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
ADVANCED CRYOGENIC SYSTEM CAPABILITIES FOR
PRECISION SHOCK PHYSICS MEASUREMENTS ON Z
D. L. Hanson, R. R. Johnston, M. D. Knudson, J. R. Asay,
C. A. Hall, J. E. Bailey, and R. J. Hickman
Sandia National Laboratories, Albuquerque, NM 87185 USA
Abstract. We have developed a general purpose cryogenic target system for precision EOS studies
with the Sandia Z accelerator short-circuit current drive. Condensation of large-area cryogenic liquid
samples is accomplished using a cryostat evaporation refrigerator, shielded for survivability in the Z
blast environment and connected by a thermal link to an expendable sample holder with active
temperature control. Accurate shock physics measurements are performed using multiple, thermally
isolated, fiber-optic-coupled VISAR interferometry, active shock breakout, and time-resolved optical
spectroscopy diagnostics. We describe variations of this cryogenic system currently under
development for three EOS measurement applications: (1) liquid D2 shock Hugoniot experiments using
magnetically driven flyer plates; (2) liquid N2 isentropic compression experiments; and (3) EOS
measurements on liquid 4He at 1.5 K.
INTRODUCTION
N2, O2, and Ar. In this paper, we will describe
variations of this cryogenic system currently under
development for three EOS measurement
applications on Z: (1) liquid deuterium (LD2) shock
Hugoniot experiments using magnetically driven
flyer plates; (2) liquid nitrogen (LN2) isentropic
compression experiments; and (3) liquid 4He (LHe)
EOS measurements.
Recently we have used the fast pulsed power
technology of the Sandia Z accelerator (20 MA
peak current in the long-pulse, short-circuit mode)
to develop new experimental techniques for
accurate EOS measurements at high pressure.
Magnetic pressure loading of material samples in
high current density electrode geometries on Z
allows the generation of continuous isentropic
compression curves previously unavailable at Mbar
pressures [1] and the launching of magnetically
driven flyer plates at velocities exceeding 20 km/s
for accurate shock Hugoniot measurements [2].
To extend the range of materials that may be
studied using these new precision EOS techniques,
we are developing a general purpose cryogenic
target system for Z. Cryogenic samples are cooled
with a cryostat evaporation refrigerator, shielded
for survivability in the Z blast environment. The
version of the system currently in use is capable of
condensing large area cryogenic liquid samples
from a variety of permanent gases with normal
boiling points above 15 K, including H2, D2, Ne,
LD2 HUGONIOT MEASUREMENTS
We have recently performed Hugoniot
measurements of LD2 in the range of 25-70 GPa
on Z [2]. The results of these experiments
challenge the controversial NOVA laser data [3,4]
which show significantly increased compressibility
compared to predictions of first principles, ab initio
EOS models for the hydrogen isotopes [5]. These
Z measurements use the technique of magnetically
driven planar flyer plate impact on relatively large
area, thick samples, with multiple redundant
diagnostics to improve accuracy and address many
of the concerns over the experimental conditions of
the laser measurements.
1141
Blast
shield
Figure 1 shows the arrangement of the LHe
cryostat used to simultaneously cool two separate
LD2 samples on the Z experiments. Survivability
of high value cryogenic components is a major
issue for cryogenic system design on Z. Following
the current pulse, more than 1 MJ of energy is
dissipated in the form of hot plasma, molten metal,
and shrapnel ejected from the load region. The LHe
cryostat is shock-mounted inside a robust stainless
steel blast shield. Cryostats in this arrangement
have survived many Z shots without significant
damage. The design of the cryocell for LD2 shock
Hugoniot measurements is shown in Fig. 2. The
LD2 sample is condensed from high purity D2 gas
at 18 psi in a cavity formed by a stepped Al pusher
plate and a sapphire rear window. This window
provides optical access for fiber-optic-coupled
VISAR, active shock breakout, and time-resolved
visible light spectroscopy diagnostic probes. The
cryocell is thermally isolated from the anode
mounting panel by a thin Nylon thermal break.
Each cryocell is cooled to about 19 K and then
warmed to 22.5 K by a combination of heater
energy and absorbed diagnostic laser light to
produce a quiescent (bubble-free) LD2 sample with
a boiling point of about 24.7 K. Spatially uniform,
constant pressure shock loading of the Al pusher is
produced by impact of the flyer plate accelerated
by magnetic pressure from a ramped current drive.
LN2
reservoir
Thermal
insulator
LHe
level
sensor
LHe
reservoir
LHe
reservoir
tail
80 K
shield
Cold
ingeiv ,
ISENTROPIC COMPRESSION OF LN2
We have recently demonstrated in studies of Cu
and Fe that magnetic pressure loading with the
ramped current pulse of the Z accelerator can be
used to accurately determine the off-Hugoniot
isentropic response of materials at high pressures
[1]. The experimental arrangement being
developed to extend this technique to an LN2
sample is shown in Fig. 3. A quiescent large area
LN2 sample is condensed from high purity N2 gas
at about 78 K in a cryocell similar to that shown in
Fig. 2. Cooling is provided by the cryostat shown
in Fig. 1, with each reservoir now filled with LN2.
In an isentropic compression experiment, the Al
pusher plate at the front of the cryocell forms part
of the current path of the short circuit load and is
gently accelerated by the magnetic pressure from a
ramped, shaped current pulse, quasi-isentropically
compressing the LN2.
Cold
j ,rfinger
/*-|^A
D2 cryocell 1 \ D2 cryocell 2
ICE panels
FIGURE 1. Static-fill LHe cryostat with double cold finger for
simultaneous cooling of two LD2 cryocells.
1142
LHe EOS MEASUREMENTS
4
He is the most fundamental of the permanent
gases. This isotope has the smallest closed-shell
atom with an extremely weak intermolecular
potential and exhibits unique quantum properties in
the liquid state. It is a component of many systems
of astrophysical, condensed matter, and ICF
interest. Very little EOS data exist for 4He at high
pressures because of the difficulty of condensing
quiescent liquid 4He samples at less than 4 K in an
environment suitable for shock physics
measurements.
We are currently developing a variation of the Z
cryogenic target system to condense superfluid
4
He-II samples at 1.5 K for He EOS measurements.
The cryostat for this system is shown in Fig. 4. To
minimize the heat load and maintain a stable
sample environment at 1.5 K, the 4He sample
holder must be surrounded by a double thermal
radiation shield consisting of an outer layer cooled
by LN2 to 80 K and an inner shield cooled by LHe
to 5 K. These shields are segmented into
overlapping sections, some of which serve as part
of the cryostat blast shield. This acts to
mechanically decouple the He sample holder from
the main LHe cryostat and allow the survival of the
most expensive cryogenic system components in
the Z blast environment. The design of the sample
holder assembly for LHe shock Hugoniot
measurements with magnetically driven flyer plates
is shown in Fig. 5. The sample holder is operated
as a continuously fed 4He evaporation refrigerator
[6]. The sample holder consists of a small LHe
bath, which controls the cooling of the sample
holder body, and a cavity at the front in which high
purity 4He gas is condensed into a quiescent LHe
sample for the experiment. LHe from the main
cryostat at 4.2 K and 760 Torr is drawn through a
flow impedance and partially evaporated to cool
the LHe bath at 3.6 Torr to 1.5 K.
*
window
M
.fter
MO
FIGURE 2. Cryocell in magnetically driven flyer plate
configuration for LD2 Hugoniot measurements.
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line
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1m
SUMMARY
ciamp
We have briefly described several of the
cryogenic sample capabilities being developed to
complement new precision EOS Hugoniot and offHugoniot measurement techniques on Z.
FIGURE 3. Cryocell design for isentropic compression of a
LN2 sample.
1143
Crywtit
blast
break
FIGURE 5. Sample holder assembly for condensing a
superfluid 4He-II sample at 1.5 K for LHe Hugoniot
measurements.
ACKNOWLEDGMENTS
Sandia is a multiprogram laboratory operated by
Sandia Corporation, a Lockheed Martin Company,
for the United States Department of Energy under
Contract DE-AC04-94AL85000.
REFERENCES
3.
4.
5.
6.
FIGURE 4. Cryostat and radiation shielding for cooling the
LHe sample holder assembly.
1144
J. R. Asay, "Isentropic Compression Experiments
on the Z Accelerator," in Shock Compression of
Condensed Matter-1999, edited by M. D. Furnish et
al, AIP Conference Proceedings 505, New York,
2000, pp. 261-266.
M. D. Knudson, et al, "Equation of State
Measurements of Liquid Deuterium Subject to
Magnetically Driven Hypervelocity Plate Impact,"
submitted to Phys. Rev. Lett.
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G. W. Collins, et al, Science 281, 1178 (1998).
T. J. Lenosky, etal, Phys. Rev. B56, 5164 (1997).
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