 6061
MgSi occlusions
Al 1100 showing grain boundaries
and tool marks
The UCSD laser-plasma and laser-material
interactions lab is used for damage tests
(Good beam diagnosis is essential to understanding damage effects)
Spectra Physics YAG laser:
2J, 10 ns @1064 nm;
800, 500, 300 mJ @532, 355, 266 nm
Peak power density ~1014 W/cm2
Class 100
cleanroom
enclosure
100 ppm accuracy
Reflectometry
Profiling
Shack-Hartmann
Damage depends strongly on the surface
morphology and angle of incidence
Surface defects in Al 6061 preferentially absorb light, causing explosive ejection of occlusions
4000x
Several shots at normal incidence, 0.2 J/cm2
Several shots at 80˚, 1 J/cm2
Measurements are ongoing: purer surfaces, steeper angles,
longer exposures, recrystallization of amorphous coatings
http://aries.ucsd.edu/Etech/ife.shtml
Analysis and initial experiments for GILMM indicate
possibility of high “damage” threshold for this
alternative idea
ABLATOR code results:
• No surface spallation damage limit predicted for
slow compression beams (20 ns). Damage limit for
fast ignitor beams (10 ps) in the 1-10 J/cm2 range
High energy density experiments:
• Large surface waves from 3 ns, 1
1mm wavelength laser impulse die out in 50 ms
J/cm2 absorbed,
Hg surface 1 ms after the laser pulse.
Wave amplitude estimate 0.81 mm
Beam-normal “damage limit”
calculations for spallation for
10 ps, 1/3 mm laser light
Liquid
Metal
Damage
Limit
[J/cm2]
Spall
Pressure
[GPa]
Al
10.23
1.47
Na
4.91
0.31
Hg
0.84
0.40
Hg surface 57 ms after the laser pulse.
No motion detectable
Outline
• Background
• Grazing incidence metal mirror research

Laser-induced damage modeling

Damage experiments

Modeling of beam degradation
• Neutron damage to transmissive optics
• Laser propagation in IFE chambers
Modeling the effects of damage on beam
characteristics helps us establish damage limits
Dimensional Defects
Gross deformations,
>l
Compositional Defects
Surface morphology,
 <l
Gross surface
contamination
Local cont amination
CONCERNS
•
•
•
•
Fabrication quality
Neutron swelling
Thermal swelling
Gravity loads
• Laser-induced
damage
• Thermomechani cal
damage
• Transmutations
• Bulk redeposition
• Aerosol, dust &
debris
MODELLING TOOLS
Optical design
software (ZEMAX)
Scattering by rough
surfaces (Kirchhoff)
Fresnel multi-layer
solver
Scattering by particles
Fresnel modeling quantifies the
tolerable level of surface contamination
metal substrate
n4, k4
n3, k3
coating
n2, k2
contaminant
q1
n1, k1
Incident
medium
- Reflectivity is reduced with increasing contaminant
thickness.
- Effect of surface contaminant is diminished at
gracing incidence.
- Carbon (k ~ n) degrades reflectivity much more
than H2O (k ~ 0).
2 nm H2O film @80o
1
Reflectivity
40o
.8
0o
80o
H20
Carbon
.8
80o
60o
.6
.4
l = 248 nm
10 nm Al2O3 coating
Al mirror
0
1
2
3
4
Contaminant Thickness (nm)
40o
20o
q1 = 0 o
Reflectivity
1
q1 = 0 o
.6
80o
.4
No film
2 nm C
.2
q1 = 0o
0
0
80o
20
40
60
80
Al2O3 Coating Thickness (nm)
Specularly reflected intensity is degraded
by induced mirror surface roughness
• The effect of induced surface roughness on beam quality was investigated
by Kirchhoff wave scattering theory.
• For cumulative laser-induced and thermomechanical damages, we assume
Gaussian surface height statistics with rms height s.
1.0
Isc
Iinc
q1
0.8
q2
q1 = 80o
0.6
70o
0.4
Isc
60o
0.2
0.1
g
 Id
Io : reflected intensity from smooth surface
Id : scattered incoherent intensity
g : (4p s cosq1/l)2
0
0
 I 0e
0.2
0.3
0.4
0.5
e.g., at q1 = 80o, s/l = 0.1, e-g = 0.97
s/l
• Grazing incidence is less affected by surface roughness
• To avoid loss of laser beam intensity, s / l < 0.01
Outline
• Background
• Grazing incidence metal mirror research

Laser-induced damage modeling

Damage experiments

Modeling surface damage effects on beam quality
 Neutron damage to transmissive optics
• Laser propagation in IFE chambers
Neutrons and g-rays create defects in SiO2
which result in photon absorption
n0
Si
Oxygen Deficient Center
(ODC, 246 nm)
Normal Site
O
Si
neutrons
Full
thermal
recovery
Si
Si
O2-
g-rays
Non-Bridging Oxygen Hole
Center (NBOHC, 620 nm)
Si
O-
Si
Si+
T < 200 C
T > 400 C
Si
O
Si
Strained Si -O Bonds
(350-750 nm)
Si
E’ Center (213 nm)
We have discovered that IFE-relevant MeV g / n° irradiation
doses of fused silica lead to significant absorption at 350 nm
Dose = 100 GRad of n° = IFE-Relevant exposure of ~2 months at LANSCE
-1
Abs.Coeff.
) -1)
(cm (cm
Coefficient
Absorption
2.0
1.5
l = 350 nm
1.0
T=426˚C
T=426
C
o
0.5
0.0
200
T=105˚C
T=105
C
o
300
400
500
600
700
800
900
1000
Wavelength (nm)
• Residual defects remaining in Si2O depend on temperature
• 620 nm band is due to the “non-bridging oxygen hole center,” while the slow rise
•
•
may be due to “strained Si-O bonds”
Under these conditions, final optics last ~1 month
May be possible to reduce absorption further by annealing at higher temperatures
(e.g., ~ 700 °C for 24 hrs)
MeV g / n° irradiation of CaF2 at room temperature
also leads to the formation of color centers
60
Mrad g-rays
11 MRad
g-rays(60
( Co),
Co),post-annealing
post-annealing
o
766 kRad
kRad n˚
766
n
0.4
-1
Abs.Coeff. (cm )
Absorption Coefficient (cm-1)
0.5
0.3
0.2
0.1
o
Annealed at 385
385˚C
Annealed
C
0.0
200
300
400
(Goal ~0.01/cm)
500
600
700
800
900
1000
Wavelength (nm)
•
•
•
•
10 MRad g-irradiation (60Co) yields no color centers for virgin sample
Absorptions at 335, 410, and 540 nm due to color centers
• Color center is a “missing fluorine” that captures an electron
Annealing removes absorption due to n0 induced color centers
CaF2 is “softened” by n0 (g-irradiation induces color center of annealed sample)
Outline
• Background
• Grazing incidence metal mirror research

Laser-induced damage modeling

Damage experiments

Modeling surface damage effects on beam quality
• Neutron damage to transmissive optics
• Laser propagation in IFE chambers
Laser propagation near or beyond the
breakdown threshold is uncertain
 Laser intensity near the target:
1013 – 1014 W/cm2
 Threshold intensity is not welldefined; laser light partially ionizes
chamber gas at any intensity
 Gas “breakdown” occurs when
plasma density is high enough that
a substantial amount of laser light is
absorbed (avalanche process).
Data for Xe, except Turcu
 Previous work: breakdown threshold defined as intensity at which visible light is emitted
from the focal spot (most of the visible light is generated by the interaction of electrons
generated by ionization of the background gas with the neutral gas atoms).
 Wavefront distortion can occur at lower (or higher) plasma densities and laser intensities,
changing the beam profile on the target. This “threshold” intensity will depend on the
required degree of beam smoothness on the target, f number of the lens, beam coherence, etc.
 Multi-species and contaminated environmental conditions further complicate the physics.
Experimental setup for laser propagation tests
Initial measurements:
• Visible light emission
from the focal spot
• Variation in laser energy
intensity and temporal pulse
shape
• Wavefront variation
Planned:
• Emission spectroscopy
25 cm
•
•
•
Design allows for interchangeable optical boards to be
inserted through the side of chamber.
Final design of the vacuum chamber is completed and parts ordered.
Emphasis of OFES program beginning to shift into chamber physics.
Outline
• Background
• Grazing incidence metal mirror research

Laser-induced damage modeling

Damage experiments

Modeling surface damage effects on beam quality
• Neutron damage to transmissive optics
• Laser propagation in IFE chambers
• Applications
The science and technology of optics and laser-material
interactions have widespread applicability to industry
• Robust high-power optics are needed in many terrestrial, airborne and
space-based applications
• Laser-material interactions research is helping to improve laser
micromachining techniques*
Governing physics is very similar to IFE
• Laser absorption in surface
• Thermal response of surface
• Liquid hydrodynamics
• Evaporation
• Unsteady gas dynamics
(including chamber environment)
• Condensation
• Laser-cluster interaction
* work
partially supported by Hewlett Packard
Closing Remarks
• This relatively young program is already producing
valuable results.
• We are prepared to accelerate progress on the key laser
driver-chamber interface issues, thanks to growing
synergisms with related OFES and DP programs.
• The field of laser interactions offers exciting
opportunities to contribute across a broad range of
leading-edge sciences and technologies.
Summary of laser driver/chamber interface work
Quality of science and technology: Scientific methods and approaches
Modeling:
•
•
Meso-scale modeling of mechanical response to repeated laser irradiation
•
•
“Kinetic modeling” of defect annealing
Modeling of the generation of sub-surface defects and their effects on
surface deformation and roughening
Optical modeling: Fresnel, Kirchhoff scattering theory, ZEMAX
Experiments:
•
•
•
•
•
Irradiation tests of fused silica and CaF2
GIMM surface damage tests
Wavefront measurements
Determination of optic darkening and effectiveness of annealing
Fabrication and characterization of several mirror types, initial damage tests
Summary of laser driver/chamber interface work
Productivity and progress:
• Coordinated as a national team: LLNL, UCLA, UCSD (with support from
GA, LANL, SNLA)
•
Experimental progress
•Creation of the UCSD laser lab, fabrication and characterization of mirrors,
initial damage test results
•Determination of optic darkening and effectiveness of annealing
•
Modeling progress
•Determination of contamination and defect limits by wavefront modeling
•Meso-scale modeling of thermo-mechanical response to repeated laser
irradiation
•Modeling the generation of sub-surface defects and their effects on surface
deformation and roughening
•
Rate of progress: this program is <2 yrs old. Some parts initiated within past year
Summary of laser driver/chamber interface work
Relevance and impact:
•
•
•
•
•
•
Driver-chamber interface is one of the top 4 issues for IFE
Contributions to optical engineering, materials science, laser-ablation fields
Publications in both fusion and non-fusion literature
5 graduate students supported by this activity
Technology transfer to laser optics and micromachining industries
Internationally unique program addressing laser driver-chamber issues in an
integrated program
Selected Publications*
*For complete list, see presentation by W. R. Meier
Damage modeling
• N. M. Ghoniem and M. S. Tillack, "Theory and Measurements of Laser-Induced Damage of Reflective
•
Metal Mirrors in IFE Environments," 10th ICFRM (2001), to be published in J. Nucl. Mater.
D. Walgraef, N. M. Ghoniem and J. Lauzeral, "Deformation patterns in thin films under uniform laser
irradiation," Phys. Rev. B 56 (23) December 1997.
Damage experiments
• M. S. Tillack, S. A. Payne and N. M. Ghoniem, "Damage threats and response of final optics for laser•
fusion power plants," 2nd Int. Symp. on Inertial Fusion Science and Applications, Kyoto Japan, Sept. 2001.
M. Zaghloul and M. S. Tillack, “Laser-induced damage to metal mirrors for laser-IFE final optics”, 19th
IEEE/NPSS Symposium on Fusion Energy, Atlantic City, Oct. 2-5, 2001.
Wavefront modeling
• T.K. Mau and M.S. Tillack, "Modeling of Mirror Surface Damage Effects on Beam Propagation in a LaserDriven IFE Power Plant", 19th IEEE/NPSS Symposium on Fusion Energy, Atlantic City, Oct. 2-5, 2001.
Laser ablation
• D. Blair and M. S. Tillack, "Particle Formation and Dynamics in the Laser Ablation Plume," submitted to
the SPIE Conference on Micromachining and Microfabrication 21 - 25 October 2001, San Francisco, CA.