Transcript Document

Optimization of plasma uniformity in laser-irradiated underdense targets
M. S. Tillack, K. L. Sequoia, B. O’Shay
H. A. Scott
C. A. Back
University of California, San Diego
9500 Gilman Drive
La Jolla, CA 92093-0438
USA
Lawrence Livermore National Laboratory
P.O. Box 808
Livermore, CA 94561
USA
General Atomics
P.O. Box 85608
San Diego, CA 92186-5608
USA
Inverse bremsstrahlung, non-LTE
Density uniformity
Under conditions of direct heating, the
value of absorption coefficient is critical
The density is uniform when Zeff is near a maximum
and hydro expansion is small (I<1014, t>1 mm)
Introduction
Objectives:
Experimental Geometry
(NIKE)
Studies of atomic processes in
laser plasma require uniform
conditions:
1.6 kJ, 248 nm
4 ns
12˚ cone angle
5x1012–5x1014 W/cm2
a) Predict the degree of uniformity
in ne and Te for directly-heated
underdense (non-LTE) targets
(note: ncr=16x1021/cm3)
Inverse bremsstrahlung in Hyades
2.5 ns
Zeff
Numerical Simulation
b) Explore the impact of physics
models on the results
I=5x1012 W/cm2
c) Propose design solutions to
improve the uniformity
It’s difficult to achieve optically
thin plasma with 2 mg/cc (5x1020)
SiO2 targets 1 mm thick @ Te<300 eV
McWhirter condition
Hyades
(ne>1.4x1014 Te1/2(DE)3 cm–3)
(Cascade Applied Sciences)
1D rad-hydro
Gray (Sesame) or multi-group diffusion
Saha or average atom ionization model
Cases analyzed
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•
•
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Helios
(Prism Computational Sciences)
1D rad hydro
5000-group computed opacities
LTE
0– 6 at% Ti dopant
2–8 mg/cc
1–2.2 mm thickness
5x1012 – 4x1014 W/cm2
non-LTE
5x1012 W/cm2
Comparison with experiment
Experimental Parameters:
The radiation mean free path
at 150 eV is several mm
Doping affects rad-hydro
2 ns
Time, ns
If t<1 mm or I>1014 W/cm2, the targets expand too quickly
Key Physics Issue: Choice of Opacity and Ionization Models
2.5 ns
}
6x1013 W/cm2
Time, ns
Most of the plasma
is non-LTE
Opacity and Ionization
Options in Hyades (pure SiO2)
Zone Coordinate, cm
Zone Coordinate, cm
SiO2 aerogel
with Ti dopant
Time, ns
High Fluence:
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•
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2 ns
2.2 mm
3% Ti dopant
2.7 mg/cc
5.7x1013 W/cm2 (248 nm)
Low Fluence:
35 photon energy groups
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•
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High Fluence Modeling Results
Non-LTE ionization balance of Ti in
2 mg/cc SiO2 (Cretin)
Energy Balance
1 mm
6% Ti dopant
2.5 mg/cc
4.6x1012 W/cm2 (248 nm)
(2.5 at%)
data courtesy of Prism Comp. Sci.
Double-Sided Illumination
Pillbox Target
2-sided illumination provides a more uniform
temperature profile at lower intensity
High IntensityBase Case Results
Indirect radiation heating from end zones also
can produce uniform temperature and density
3 mg/cc
SiO2
Laser
1 mm thick
2.6 mg/cc SiO2
Same total laser input
(2 x 2.5e12 or 2 x 3e13)
Helios predicts much
higher temperatures
Hyades 35-group,
non-LTE avg. atom
2.6 mg/cc
SiO2 3% Ti
0
3 mg/cc
SiO2
1 mm
Higher laser intensity gives higher, slightly flatter
temperature and faster, stronger ionization
Laser
2
160
140
100
Te, eV
Te, eV
120
80
1
2
3
4
60
40
20
ns
ns
ns
ns
2.5 ns
2.5 ns
0
-0.20 -0.15 -0.10 -0.05
0.00
0.05
0.10
0.15
0.20
Time, ns
Zeff
Ne, 1020 cm-3
Te, eV
R, cm
I=6x1013 W/cm2
Time, ns
Time, ns
Time, ns
Conclusions:
• In this regime, results are sensitive to models used
R, cm
• LTE and non-LTE results are quite different
• Doping has a significant effect on the radiation hydrodynamics
I=6x1013 W/cm2
• Double-sided and indirect illumination both show promise
Time, ns
• More data are needed to help understand the underlying physics