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
For special copyright notice, see page 1038.
AN INVESTIGATION INTO THE INITIATION OF
HEXANITROSTILBENE BY LASER-DRIVEN FLYER PLATES
M. W. Greenaway3, M. J. Gifford3, W. G. Proud3, J. E. Field3, S. G. Goveas"
a
Physics and Chemistry of Solids, Cavendish Laboratory, Madingley Road,
Cambridge, CB3 OHE, United Kingdom
h
AWE, Aldermaston, Reading, RG7 4PR, United Kingdom
Abstract. An investigation into the shock sensitivity of hexanitrostilbene (HNS) has been carried out.
A Q-switched Nd:YAG laser was used to launch miniature flyer plates from substrate-backed
aluminium films. The impact produces a shock with duration of the order of 1 ns and pressure of the
order of 10 GPa. The explosive samples were pressed into PMMA cylinders to 65-78 % theoretical
maximum density. The threshold laser pulse energy required to produce a flyer with sufficient velocity
to cause detonation was found. A high-speed camera was used to record the entire event. Initial
curvature of the streak record, for impacts just below the detonation threshold, showed that reaction
started inside the column. This feature was not seen in a previous study1. It was found that
conventional HNS, with a mean particle size of approximately 25 Jim, could not be detonated while fine
grained HNS (sub-micron particle size) would detonate.
INTRODUCTION
The desire to enhance safety provides the main
motivation behind the development of techniques
such as the laser-driven flyer plate. More
sophisticated detonators, like this one, can use
more insensitive explosives while retaining
similar output characteristics.
A technique for imparting very short duration,
high pressure shock waves into energetic materials
has been developed.
The system uses a high power laser to drive
minature plates of aluminium at velocities up to 8
km/s2. The primary motivation for the development
of this technique has been the initiation of
explosives. Other authors have reported similar
systems for high strain rate material testing and for
the ground-based simulation of micrometeorite
impact3.
This study was a preliminary investigation to
determine whether this system could be used to
initiate hexanitrostilbene (HNS). The main aim was
to determine the influential parameters and compare
with the findings from a study on the explosive
pentaerythritol tetranitrate (PETN)1. For these
reasons no statistical methods, for finding thresholds
and suchlike, were employed.
The shock-to-detonation transition (SDT)
The laser-driven flyer plate is really an extension
of the exploding foil initiator (EFI) and the slapper
detonator. Here, a metallic plasma is formed by
depositing a large current through a small copper
bridge. The exploding bridge fires a kapton flyer in
a similar manner to the laser-driven flyer. A barrel
is often used to give directionality to the flyer. The
attraction of shock initiation is its promptness and
reproducibility. The flyer is a means for imparting a
shock wave into the explosive. The otherwise
decaying shock is supported by the chemical
reaction of the detonation. In this manner, a
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A preliminary investigation into the sensitivity of
PETN to this type initiator has been carried out 1 .
This study showed the importance of grain size on
the initiation threshold by comparing a fine grain
and coarse grain variant.
Hexanitrostilbene (HNS) was selected for this
study with a view to enhancing our understanding of
the initiation mechanisms involved. Its importance
as a secondary explosive and availability as a fine
and coarse grain material made it an ideal choice.
Three forms of the energetic material were tested
with this system. HNS IV is the fine grain form of
the material, supplied in its pure form and with
pressing additives. The pressing additives were zinc
stearate and graphite and contributed approximately
1% of the total mass. HNS II is the coarse grain
version and has a grain size of the order of 25 jum.
The charges were pressed into PMMA
confinements 5 mm deep and 5 mm diameter. The
columns were polished with 2500 grade SiC paper
to provide a consistent surface finish. The charges
were backed with a brass witness plate and held 100
|im from the metal film on the impact (front) side.
A schematic of this set-up is given in Fig. 2.
detonation could be defined as a reaction supported
shock wave. Thus reaction must start within the
temporal shock width in order for the support to
remain. This is where a large dependence on the
grain size of the energetic is introduced.
The pressure of the shock wave must also be
sufficient to start the reaction. The pressure
threshold is dependant on the production of ignition
hot spots within the material. The key mechanisms
are granular friction, shear deformations and the
compression of gas spaces. These hot spot
mechanisms must occur during the shock
compression in order for the reaction to be
supported.
Since the technique was first reported4, much
interest has surrounded its development including
research in this laboratory5"7 and elsewhere8"10.
EXPERIMENTAL
A Nd:YAG laser operating at the fundamental
wavelength is used to deliver horizontally polarised
pulses of optical radiation up to 1 J in approximately
9 ns (FWHM). Energy modulation is achieved by
incorporating a variable angle half-wave plate and
polarising beamsplitter.
A plano-convex lens is used to focus this optical
energy onto a substrate-backed aluminium film.
This energy is readily absorbed in the metal, which
generates a hot plasma at the substrate-film
interface. Rapid expansion of this plasma blasts off
the remaining depth of film as a miniature flyer
plate. This mechanism is illustrated in Fig. 1. For
the purpose of this research, the flyers were 1 mm in
diameter and approximately 5 jtim thick. The key
properties of these flyers have been measured and
the results are reported elsewhere2.
100 urn
Brass
Witness
plate
Metal
film
Substrate
HNS charge
FIGURE 2. A schematic of the substrate-charge arrangement.
The initiation event was recorded by a Hadland
Imacon 790 electronic image converter camera.
Plasma
Incoming
light pulse
PMMA confinement
Flyer
RESULTS
The aim of these experiments was to determine
the minimum amount of optical energy required to
produce a flyer of sufficient velocity to induce
detonation.
Metal film
Substrate
FIGURE 1. A schematic of the flyer launch mechanism.
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The results are given in Fig. 3 where a solid
marker indicates a go (i.e. successful initiation) and
a hollow marker a no go.
Pure HNS IV at 70% theoretical maximum
density (TMD) showed the greatest sensitivity, in
this study, with a threshold in the region of 250 mJ.
A small drop in TMD to 65% yields a substantial
rise in the threshold, as can be seen in comparing
the first and second columns of data. HNS IV with
the additives and pressed to approximately 65%
TMD shows a threshold of about 350 mJ. The
contribution of the additives on the sensitivity
cannot be extracted from this data but it does not
appear to be hugely influential.
propagates outward as a spherical wave of constant
velocity.
Using this model, the detonation in Fig. 4 is found
to have broken out 750 fim inside the column. This
is in good agreement with the impact site found on
charges that did not react just below the threshold.
A clear correlation between the depth at which
detonation breaks out and the charge density is
evident. Higher density charges showed less
hooking, indicating that reaction has begun on or
just below the surface. In some cases, shots just
below the threshold showed some evidence of
limited reaction.
CONCLUSIONS
This laser-driven flyer plate system was found to
be capable of initiating fine grain HNS. A key aim
of this study was to compare HNS with another
important secondary explosive, PETN. HNS was
found to be more insensitive to this type of initiator.
In both cases, the sensitivity of the material was
heavily dependent on particle size. The coarse grain
HNS II could not be initiated, just like the PETN
equivalent.
The initiation process for this type of loading has
to be by a hot spot mechanism which can respond
extremely rapidly. In general, this suggests that a
mechanism associated with a rapidly collapsed gas
space is more likely than a sheer or friction process.
The large dependence on particle size is explained
by the size of the critical hot spots. The shock width
is of the order of a few microns, this is comparable
to the grain size (and hence gas spaces) of the fine
materials but is much shorter than the grain size of
the coarse grain materials. Thus, in the case of the
latter, the gas spaces cannot be entirely collapsed
during the short duration of the shock. This is
illustrated in Fig. 5. In addition, we have better
mixing of the hot spots and explosive particles
with a fine grain material. These differences offer
an explanation as to why the fine grain materials are
more sensitive to these very short shocks.
FIGURE 3. A scatter plot of the results for four different
variations of HNS.
Perhaps the most interesting result comes from the
photographic record. Non-linear streak records
occurred for low density samples. An example is
given in Fig. 4. This hooking nature indicates that
detonation is breaking out inside the column rather
than on the surface at the impact site. This was not
seen in the previous study with PETN. Higher
density charges did not show this effect.
FIGURE 4. Streak record of the detonation of HNS IV.
Using a simple curve fitting macro, the depth at
which reaction broke out is found. This macro
assumes reaction breaks out at a single point and
1037
3. Tighe, A., Ground based simulation of orbital debris
using laser driven flyer plates, PhD Thesis, University
of Southampton, 2000.
4. Sheffield, S. A. and Fisk, G. A., "Particle velocity
measurements in laser irradiated foils using ORVIS",
Shock Waves in Condensed Matter - 1983, edited by J.
R. Asay, R. A. Graham and G. K. Straub, pp. 243-246,
North-Holland, Amsterdam, 1984.
5. Dickson, P. E. and Field, J. E., Laser initiation of fast
reactions, 1994, Cavendish Laboratory internal report
(unpublished).
6. Watson, S. and Field, J. E., J. Phys, D: AppL Phys. 33,
170-174(2000).
7. Watson, S. and Field, J. E., J. AppL Phys. 88, 38593864 (2000).
8. Paisley, D. L., "Laser-driven miniature flyer plates for
shock initiation of secondary explosives", Shock
Compression of Condensed Matter - 1989, edited by S.
C. Schmidt, J. N. Johnson and L. W. Davidson, pp.
733-736, Elsevier, Amsterdam, 1990.
9. Trott, W. M. and Meeks, K. D., J. AppL Phys. 67,
3297-3301 (1990).
10. Frank, A. M. and Trott, W. M., "Investigation of thin
laser-driven flyer plates using streak imaging and stop
motion microphotography", Shock Compression of
Condensed Matter - 1995, edited by S. C. Schmidt and
W. C. Tao, pp. 1209-1212, American Institute of
Physics, Woodbury, New York, 1995.
11. Greenaway, M. W., Proud, W. G., Field, J. E.,
Goveas, S. G. and Drake, R. C., "The high power
transmission characteristics of fused-silica optical
fibres" in Laser-Induced Damage in Optical Materials,
edited by G. J. Exarhos, A. H. Guenther, M. R.
Kozlowski, K. L. Lewis and M. J. Soileau, SPIE 4347,
Boulder CO, 2000, pp. 599-607.
FIGURE 5. The shock induced reaction in a relatively large
grain explosive (left) and a relatively small grain explosive
(right).
A hooking of the streak record indicated that
reaction breaks out inside the column. This was not
seen with PETN. The degree of hooking being
heavily dependant on the density of the charge.
Shots carried out at high density showed much less
prominent hooking.
Improvements to the flyer generation process are
under investigation. The use of fibres to deliver the
optical energy to the launch substrate is one such
idea currently under research in this laboratory. The
attraction of this technique is the improved spatial
profile that can be achieved using a multimode
fibre. The main difficulty is reliably carrying such
large power densities down the fibre. The
preparation of the fibre and the coupling method and
procedure are the key issues. Such a system has
been successfully developed in this laboratory11.
ACKNOWLEDGMENTS
The equipment was purchased on grants from
AWE Aldermaston and the Engineering and
Physical Sciences Research Council (EPSRC).
Thanks also go to Dr. S. Watson, Dr. R. C. Drake
and Dr. J. Andrew for some useful discussions.
(c) British Crown Copyright 2001 /MOD
REFERENCES
This document is of United Kingdom origin and contains
proprietary information which is the property of the Secretary of
State for Defence. It is furnished in confidence and may not be
copied, used or disclosed in whole or in part without prior
written consent of the Director Commercial 2, Defence
Procurement Agency, Ash 2b, MailPoint 88, Ministry of Defence,
Abbey Wood, Bristol, BS34 8JH, United Kingdom.
1. Watson, S., Gifford, M. J. and Field, J. E., J. AppL
Phys. 88, 65-69 (2000).
2. Watson, S., The production and study of laser-driven
flyer plates, PhD Thesis, University of Cambridge,
1998.
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