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
INITIATION OF PETN POWDER BY PULSE LASER ABLATION
Kunihito Nagayama*, Kazunari Inou*, and Motonao Nakahara**
^Department of Aeronautics and Astronautics, Faculty of Engineering,
Kyushu University, Hakozaki, Fukuoka 812-8581, JAPAN
** Department of Computer and Communication Engineering, Faculty of Engineering,
Fukuoka Institute of Technology, Fukuoka 811-0295 Japan
Abstract We developed a laser-driven method of initiating a secondary high explosive charge, based on
the energy conversion from the pulsed laser energy to the shock wave energy. Enhanced energy conversion was found from optical to mechanical by an intentionally roughened surface of a transparent medium
through which laser beam propagates. In this study, a PMMA plate is used, one of whose surface was intentionally roughened. Aluminum coating of less than 1 ^im thickness was made to further enhance the
laser absorption at the surface. Observed air shock wave indicates that it is very strong with Mach number
larger than 30. Nd:YAG laser of 4 ns duration, 1064 nm wavelength and of up to 200 ml was used as an
energy source. Detonation emission of PETN powder was recorded by an image converter streak camera.
Delay of 200-300 ns in the initiation was detected.
INTRODUCTION
can be initiated by a extremely thin foil of 3-5 \im
accelerated up to 8 km/s.
In this study, we propose a new energy deposition method on explosive through the generation of high-temperature metal plasma due to the
pulse laser ablation of vacuum deposited thin metal
layer on a roughened polymer surface.2. 3 We have
found an enhanced absorption of pulse laser energy
by a roughened surface.2, 3 High-pressure shock
wave generation was observed in an ambient medium. This is evidenced by the laser shadowgraphy
technique. Such effects may find several applications. One application is medicine as a well controllable tool of short pressure pulse for various kinds
of microsurgeries.
Strength of shock wave produced in these
conditions is further enhanced, if a thin metal layer
is deposited on the roughened surface. In such
cases, laser energy transfer to the desired position
might be made through use of plastic or glass optical fiber. End surface of the optical fiber is intentionally roughened and then metallized. Deposition
of the laser energy through the fiber causes an intense ablation of the metal layer. Then, we have
By focusing very intense laser beam onto solid surface, laser energy absorption at the surface
layer causes extreme high temperature and high
pressure which drives explosive burst of plasma and
particles called laser ablation. Such process can be
applied to various physics experiments as well as
engineering and medical applications. Laser pulse
energy is one of the promising source of energy to
be used to initiate high explosive charge without
danger of accidental explosion by electrical noise.
Accurate timing of initiation can be expected by the
proper choice of energy deposition method. This is
a simple extension of the impact test. Various attempts have been made by focusing the laser energy
directly on the explosive charge. In this application,
infrared and ns duration laser is preferable. Since
most of the explosives are not a good absorber of
light, direct absorption of laser energy by explosives
is not expected to be very efficient. Watson et al
have studied an experimental feasibility of initiating
PETN powder by pulse laser driven thin metal foil.*
They claimed that at least 1 jiun grain PETN powder
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shadowgraphs of air shock waves by focusing a
Nd:YAG laser beam through PMMA plate to air.
Air breakdown-induced shock front by laser focus
can be seen in Fig. l(a). In this case, laser beam
transmits through smooth PMMA surface, although
a small portion of laser energy is absorbed at the
surface causing a weak pressure wave in air and in
PMMA shown clearly in Fig. 1. When the PMMA
surface is roughened, one notices that a very strong
absorption of laser energy causes a much stronger
shock wave in air and in PMMA emanating from
the roughened interface, as shown in Fig. l(b). In
this case, a small amount of laser beam transmitted
through the interface causes a weak air breakdown
and shock wave front. This is also seen in Fig. l(b).
studied initiation of high explosive charge by this
process.
In this report, we will give an evidence of the
effect of the surface roughness to absorb laser energy at the surface resulting in the generation of highpressure shock wave in air. Preliminary explosive
initiation test results are described, and discussed.
SHOCK WAVE GENERATION BY LASER
ABLATED fflGH-TEMPERATURE METAL
PLASMA
To apply this phenomena to the explosive initiation, laser beam energy is provided through the
transparent fiber medium to the roughened and
metallized surface. This condition is different from
the common situation encountered in pulsed laser
deposition of thin films. The most important difference is that the laser fluence should be less than
the ablation threshold fluence of the laser transmitting medium, but be larger than the ablation
threshold fluence of the roughened and metallized
surface layer.
We first observed the shock wave propagation
in air produced by the laser ablation of the roughened layer. Figure 1 (a) and (b) shows the laser
(a)
A!
1 QQnm
time = 536 us
(b)
AI
(a)
= 524 us
(C)
(b)
Al
FIGURE 1 Shadowgraphs of shock waves generated at the PMMA-air interface, (a) smooth interface,
(b) intentionally roughened interface. Nd:YAG laser pulse of 4 ns duration, 180 mJ/pulse is focused on
the interface with a spot radius of about 2 mm.
= SCO ns
FIGURE 2 Shadowgraphs of shock waves generated at the PMMA-air interface with thin aluminum
coating, (a) Al thickness 100 nm, (b) 300 nm, and
(c) 420 nm, respectively.
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lumination condition. Even so, the shock front position in three cases is found to be almost identical.
Figure 3 shows the x-t diagrams of four different cases with different Al layer thickness and no
layer. Roughness effect is apparent, but the Al layer
thickness has almost no effects on the x-t diagram of
air shock propagation. Shock velocity in air exceeds over that in PMMA in an early stage, but decays very fast. Shock in PMMA decays very fast as
well especially at an early stage approaching rather
steady propagation. No appreciable difference is
seen in the x-t diagram for shock front in PMMA.
Further enhancement of laser absorption is realized by depositing a thin metal layer on the roughened interface of transparent medium with air. In
this study, aluminum layer is deposited. A series of
shadowgraphs are taken with a thickness of alumi1200
INITIATION OF VERY SLENDER PETN
POWDER EXPLOSIVE COLUMN
Experimental results in the previous section
reveals that high-temperature metal plasma can be
generated by pulse laser ablation of thin metal layer
on a roughened surface. This high-energy density
flow is used to initiate high explosive charge of extremely small amount. In the present situation,
scenario of the explosive initiation by pulse laser
energy is described as follows: (i) laser energy pulse
is converted to the high energy plasma flow by the
absorption of laser energy by metal layer on a
roughened surface; (ii) high-temperature high-pressure metal plasma compresses and heats up the high
explosive powder grains; (iii) some kind of local hot
spot formation takes place, that leads to the initiation of high explosive layer.
We have tested the initiation test of PETN
powder charged into very thin layer of 0.5 mm to 1
mm. Total amount of PETN charge is less than 1020 mg per shot. This extremely small amount of explosive is supposed to be far less than the limiting
size of DDT to steady detonation. Previous data on
the impact initiation tests suggests the high sensitivity of powdered PETN. Impact test using very thin
foil, however, showed dependence of grain size of
PETN due to the finite duration of high pressure exerted on the grain.
Figure 4 shows the streak photographs of self
emission due to laser induced reaction of PETN thin
layer. Although thickness of the explosive layer is
very thin, explosion is observed for the laser focus
onto less than 2 mm diameter. Explosion is determined by the streak record of self luminous front
#8300(0iim, 100nm,300nm -420n
0.0
1.0
2.0
3.0
4.0
Shock front position - mm
FIGURE 3 The x-t diagram of shock fronts in air
for four cases, (a) Rough surface, no Al layer, (b)
Rough surface, 100 run Al thickness, (c) Rough surface, 300 nm Al thickness, and (d) Rough surface,
420 nm Al thickness, respectively.
num layer as a parameter. Figure 2 shows comparison of resultant shock waves with different aluminum layer thickness. In these experiments, PMMA
surface is intentionally roughened and aluminized.
Since the figures in Fig. 2 are different only in the
Al thickness, all the figures in Fig. 2 can be compared with Fig. l(b). Much stronger shock wave
generation is seen compared with the picture in Fig.
l(b). Air shock velocity in these cases exceeds 10
km/s. This is evidenced by the difference in the
shock front position at similar delay time. As seen
in Fig. 2, difference in Al layer thickness has no appreciable effects on the shock front position. A dark
undulation observed inside the shock wave front in
air is created due to a small aperture inserted in optics of shadowgraphy experimentation. Comparing
three pictures in Fig. 2 with different Al thickness,
quite different feature in an area inside the air wave
front is seen. In Fig. 2(c) of 420 nm Al thickness,
granular density distribution strongly suggests an incomplete evaporation of Al layer by the present il997
of steady detonation. Detonation velocity estimated
by the slope of the streak photography is 4 km/s,
while the published data of density dependence of detonation velocity in powdery PETN is
4.5-4.8 km/s. Agreement of these two values suggests that the emission observed in this study seems
to be due to almost steady detonation wave front
even in an extremely slender explosive column.
We have tested sensitivity to study the laser
energy dependence of initiation. In this experiment,
laser pulse of 200 ml was focused to 0.5 mm thick
PETN layer with different diameter. Very simple
scaling suggest the correspondence of the conditions
with the same value of laser fluence, but detonation
sensitivity of the present tests cannot be described
by the threshold value of laser fluence.
propagation, sound, smell, and broken assemblies.
In some cases, incomplete detonation, half detonation was observed. All the evidences in this case is
intermediate between complete detonation and no
detonation. As seen in Fig. 4, detonation reaction is
found to be delayed 200-300 ns after laser ablation.
In the photographs, intense flash due to laser ablation is recorded in the streak photographs through
the opaque powder explosive layer. This is due to
the extreme intensity of light flash of ablation.
Light intensity of ablation is brighter than the self
emission of detonation. This is attributed to be the
extreme high temperature of ablated metal plasma.
It is shown in Fig. 4 that detonation front proceeds
almost at constant velocity, although the cross section of explosive is only 1mm2. This result is surprising in the sense of existence of limiting diameter
CONCLUSION
We have proposed a new energy conversion
mechanism from the laser pulse energy to shock
wave energy by the intentional roughening of the
energy deposition surface and thin metal coating. It
is found that surface roughness and thin metal layer
is very effective for the enhanced absorption of laser
energy and produces a very strong shock wave in
ambient media including a transparent medium
through which laser pulse transmits. We have succeeded in the detonation of very slender PETN powder column almost in the steady propagation mode.
* AI
ACKNOWLEDGEMENTS
Authors wish to thank Ms. S. Hatano and Mr.
Y. Mori of Kyushu University, and Mr. Murakami
for their help of experiments. They also wish to
thank Asahi Chemical Industry for providing PETN
explosives.
REFERENCES
1.
S. Watson, M.J. Gifford and IE. Field, J. Appl.
Phys., 88, 65 (2000).
2. Nakahara M, Nagayama K, Proc. 21st Shock
Wave Symp. In Great Keppel, Australia. 1997,
vol. 2, (1998) p. 801.
3. M. Nakahara, and K. Nagayam, J. Materials
Processing Technology, 85, 20 (1999).
FIGURE 4 Streak photograph of delayed detonation of
PETN powder of 10 mg. Nd:YAG laser pulse of 180
ml is focused to about 1.5 mm diameter.
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