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FORM 836 (10/96)
LA-UR-01-3375
Approved for public release;
distribution is unlimited.
Title:
EJECTA PARTICLE SIZE DISTRIBUTIONS FOR
SHOCK LOADED SN AND AL TARGETS
Author(s):
D.S. Sorenson, R.W. Minich, J.L.Romero,
T.W. Tunnell and R.M. Malone
Submitted to:
http://lib-www.lanl.gov/la-pubs/00796462.pdf
EJECTA PARTICLE SIZE DISTRIBUTIONS FOR SHOCK LOADED SN
AND AL TARGETS
D.S. Sorenson
1
, R.W. Minich
2
, J.L.Romero
3
, T.W. Tunnell
4
, R.M. Malone
4
1
Los Alamos National Laboratory, Los Alamos, NM 87544
2
Livermore National Laboratory, Livermore, CA 94550
3
University of California, Davis, CA 95616
4
Bechtel Nevada, Los Alamos, NM 87544
Abstract. When a shock wave interacts at the surface of a metal sample “ejected matter” (ejecta) can be
emitted from the surface. The mass, size, shape, and velocity of the ejecta varies depending on the initial
shock conditions and the material properties of the target. To understand this phenomena, experiments have
been conducted at the Pegasus Pulsed Power Facility (PPPF) located at Los Alamos National Laboratory
(LANL). The facility is used to implode cylinders to velocities of many mm/msec. The driving cylinder
impacts a smaller target cylinder where shock waves of a few hundreds of kilobars can be reached and ejecta
formation proceeds. The ejecta particle sizes are measured for shock loaded Sn and Al metal samples using
an in-line Fraunhofer holography technique. The distributions will be compared to calculations from 3 and 2
dimensional percolation theory.
INTRODUCTION
Metals under shock-loaded conditions can lead to
complex phenomena depending on the properties of
the material and initial shock conditions. The
phenomena being reported here involves particle
ejection which results from a shock wave interacting
at a metal vacuum (gas) interface. For these
experiments, particle sizes are measured using
holography. Ejecta experiments similar to these
have been performed at other facilities [1,2,3,4,5]
however, only a few measurements of particle sizes
have been carried out. The experiments were
performed at the Pegasus Pulsed Power Facility
(PPPF) [6,7] at Los Alamos National Laboratory.
Particle size distributions resulting from microjet
production will be presented for shock loaded
Al (6061 T-6) and Sn metal samples. The shock
pressures generated in the samples were 300 and
400 kbar respectively. The measured particles size
distributions will be compared with predictions from
percolation theory.
Many phenomena in nature involve fragmentation
from very small (nuclei) to large (planets) scale
systems [8, 9, 10, 11]. A useful model that has been
used to describe the fragment size distributions is
based on percolation theory. For example,
fragmentation of heavy nuclei resulting from
high-energy nucleus-nucleus interactions [9,10] are
described well using this theory. Percolation theory
predicts simple power laws with values that depend
on the dimensionality of the system being
investigated. For example, the powers change
depending if the system is one, two or
three-dimensional [12] and thus provide insight into
the dimensionality of the system being investigated.
We will use predictions from percolation theory in
the interpretation of the data.
EXPERIMENTAL PROCEDURE
For the Al and Sn experiments, a 400-micron
thick target cylinder measuring 3.0 cm in diameter is
impacted with another cylinder producing a pressure
of 300 kbar for the Al experiment and 400 kbar for
the Sn experiment. In both cases, the metal surface
was prepared initially with a surface finish of
20 m” R
a
. Grooves were subsequently machined into
the target at various locations azimuthally around
the target. Fig. 1 shows a cross section of these
grooves which were machined into the Al and Sn
cylinder at two depths of 20 and 100 (not shown)
microns and groove angles of 60 and 120 degrees.
The grooves establish the initial surface conditions
for the formation of microjets and the subsequent
fragmentation. In addition, the grooves allow some
control over the amount of energy available for the
fragmentation process. This is manifested in the
increased microjet velocities for smaller groove
angles.
For these experiments, the impacting cylinder is
used to set up the initial shock wave conditions in
the metal sample. The resulting shock wave that is
setup in the metal sample then interacts at the Al
(Sn)-vacuum interface producing microjets at the
groove locations. The microjets produced then pass
through a 1.5 diameter tantalum cylindrical mask.
The measurement of the particles is carried out
using an in-line Fraunhofer holography technique
[13,14,15] which records a 3 dimensional image of
the particles over a cylindrical volume 15 mm in
diameter and 6 mm long. The particle distributions
are later reconstructed [16] from the hologram.
Resolution of the system is approximately 1.5
microns.
RESULTS FOR SHOCK LOADED AL AND SN.
Fig. 2 shows an image of the hologram
(viewed as a shadowgram) obtained from the
shocked Al experiment. The holographic film
records the phase information used to reconstruct the
ejecta particles, and a DC term which forms a
shadowgram of the microjets. The shadowgram of
the ejecta and the various calibration markers are
clearly observable in the figure, whereas the
interference patterns used in the reconstruction of the
individual particles are not visibly discernable. The
figure is viewed into the source of the laser beam
where the outer part of the image corresponds to just
inside the 1.5 cm diameter collimator. The figure
shows micro jets originating from two groove angles
of 60 and 120 degrees. The microjets from the 120
degree V grooves span the angular region shown on
the figure from 7.5 degrees to 112.5 degrees
incrementing every 15 degrees. The 60 degrees V
grooves span the angular range 137.5 through 242.5
degrees incrementing every 15 degrees. Also, clearly
shown in Fig. 2 are the three posts at 125, 255, and
355 degrees marking the center axial position of each
masked region. In addition, six 12 micron diameter
tungsten wires are strung in the collimator for further
calibration.
Figure 2. Image of hologram viewed as a shadowgram. 12
mciron diamter calibration wires are shown along with 75 micro
n
diamter pegs at 125, 255, and 355 degrees. Microjets produce
d
from the 60 and 120 degree grooves are clearly seen.
20 mm
35 mm
70 mm
120 deg
60 deg
Figure 1. Cross section of grooves that were machined into the
Al and Sn cylindrical targets
10
1
10
100
1000
16
16
2
Number of particles
Particle Diameter (microns)
1
10
100
1000
Al Target: 120 degree V grooves
Fit to Data. Exponent = -2.94
Percolation Calculation 2-D
Al Target: 60 degree V grooves
Fit to Data. Exponent = -5.78
Percolation Calculation 3-D
Figure 3. Particle size distributions for Al experiment. Top plot
is for 60 degree V groove, and bottom is for 120 degree V
grooves. Solid curves are fits to the data using a power law.
Percolation calculations are shown as the open circles, and the
data is shown as the solid squares.
10
1
10
100
2
Number of particles
Particle Diameter (microns)
1
10
100
Sn Target: 120 degree V grooves
Fit to Data Power = -5.1
Percolation Calculation. 3-D
Sn Target: 60 degree V grooves
Fit to Data Power = -5.19
Percolation Calculation. 3-D
Figure 4. Particle size distributions for Sn experiment. Top
plot is for 60 degree V grooves, and bottom is for 120 degree V
grooves. Solid curves are fits to the data using a power law.
Percolation calculations are shown as the open circles, and the
data is shown as the solid squares.
The Al ejecta particles distributions are shown in
Fig. 3 for the 60 and 120 microjets. The shape of
the distribution can be fit well using a power law
function. The powers that fit the data are –5.78 for
the 60 degree microjets, and –2.94 for the 120
degree microjets. The error bars shown are due to
two sources. The first source of error has to do with
finite resolution in the reconstruction system and
noise associated with the recording of the hologram
and reconstructing the data. This error is typically
+/- 0.5 micron. The second error has to do with the
low signal to noise ratio that is encountered at our
resolution limit between 1.5 and 2.5 microns in
diameter. Also shown in the figure are percolation
calculations shown as the open circles. The
calculation in the top figure was done using a 3-D
cubic lattice, whereas the bottom figure required
using a 2-D lattice. Both calculations were
performed at the critical probabilities of 0.25 and
0.5 respectively. The dramatic change in the slope
for the two cases suggests that the physics for the
fragmentation may be quite different. In terms of
percolation theory, the 60 degree V groove data is
consistent with a 3-D breakup, whereas the
120 degree V groove data is undergoing a 2-D
breakup. Furthermore, since the percolation
calculations were done at the critical probability, this
suggests the metal may be near a phase transition.
Fig. 4 shows the particle distributions for the
shocked Sn experiment. The initial metal surface
conditions were identically to the Al case, but the
shock pressure in the Sn sample was 400 kbar. With
this shock pressure, it was calculated to be well
above melt conditions. The distributions shown in
the figure both are fit well with a power close to –5.
The data agrees well with 3-D percolation
calculations at the critical probability
DISCUSSIONS AND CONCLUSIONS
In this report we have presented ejecta particle
size distributions for shock loaded Sn and Al
samples. The measurements were accomplished
using an in-line Fraunhofer holography technique.
Grooves were machined into the target sample
producing well characterized initial conditions for the
production of microjets. The microjets fragment
giving rise to particle distributions that were
measured holographically. Within the context of
percolation theory, we have fit these ejecta particle
distributions with both 3-D and 2-D models. The Al
data shows a dramatic difference in the distribution
shapes going from the 60 to 120 degree V grooves.
This we understand to be consistent with the
difference between a 3-D and 2-D fragmentation
process. Possibly, the Al in the latter case is
undergoing spallation. The Sn data for both grooves
gives very similar shapes for the distributions
consistent with 3-D fragmentation. The
implications of these results are being further
investigated
ACKNOWLEDGMENTS
The authors would like to acknowledge B. C.
Froggett, R.L. Flurer, and J. Roberts who ensured
that the optical relay system and laser systems were
properly setup for recording the hologram. The
target assemblies were provided and built by W.E.
Anderson, J. Bartos, and the rest of the fabrication
team. In addition, many people were involved in
setting up and recording data. These included D.T.
Westley, C.Y. Tom, G. Allred, as well as the facility
operations support team at the Pegasus Pulsed
Power Facility headed by J. Cochrane, D.W.
Scudder, and R. R. Bartsch. Other measurements
were done to determine the dynamics of the target
assembly. These were carried out by D.V. Morgan,
D. Platts, J. Stokes, B. Broste, P. Rodriguez, and L.
Veeser. We also acknowledge theoretical support
from H. Lee, M. G. Sheppard, R.L. Bowers, R.
Gore, G. Bazan, and A. J. Scannapieco. This work
was supported by the High Energy Density Program
and funded by the United States Department of
Energy.
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