Optimization of plasma uniformity in laser-irradiated underdense targets
M. S. Tillack, K. L. Sequoia, B. O’Shay
University of California, San Diego9500 Gilman DriveLa Jolla, CA 92093-0438USA
H. A. Scott
Lawrence Livermore National LaboratoryP.O. Box 808Livermore, CA 94561USA
C. A. Back
General AtomicsP.O. Box 85608San Diego, CA 92186-5608USA
Objectives:
Studies of atomic processes in laser plasma require uniform conditions:
a) Predict the degree of uniformity in ne and Te for directly-heated underdense (non-LTE) targets
b) Explore the impact of physics models on the results
c) Propose design solutions to improve the uniformity
Under conditions of direct heating, the value of absorption coefficient is critical
If t<1 mm or I>1014 W/cm2, the targets expand too quickly
Key Physics Issue: Choice of Opacity and Ionization Models
Inverse bremsstrahlung, non-LTEIntroduction
Inverse bremsstrahlung in Hyades
Comparison with experiment
Hyades (Cascade Applied Sciences)1D rad-hydroGray (Sesame) or multi-group diffusionSaha or average atom ionization modelHelios
(Prism Computational Sciences)1D rad hydro5000-group computed opacities
Numerical Simulation
Experimental Geometry
(NIKE)1.6 kJ, 248 nm4 ns12˚ cone angle5x1012–5x1014 W/cm2
McWhirter condition
The density is uniform when Zeff is near a maximum and hydro expansion is small
(I<1014, t>1 mm)
Density uniformity
(ne>1.4x1014 Te1/2(E)3 cm–
3)
It’s difficult to achieve optically thin plasma with 2 mg/cc (5x1020) SiO2 targets 1 mm thick @ Te<300 eV
(note: ncr=16x1021/cm3)
Pillbox Target
Most of the plasma is non-LTE
Cases analyzed• 0– 6 at% Ti dopant
• 2–8 mg/cc
• 1–2.2 mm thickness
• 5x1012 – 4x1014 W/cm2
Experimental
Parameters:
High Fluence:
• 2.2 mm
• 3% Ti dopant
• 2.7 mg/cc
• 5.7x1013 W/cm2 (248 nm)
Low Fluence:
• 1 mm
• 6% Ti dopant
• 2.5 mg/cc
• 4.6x1012 W/cm2 (248 nm)
LTE
non-LTE
2.5 ns
Non-LTE ionization balance of Ti in 2 mg/cc SiO2 (Cretin)
2 ns
data courtesy of Prism Comp. Sci.
(2.5 at%)
The radiation mean free path at 150 eV is several
mm
Hyades 35-group, non-LTE avg. atom
Helios predicts much higher temperatures
Double-Sided Illumination
1 mm thick2.6 mg/cc SiO2
Same total laser input(2 x 2.5e12 or 2 x 3e13)
2-sided illumination provides a more uniform temperature profile at lower intensity
I=6x1013 W/cm2
Te,
eV
Time, ns
I=5x1012 W/cm2
Time, ns
Indirect radiation heating from end zones also can produce uniform temperature and
density
3 mg/cc SiO2
2.6 mg/cc SiO2 3% Ti
3 mg/cc SiO2Laser Laser
0 1 mm 2
0
20
40
60
80
100
120
140
160
-0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0.20
R, cm
Te, eV
1 ns
2 ns
3 ns
4 ns
Time, ns
R,
cm
Ne,
10
20
cm
-3
Time, ns
Time, ns
Te,
eV
2 ns
35 photon energy groups
Doping affects rad-hydroOpacity and Ionization Options in Hyades (pure
SiO2)
High Fluence Modeling Results
High IntensityBase Case Results
I=6x1013 W/cm2
Zeff
Time, ns
Higher laser intensity gives higher, slightly flatter temperature and faster, stronger
ionization
2.5 ns
2.5 ns
Conclusions:
• In this regime, results are sensitive to models used
• LTE and non-LTE results are quite different
• Doping has a significant effect on the radiation hydrodynamics
• Double-sided and indirect illumination both show promise
• More data are needed to help understand the underlying physics
SiO2 aerogel
with Ti dopant
}
Energy Balance
2.5 ns
Zeff
5x1012 W/cm2
Time, ns
Zon
e C
oord
inate
, cm
6x1013 W/cm2
Time, ns
Zon
e C
oord
inate
, cm
