Evolution of electron temperature and electron density in indirectly driven spherical implosions
N. C. Woolsey,* B. A. Hammel, C. J. Keane, A. Asfaw, C. A. Back, J. C. Moreno, and J. K. Nash
University of California, Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, California 94551
A. Calisti, C. Mosse
´
, R. Stamm, and B. Talin
URA 773 case 232, Universite ´ de Provence, Centre St. Je ´ro ˆme, 13397 Marseille Cedex 20, France
L. Klein
Department of Physics, Howard University, Washington, DC 20059
R. W. Lee
Department of Physics, 366 LeConte Hall – 7300, University of California, Berkeley, California 94720
Received 29 July 1996; revised manuscript received 4 February 1997
Using spectroscopic measurements to extract electron density and temperature, we construct simulation-
independent time histories of the assembly and disassembly phase of an imploding core. To achieve this, we
show the hot dense plasma produced by indirectly imploding a gas-filled microsphere is a reproducible and
reliable plasma source. We further show that this plasma is suitable for detailed hydrodynamic and spectro-
scopic studies, and that the plasma provides a useful testbed for nonlocal thermodynamic equilibrium plasma
studies at extreme conditions. S1063-651X9709908-X
PACS numbers: 52.70.La, 52.25.Nr, 52.50.Lp, 52.58.Ns
The study of hot density plasmas is of interest to those
studying stellar atmospheres, inertial confinement fusion
plasmas, strongly coupled plasmas, and radiative properties
at extreme conditions 1. The production and diagnosis of
such plasmas is made difficult by their inherently transient
nature, their small spatial extent, and the need to verify that
the plasmas are hydrodynamically stable and reproducible.
Here we present the spectroscopic results from a hydrody-
namically stable reproducible high-energy density plasma,
and use it to track the implosion dynamics in a manner that is
independent of hydrodynamic simulation. The plasma pro-
duced is contained within an implosion core of a plastic mi-
crosphere prepared to have a small amount of seed gas that is
used as a probe of the hot dense matter. Although there have
been previous reports of single, peak high-density results
from implosion cores, the emission is dominated in those
plasmas by effects of nonsphericity of the implosions, hydro-
dynamics instabilities, laser nonuniformities, or laser-plasma
instabilities. The results reported here are derived from the
first stable and reproducible hot dense plasmas, and therefore
can be used to address questions on both the formation of
these plasmas and the effects of the extreme conditions on
ions embedded in them.
In these plasmas, where extreme conditions such as 1
10
24
cm
-3
and 1000 eV can be reached, a medium-Z dop-
ant, in the present case Ar, is introduced in trace amounts
into the low Z , i.e., D
2
, gas-filled core contained in a spheri-
cal shell of CH plastic 2,3. The dopant concentration is
made small enough to minimize any perturbation, due to
radiative cooling, of the hydrodynamics of the target, while
keeping the concentration high enough to produce observ-
able x-ray emission. Further, since spectral line shapes are
the primary diagnostic, it is essential to maintain optically
thin spectral lines; with these considerations the concentra-
tion of the Ar dopant is controlled to ensure that the K -shell
1–3 transitions, centered at 3.365 and 3.150 Å, respectively
4, have optical depths of 0.2–0.4. In the following we use
the detailed Stark broadening calculations of the complete
profile, i.e., the Ar XVII 1 s
21
S –1 s 3 p
1
P He2,3, to-
gether with the Li-like 2 l 3 l ' and 3 l 3 l ' satellites to the He
transition, to determine the temperature and density history
of the implosion core 2,5. The experimental parameters are
fixed to drive the microsphere implosion to radial compres-
sion ratios of 6, which allows us to map the sphericity of
the implosion by x-ray imaging, and limits the growth of
hydrodynamic instabilities that occur at high aspect ratios.
To provide the diagnostic information a N
e
- T
e
grid of
theoretical line profiles including the dielectronic satellites
was generated using TOTAL-II 6; these profiles are con-
volved with the instrument response, and compared directly
to the experimental data. Efficient fitting of experimental
data with the predicted line shapes is ensured by interpolat-
ing across the N
e
- T
e
grid 7. The shape of the He transi-
tion, ignoring the Li-like satellites, determines the N
e
, while
fitting the satellites and the He line determines the T
e
.
Fitting the Li-like satellites on the low-energy side of He
line is an accurate T
e
diagnostic below 800 eV.
The implosions are indirectly driven using a soft-x-ray
radiation source created with NOVA 8. Five beams, sym-
metrically arranged, enter each end of a 1.6-mm-diameter,
2.55-mm-long gold cylinder, the hohlraum target, through
0.8-mm-diameter entrance holes. Each beam has a tempo-
rally square 1-ns pulse shape and 2 kJ of energy at a wave-
length of 0.353 m. The beams strike the interior wall cre-
ating a plasma that radiates efficiently at soft-x-ray energies.
*Present address: School of Mathematics and Physics, Queen’s
University of Belfast, Belfast BT7 1NN, Northern Ireland. Elec-
tronic address: n.woolsey@qub.ac.uk
PHYSICAL REVIEW E AUGUST 1997 VOLUME 56, NUMBER 2
56 1063-651X/97/562/23144/$10.00 2314 © 1997 The American Physical Society