CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
RADIATIVE SHOCK EXPERIMENT USING HIGH POWER LASER
M. Koenig1, A. Benuzzi-Mounaix1, N. Grandjouan1, V. Malka1, S. Bouquet2, X. Fleury2,
B. Marchet2, Ch. Stehle3, S. Leygnac3, C. Michaut3, J.P. Chieze4, D. Batani5, E. Henry5,
T. Hall6
1
Laboratoire pour I 'Utilisation des Lasers Menses, UMR 7605, CNRS - CEA - Universite Paris VI - Ecole
Polytechnique,, 91128 Palaiseau Cedex, FRANCE
2
CEA DRIF, BP 12, 91680 Bruyeres-le-Chdtel, FRANCE
3
Departement d'Astrophysique Stellaire et Galactique, Observatoire de Paris, 92 Meudon Cedex, FRANCE
4
CEA, Saclay, DSM/DAPN1 A/Sap, 91191 Gif-sur-Yvette cedex, France
5
Dipartimento di Fisica (G. OcchialinV, Universita di Milano-Bicocca and 1NFM, Via Emanueli 15,
20126 Milano, Italy
6
University of Essex, Colchester CO4 3SQ, United Kingdom
Abstract. High power lasers are nowadays tools that can be used to simulate some astrophysical
phenomena. In this experiment, the physical parameters have been chosen in order to reproduce
radiative shock conditions. The targets were made of a small cell (1 mm3) filled with Xenon at low
pressure (< 1 atm). On the laser side, we had a three layers pusher optimized for reaching conditions
where the launched shock in Xe is radiative. Rear side and transverse diagnostics allowed to determine
shock and precursor velocities, electronic density along the shock propagation. Comparison with
numerical simulations and models are presented. These experiments were performed with the LULI
laser.
hydrodynamic code simulations.We used a Xenon
INTRODUCTION
gas cell to observe this radiation phenomenon [2].
Radiative shocks are present in advanced star
envelopes and contributes to their mass losses.
They also exist in pulsed star atmospheres (Miras,
RR Lyrae, ...). Laboratory measurement of the
evolution and properties of such shocks will
provide a better understanding of these stellar
objects.
Radiative shocks are present either in dense
and hot plasmas where matter is at the local thermal
equilibrium (LTE) or in dilute atmospheres where
the plasma is at non-LTE. In both cases, a radiative
precursor appears before the shock front.
In this experiment, we intend to observe such a
feature using the nanosecond laser pulse of the
LULI laboratory. Target design is based on semianalytical model [1] predictions and full radiative
RADIATION PHYSICS AND TARGET
DESIGN
In a radiative shock, radiation emitted by the
hot compressed part of the fluid has a nonnegligible effect up or downstream in the flow. This
happens only for sufficiently high Mach numbers.
Compression ratios are then higher than with
classical shocks. When radiative processes are
dominant, they modify the structure of the waves.
At the shock front, temperature and density
discontinuities are smoothed and may disappear
under radiation effects.
Recent analytical work [1], adding the effects
of radiation energy and pressure to conservation
laws for mass, momentum and matter energy
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derived "generalized" Rankine-Hugoniot relations.
In this model, matter is a perfect gas radiating like a
black body at local thermal equilibrium (LTE). The
radiative properties of the shocks appear when
radiation and matter have identical pressures. To
reach this condition the shock velocity should
exceed a critical velocity defined by:
produced with less than a hundred joules of pulsed
laser light. An optimized three layers pusher drives
the shock in Xenon gas initially at rest in a small
quartz tube. A detailed comparison with FCI1
(CEA internal code) was performed to validate
MULTI as an every day tool to understand and
reduce experimental data. The pusher is composed
of a polyethylene ablator (2 fim), a titanium X-ray
screen (3 (im) and a polyethylene foam accelerator
(25 Jim). The laser pulse, sent on the ablator side of
the pusher, drives a shock in xenon which is at 1/5
of the atmospheric pressure.
The foam-xenon interface that acts as a piston
travels with a velocity of 70 km/s. When radiation
is included, we observe a long precursor as shown
in figure 1.
(1)
y is the usual ratio of specific heat for the perfect
gas, jiis the atomic mass, k is the Boltzmann
constant, a is the Stefan-Boltzmann constant, c is
the speed of light and n is the particle density. For
instance, a shock propagating in hydrogen gas at 35
km/s would not have radiative properties if matter
density is higher than 1014 cm"3. Experimental
diagnostics should have access to the structure of
shocks in order to demonstrate their radiative
properties. This implies to that we choose to drive
the shocks in a light and transparent medium (a gas)
with an atomic mass ji as high as possible (Xenon).
21
10 -
Ne
\
EXPERIMENTAL SET-UP
Experiments have been performed using three
of the six available beams of the LULI's Nd-glass
laser. The beams were converted at X = 0.53 jim,
with a maximum total energy £2^ ~ 100 J focused
on a same focal spot. The laser pulse was a square
pulse with a rise time of 120 ps giving a full width
at half maximum (FWHM) of 720 ps. Each beam
was focused with a 500 mm lens. We used phase
zone plates (PZP) [4], in order to eliminate large
scale spatial modulations of intensity and obtained
a flat intensity profile in the focal spot [5].
Characteristics of our optical system (lens+PZP)
were such that our focal spot had a 500 jum
FWHM, with a ~ 250 jum diameter flat region at
the center. Spatially averaged intensities between
4-6 1013 W/cm2 were obtained, depending on the
laser energy.
The diagnostics used in this experiment are
given in Figure2. The self emission diagnostic
consisted of a streak camera recording the emitted
light from the rear surface of the target at shock
breakout.
We had two line VISAR [6] with different
sensitivities to infer the shock velocity in the foam
and in the Xenon. Finally we implemented a MachZehnder interferometer to determine the electronic
density along the shock propagation. Two streak
camera were used, one looking at the fringes
(LONG), the other one for a transverse image at a
given position in the gas (TRANS).
r
foam-xe interface
shock front
20
10 19
precursor.
10 18
1017
0
mm
0.3
0.6
0.9
FIGURE 1. Effect of radiation in shocked Xenon given by
simulations. Dash and plain lines corresponds to the non
radiative and radiative case respectively.
A more quantitative design of the whole
experiment has been done with the radiation
hydrocode MULTI [3]. A radiative shock wave is
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deceleration), then to the right (small acceleration)
and finally back to the left.
This feature is quite similar to the hydro code
prediction of the piston velocity (figure 4.). These
three stages correspond to the first shock breaking
through at the foam-xenon interface, then a second
shock is coming due to reflection on the pusher
interface, finally the piston slows down .The mean
measured velocity is approximately 67 km/s.
Code computed velocities are in good
agreement with these experimental values and
suggest (see Fig. 1) that the shock in xenon is
strong enough to have a precursor. According to the
expected shock temperatures (10-15 eV), the shock
front in xenon is subcritical (Ne ~ 1021 cm"3) when
the precursor zone is much less than critical (Ne <
1020cm-3).
•••-——'
FIGURE 2. Experimental set-up
200jan
FIGURE 4. Foam-Xenon interface velocity. Plain and dashed
lines are ID simulations and experimental data respectively
Indeed, according to equation (1), the radiative
process becomes important when the shock velocity
is high enough for a given initial pressure. With our
experimental parameters, Dcrit ~ 25 km/s. In the
experiment, the measured shock velocity is twice
this value so we are confident to be in a regime
where radiation effects in the shock are important.
To the precursor, whose position is defined by
a change in density (fringe shift), we can associate a
much higher velocity than the shock velocity. A fit
of the trajectory, represented by the plain curve in
figure 5, gives an initial velocity -140 km/s. It is
again very close to the hydro code prediction with a
FIGURE 3. VISAR image for a Pa/5 filled gas cell.
With the VISAR, we could determine the
piston velocity which drives the shock in the xenon.
As we can see in figure 3, the fringes shift to the
left side (velocity jump associated with a small
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time decrease due to piston velocity slowing down
and possible 2D effects. Those effects can be
assessed by our transverse diagnostic. Here we are
looking at one longitudinal position in the cell (~
100-200 mm from the foam interface) with spatial
information in the transverse direction.
The fringe shift in figure 5 is directly related to
the electronic density Ne per plasma length; in our
case one fringe corresponds to ~ 4.5 1021 e/cmVjim. Assuming a 200 |im plasma created by
the precursor, one can deduce the variation of Ne
versus time (figure 6.).
1022
1021
1020
1019
1018
500JOB
1017
FIGURE 6, Electronic density vs time at a given position inside
Xenon (200 um from the foam interface). Dashed and plain lines
are experiment and simulations results respectively.
CONCLUSIONS
In this experiment, we observed a radiative
precursor preceding a strong shock wave in the
xenon gas cell. The results are in general agreement
with numerical simulations or semi-analytical
models. However the low number of shots cannot
give us a detailed picture of the radiation effects in
our experimental conditions.
ACKNOWLEDGEMENTS
The authors would like to thank F. Gex
(OPM/GASGAL), L. Poles (CEA/VALDUC), B.
Cathala (CEA/CESTA) for their fundamental
contribution to the target fabrication. Also Ph.
Moreau (LULI) has to be mentioned for his
contribution to the success of the experiment.
FIGURE 5. Interferometry in Xenon along the laser axis.
Dashed line and solid lines correspond to shock front and
precursor trajectories respectively
We observe a reasonable agreement between
hydrodynamic simulations and the experimental
results. However at late time, one can expect 2D
effects to become important.
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1370
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