IFE/P6-10
1
Counter Implosion of 500-µm Diameter CD
Shell and Fast Heating of its Core Plasma
by Tailored DPSSL-Pumped Laser
Y. Nishimura1,2 , Y. Mori1 , Y. Kitagawa1 , R. Hanayama1 , K. Ishii1 , S. Nakayama1 , T.
Sekine2 , T. Kurita2 , N. Sato3 , T. Kawashima3 , O. Komeda4 , T. Kondo4 , M. Fujine4 , H.
Azuma5 , T. Hioki5 , T. Kajino5 , T. Motohiro5 , S. Oshima5 , A. Sunahara6 , E. Miura7 and
Y. Sentoku8
1
The Graduate School for the Creation of New Photonics Industries
Toyota Technical Development Corporation
3
Hamamatsu Photonics, K. K.
4
Advanced Material Engineering Div., TOYOTA Motor Corporation
5
TOYOTA Central Research and Development Laboratories, Inc.
6
Institute for Laser Technology
7
National Institute of Advanced Industrial Science and Technology
8
Department of Physics, University of Nevada, Reno
2
Corresponding Author: [email protected]
Abstract:
For the purpose of high-repetitive implosion and heating of 500 µm-diameter shell targets,
we, for the first time, developed a fast-ignition scheme tailored pulse Diode Pumped Solid
State Laser (DPSSL). The tailored implosion beam consists of a preceding foot pulse ”nsBeam” from KURE-1 (4.4 J, 1053 nm, 15.2 ns) and a main chirped pulse from HAMA (1.4 J,
800 nm, 300 ps). The direct heating beam ”fs-Beam” is a pulse-compressed beam ”ps-Beam”
from HAMA (1.0 J, 800 nm, 192 fs). We divide each beam into two counter beams. The
target is a deuterated polystyrene (CD) shell-target of 500 µm in diameter and 7 µm in
thickness. Two implosion beams successfully imploded the shell to form a core plasma,
which two fs-Beams heated and yielded DD neutrons of 104 n/4πsr. STAR1D hydrocode
well predicted the results.
1
Introduction
We are developing the key technologies to construct of a compact inertial fusion energy
experimental reactor for integrated experiment (we are calling ”CANDY”), a fast ignition
scheme laser fusion machine. For this purpose, we first developed the DPSSL“ KURE-1 ”
pumped 4 J HAMA laser. A 4 J/ 0.4 ns output of HAMA was divided into four beams,
two of which counter illuminate double deuterated polystyrene foils separated by 100 µm
IFE/P6-10
2
for implosion. The remaining two beams, compressed to 110 fs for fast heating, illuminate
the same paths. The heating pulses heat the imploded core, emitting X-ray radiations >
40 eV and yielding some 103 thermal neutrons [1, 2]. Although the HAMA energy is large
enough to implode and heat the 100 µm-separated double foils, it is too small to implode
a 500 µm-diameter shell-target. Only a pulse-shaped long foot pulse, that is to say the
tailored pulse, can implode such a large shell target.
So that, as a long foot pulse, we have newly added a fundamental wave beam from
KURE-1 into the HAMA imploding beam. Thus we succeeded in imploding and core
heating of the CD shell-target of 500 µm in diameter and 7 µm in thickness. In this paper,
we describe the tailored HAMA laser system for the shell implosion and direct-heating
and the preliminary results, as well as the simulations.
2
Experimental procedure
KURE-I laser
(DPSSL)
ω0 = 1053 nm
ns-Beam
ps-Beam_1
Beam
combiner
2ω = 527 nm
BEAT laser
(Seed laser)
fs-Beam
ns-Beam_1
Ti: sap
Beam
amplifier
splitter
(Four-pass)
HAMA laser
ps-Beam
ω = 820 nm
fs-Beam_1
Beam
splitter/
combiner
ps-Beam
ns-Beam
fs-Beam_2
Pulse
compression
(a)
fs-Beam
ps-Beam_2
ns-Beam_2
(b)
FIG. 1: (a) The block diagram of laser system. (b) Pulse shape of combined ns-Beam,
ps-Beam and fs-Beam.
The block diagram of laser system shows in figure 1 (a). The laser system was composed of a Titanium:Sapphire BEAT laser [3] as a seed, the diode-pumped solid state
laser KURE-1 (constructed at HAMAMATSU Photonics, K. K.)[4, 5] (4.4 J in energy,
ω0 = 1053 nm in wavelength and 15 ns in pulse width), and HAMA laser of amplified by
the second harmonics of KURE-1 laser. The HAMA laser beam is divided into two laser
beams by using the beam splitter. One is ”ps-Beam” (1.14 J, ω = 820 nm, 300 ps), other
is ”fs-Beam” (0.84 J, ω = 820 nm, 110 fs) which was made pulse compression. Moreover,
”ns-Beam” (2.6 J, ω0 = 1053 nm, 15 ns) is the fundamental wave which remained when
KURE-1 laser amplified with HAMA laser. Those laser beams are divided into a total
of six beams by the beam splitter arranged in a compressor chamber, and condensed by
using the two off-axial parabolic mirror. Pulse shape of combined three-beams shows in
Fig. 1 (b). By combining ns-Beam and ps-Beam, we made a tailored implosion beam
with a long foot pulse. Fs-Beam is a direct heating beam. The specification of each beam
is summarized in Table I.
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TABLE I: Specification of laser system
Output energy [J]
Wavelength [nm]
Max. repetition rate [Hz]
Pulse width (FWHN)
ns-Beam
2.6
1053
ps-Beam ps-Beam
1.14
0.84
820
10
300 [ps]
110 [fs]
15 [ns]
The X-ray streak camera (XSC: Hamamatsu Photonics: C4575-03) observed the emissions related to the implosion and direct heating of CD shell-target. From the 50 µm slit,
we estimated the spatial resolution to be 54 µm. The observation angle is normal to
the laser illumination axis, and elevation angle tilted at 30 degree to the target hole.
The image is magnified 7.0 times. We used a full-window range of 30 ns. The 30 nm
in thickness Au-coated 100 nm in thickness parylene-N cathode detects photons in the
region from 20 eV ∼ 5 keV. This cathode detects emissions not less than 20 eV. On the
hand, The X-ray pinhole camera (XPC: Hamamatsu Photonics: C880-21CD) observed
the emissions from direct heating core, and it attached to the side opposite to XSC. The
image is magnified 4 times. Moreover, this XPC is attached a 100 eV bandpass filter of
combined with 59.3 nm-thickness Zr-coated and 118.6 nm-thickness Si-coated, because of
core temperature estimates by this experiment.
3
3.1
Results and discussion
Simulation of hydrodynamics
ps-beam fs-beam
Radius [mm]
ns-beam
0.25
0.20
0.15
0.10
0.05
0
(a)
10
20
30
Time [ns]
40
50
(b)
FIG. 2: (a) The counter-irradiattion beams layout on the target: K is ns-Beam, L is
ps-Beam and S is fs-Beam. (b) Hydrodynamic flowchart by STAR1D.
IFE/P6-10
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We performed the STAR1D simulation by this laser beams condition [6]. Irradiation
layout of six beams with which the CD shell-target shows in Fig. 2 (a). As shown in
Fig. 2 (a), we used the target which made a hole on both sides of 200 µm-diameter in CD
shell-target of 500 µm in diameter and 7 µm in thickness. The reason for having made
the hole is for observing the X-ray emission from a core with the XSC and the XPC. An
irradiation size on the target of ns-Beam was set to 230 µm, and ps-Beam and fs-Beam
were set to 97 µm. As shown in Fig. 2 (b), STAR1D predicts the tailored pulse implosion
and fixes each irradiation timing of ns-beam, ps-beam and fs-beam. This simulation figure
shows that, implosion and direct heating are able to do by using this laser beams.
3.2
Tailored pulse implosion and direct heating
Space
Intensity [a.u.]
0
Red : Core
Sweep time [ns]
Sweep time
5
Blue: Ablation
10
15
20
25
30
500 μm
(b)
(a)
Space
Intensity [a.u.]
0
Red : Core
Sweep time [ns]
Sweep time
5
Blue: Ablation
10
15
20
25
30
500 μm
(c)
(d)
FIG. 3: The observation result by X-ray streak camera. (a) X-ray streak image of irradiation with ns-Beam and ps-Beam (tailored pulse implosion). (b) X-ray emission profile of
(a). (c) X-ray Streak image of irradiation with ns-Beam, ps-Beam and fs-Beam (tailored
pulse implosion and direct heating). (d) X-ray emission profile of (c).
IFE/P6-10
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The implosion must make the core bright. In order to check whether the tailored
pulse makes the implosion or not, we observed an X-ray emission by using a XSC. Figure
3 (a) shows that the core is formed at center. From (a), fs-Beam is delayed 2.6 ns from
ps-Beam.
Intensity of Core [a.u.]
Comparison of Fig. 3 (c) with Fig. 3 (a) seems to show that the core is directlyheated by fs-Beam irradiation. The intensity in Figs 3 (b) and (d) of the red line is X-ray
emission from the core, which is increasing 2.3 times by fs-Beam irradiation. The XSC
image in Fig. 3 (d) seems to show that the core have shone immediately after fs-Beam
irradiation. The results lead us to that the fast electron or the shock wave reached the
core and the core is directly-heated by fs-beam irradiation without corn guide. In this
timing, the neutron scintillators detect 104 n/4πsr neutrons.
40
Delay time of fs-Beam to ps-Beam [ns]
FIG. 4: The X-ray emission from a core distribution when the irradiation delayed time
of ps-Beam and fs-beam.
We performed the optimization of directly-heat timing of fs-Beam irradiation. To psBeam, fs-Beam delayed irradiation with 1.9 ns, 2.3 ns, 2.6 ns, 2.7 ns, 2.8 ns and 3.1 ns in
Fig. 4. In this figure, the X-ray intensity distribution and STAR1D flowchart simulation
figure are overlapped, and estimated. This figure shows that the direct heat timing is 1.3 ns
late, which means that the velocity of implosion is slower than a simulation prediction.
However, the simulation result explains well the experimental results. We need to optimize
the tailored pulse shaping.
IFE/P6-10
3.3
6
Estimation of core temperature
We measured the X-ray emission from a direct heating core by using XPC, and estimated
core temperature. The CD shell-target has holes to observe X-ray emission from core
below the K-absorption-edge of Carbon (284 eV) in 7 µm thickness. Then, we observed
X-ray emission from a direct heating core by using the 100 eV bandpass filter which
combined Zr (59.3 nm-thickness) and Si (118.6 nm-thickness). The transmittance of the
filter shows in Fig. 5 (a), when an implosion core is directly heated with fs-beam, the XPC
image which uses this filter is shown in Fig. 5 (b). This figure shows that, the energy
of the X-ray emission from a core could presume to be 60 ∼ 110 eV, and the electron
temperature of the core estimated approximately 40 eV.
#66
emission from core
Transmittance of Zr/Si filter [x 100 %]
Band pass domain
CD-shell diameter 500 μm
emission from target ablation
Photon energy [eV]
(a)
laser irradiation direction
(b)
FIG. 5: (a) The transmittance of the filter: This filter has the bandpass characteristic of
60-110 eV. (b) The observation result of X-ray pinhole image: Luminescence from a core
is observed through the filter. A cross-shadow is a mesh wire of filter.
4
Conclusions
We described for the first time a tailored pulse DPSSL to implode the CD shell-target
of 500 µm in diameter and 7 µm in thickness, and its direct core heating. Laser fusion
experiment of fast-ignition scheme was able to performed in our experimental scale, when
combining the tailored pulse implosion beam and the direct heating beam irradiated with
an optimal timing to CD shell-target.
The intensity of the X-ray emission from an implosion core increased by 2.3 times by
irradiating with fs-Beam after 2.6 ns from ps-Beam irradiation. And neutron scintillators
detect 104 n/4πsr neutrons. The STAR1D hydrocode simulation explained well our experimental results. The estimated electron temperature of the core from X-ray emission
7
IFE/P6-10
energy from an implosion core was approximately 40 eV.
Finally, we are going to investigate physical parameters, such as core density, in future
research and development. And, we are planning to construct the ”CANDY” by using
tailored pulse implosion and direct heating with fast-ignition scheme.
References
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