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
TIME-RESOLVED MEASUREMENT OF THE LAUNCH OF
LASER-DRIVEN FOIL PLATE
Hongliang He, Takamichi Kobayashi, and Toshimori Sekine
Advanced Materials Laboratory, National Institute for Materials Science (NIMS),
1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
Abstract. Foil plates (or mini-flyers) have been launched to high velocity by a tabletop laser system,
and their velocity histories have been measured by a push-pull type VISAR coupling with an
electronic streak camera for recording the quadrature outputs. Typical results are presented for Al (10
[a,m-thick), Pt (10 jim), Au (5 ^tm) and Cu (5 [im) foil plates at laser intensities ranging from 20 to
400 GW/cm2. For the 10 [i in-thick Al, velocities over 13 km/s within about 30 ns have been detected
with a time resolution of -300 ps and -2% error for the peak velocity.
INTRODUCTION
within a time duration on the order of nanosecond,
and (2) the small size of mini-flyer, which is
normally hundreds of ^im or 1-2 mm in diameter.
Previously, the time-resolved velocity profiles have
been measured for the foil plates tamped with a
substrate at low laser irradiance (<5 GW/cm2).1'2
Probing the velocity at higher laser intensity is not
satisfactory yet. In this report, we present a
measurement on Al, Pt, Au and Cu foil plates
irradiated at laser intensities up to 400 GW/cm2. An
optical system that couples with an electronic streak
camera (ESC) for recording the quadrature outputs
of push-pull type VISAR has been established,
which provides high time resolution (300-500 ps)
and high accuracy in the velocity history
measurement.
Dynamic high pressures up to tera-pascal (TPa)
have been generated with the direct irradiation of
strong laser beam on condensed matter in the past
decade. This progress offers a new opportunity to
study material property under the extreme condition
in laboratory. To improve the shock compressing
condition of the target sample, an indirect
irradiation idea, that we call laser gun, is now being
proposed. Analogous to a propellant gun, a foil
plate (or mini-flyer) is launched to high velocity
driven by pulsed laser beam, and then impacts with
a static target. This flyer-impact system may
provide a well-controlled, stable shock wave
compression for the sample, and also a significant
preheating of the sample may be avoided, which is
inevitable in the direct irradiation experiments. In
order to well characterize the performance of laser
gun, a time-resolved measurement on the launch of
foil plate is required. A challenge of this
measurement, however, comes from: (1) the foil
acceleration process being extremely fast that the
velocity rises up to a few km/s or even more
EXPERIMENTAL TECHNIQUE
To drive the foil plate, a tabletop-type compact
laser system (Q-switched Nd:YAG laser) was used.
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The pulsed laser beam with wavelength of 1064
nm, pulse duration (FWHM) of 10 ns and a
Gaussian spatial profile was focused onto a foil
plate located inside a vacuum target chamber. The
focused beam diameter of the driving laser was ~2
mm. Irradiation intensity was from 20 to 400
GW/cm2 with the changes of laser energy from 8 to
100 J.
To measure the foil plate's velocity history, a
push-pull type VISAR was employed. Compared
with other velocity interferometers, such as FP3
(Fabry-Perot interferometer) and ORVIS4 (optically
recording velocity interferometer system), the pushpull type VISAR has great advantages that it
provides almost a continuous and an excellent
accuracy in velocity measurement because of the
usage of four quadrature outputs with special phase
difference.5'6 In the conventional use of push-pull
type VISAR, a PMT system, that consists of four
photomultiplier tubes (PMTs) and one or two
oscilloscopes, is used to record the quadrature
outputs.5'6 It provides a time resolution normally
about 2-3 ns. To achieve higher time resolution, in
this work the quadrature signals have been recorded
by an optical system, that we call ESC system.7'8
The VISAR's output signals have been transmitted
through an optical fiber directly to an ESC. A fast
rise-time ESC (C5680 streak camera, HAMAMATSU PHOTONICS K. K, Japan) is used, which
provides a time resolution less than 50 ps by using a
sweep unit M5677. Considering also the time
dispersion in optical fibers and the VISAR's
intrinsic rise time, a time resolution of about
300-500 ps is obtained for the present recording
system.
1A
IB
2A
Shot No.00616s5
w
180° phase difference
'E
.n
03
_L
20
40
60
80
100
Time (ns)
(b)
Snot No
1A-1B
- 00616S5
90° phase difference
c
13
_d
03
RESULTS AND DISCUSSION
Figure la shows a typical interferential fringe
pattern recorded by the ESC system from a 10 jumthick Al foil plate irradiated at 340 GW/cm2 (shot
No. 00616s5). The time resolution is about 300 ps
given the Velocity Per Fringe (VPF) constant of
10.05 km/s/fringe. Time axis increases from the top
to the bottom and the total record is 100 ns. The
four outputs of push-pull type VISAR,61 A, IB, 2A,
and 2B, are recorded simultaneously, and fringe
patterns are evident by a variation of light intensity
with time. The vertical dark lines in outputs 1A, 2A
0
20
40
60
80
100
Time (ns)
(c)
FIGURE 1. VISAR record and analysis in shot 00616s5. (a)
Fringe pattern recorded by the ESC system from 10//m-thick Al
foil plate driven at 340 GW/cm2; (b) Digitized data of outputs 1A
and IB; (c) Subtractions of (1A-1B) and (2A-2B).
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15
AI10
Pt 10 \w\
380 GW/cm 2
12
340 GW/cm 2
"c/r
377 GW/cm 2
^CO
"E
9
280 GW/cm 2
1
6
130 GW/cm 2
1=
o
_o
272 GW/cm 2
167 GW/cm 2
CD
66 GW/cm 2
28 GW/cm
20
40
60
80
38 GW/cm 2
2
0
100
20
40
60
Time (ns)
Time (ns)
(a)
(b)
Au 5 \m\
80
100
- Cu 5 (im
270 GW/cm 2
258 GW/cm 2
^co
IE
168 GW/cm 2
117 GW/cm
2
102 GW/cm 2
o
JO
0)
59 GW/cm 2
55 GW/cm 2
22 GW/cm 2
20
40
60
80
23 GW/cm 2
100
20
40
60
Time (ns)
Time (ns)
(c)
(d)
80
100
FIGURE 2. Velocity profiles of Al IQwm-thick, Pt lO^m-thick, Au 5/im-thick and Cu 5 //in-thick foil plates.
These results demonstrate the excellent push-pull
phase relationship among these quadrature outputs.
The velocity history, v(t), has been computed
from the VISAR fringe count, F(t), with the
equation,5'6
and 2B are due to the break of fibers or separation
at the time of fabrication. The output records are
analyzed with an image readout system that yields
the data by digitizing the image with 1024 intervals.
The time relationship between each interval has
been calibrated by the manufacture and no
interpolation has been made during our data
reduction. Figure Ib is the digitized outputs of 1A
and IB, showing a 180° phase difference. Figure Ic
indicates the subtractions of outputs (1A-1B ) and (
2A-2B), and they exhibit 90° phase relationship.
v(t--T) = VPFxF(t),
(1)
where T is the delay time of VISAR's etalon, and
F(t) can be known from,
F(t) = 9(t)/2jc,
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(2)
ACKNOWLEDGEMENTS
and
9(t) = tan'1 [D2(t)/D!(t)].
(3)
D](t) and D2(t) are the amplitudes of the two fringe
data sets shown in Fig.lc. D^t) is the subtraction of
(1A-1B), and D2(t) is (2A-2B). They possess the
same amplitude and 90° phase difference. This
calculation has been done with a computer
program9 and the result is presented in Fig.2a as
indicated by 340 GW/cm2. Given that about 1.1
fringes were recorded (Fig.lc), the accuracy of peak
velocity is about 2% in this shot.10
Typical velocity profiles are presented in Fig. 2
for the foil plates Al (10 [i in-thick), Pt (10 JAHI),
Au (5 jim) and Cu (5 ^im), respectively. Repeated
experiments have been conducted at the fixed laser
conditions to check the reproducibility. Good
coincidence in a series of shots is evident. The
difference of acceleration property among the four
kinds of foil plate is significant. 10 jim-thick Al
foil plate appears to be the most favorite one to
generate high velocity under the same laser
intensity. A terminal velocity increase from 1.5 to
13 km/s has been observed as the laser intensity
increases from 28 to 380 GW/cm2. 5 ji m-thick Cu
foil plate is the second candidate to generate high
velocity, and then Au (5 ^im) and Pt (10 jim),
respectively (Fig.2).
In terms of shock impedance, Pt flyer is the
highest, then Au, Cu, and Al is the last. However,
since Al flyer has significantly higher acceleration
velocity than the rest of three, it can generate much
stronger dynamic pressure under the same driving
laser intensity. With the present tabletop laser
system, a shock pressure of -500 GPa can be
generated in a Pt target sample by using an impact
of 10 ji m-thick Al foil plate at -13 km/s. Higher
shock pressures may be produced by reducing the
laser beam diameter down to less than 2 mm or
using thinner foil plate. However, the twodimensional effect in pressure generation will be
more significant.
The authors gratefully acknowledge L. M. Barker
at Valyn International for the technical support.
This research was supported by a grant from the
COE project in Advanced Materials Laboratory,
National Institute for Materials Science (NIMS),
Japan.
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