J. Plasma
J.
Plasma Fusion
Fusion Res.
Res. SERIES,SERIES,Vol.
Vol.6 6(2004)(2004)337–340
000–000

X-Ray Spectroscopic Measurements of Energy Transport
in Ultra-Intense Laser Produced Plasma
INUBUSHI Yuichi, NISHIMURA Hiroaki, MATSUI Ryoji, OCHIAI Masayuki, KAWAMURA Tohru,
KODAMA Ryosuke, TANAKA Kazuo A., MIMA Kunioki, NAGATOMO Hideo, JOHZAKI Tomoyuki,
KITAGAWA Yoneyoshi, FUJITA Hisanori, JITSUNO Takahisa, MIYANAGA Noriaki,
NORIMATSU Tkakayoshi, NAGAI Keiji and IZAWA Yasukazu
Institute of Laser Engineering, Osaka University, Suita 565-0871, Japan(Received: 9 December 2003 / Accepted: 20 March 2004)

Abstract
Experimental study on energy transport in ultra-high intensity laser plasma was made using a PW laser system.
X-ray emission from a triple-layer target, involving chlorine as a seed material for the spectroscopy, was observedwith x-ray spectrographs and monochromatic imagers to provide a temperature profile in the depth of the target andlateral extension of the heated region. It was found that a very shallow region (~0.5 µm from the target surface) was
heated up to 650 eV but the temperature of deeper region (up to 5 µm) was around 100 eV. These depths are much
shorter than those expected from the classical penetration of the hot electrons of ~500 keV predicted by calculations.
Keywords:
x-ray spectroscopic measurement, ultra-high intense laser, energy transport, hot electrons

1. Introduction

focusing and plasma channel formation occur under such a
deep relativistic regime, much better energy transport will beexpected. Therefore, spectroscopic measurement at the deep
relativistic laser intensity is needed to improve understandingof energy transport and deposition mechanism associated withthe fast ignitor experiments.
In this paper, we will show x-ray spectroscopic
measurements of a chlorine-doped triple-layer target irradiatedby an ultra-intense laser at the intensity over 1019 W/cm2. We
observed shifted Kα lines and resonance lines such as Heα,Lyα from the tracer simultaneously in order to cover a wide
range of temperatures. The thickness of over-coating layer
facing to the side of laser incidence was varied to derivetemperature profile in the target depth. In addition, lateralextension (the direction along the target surface) of the heatedregion was observed using x-ray monochromatic imager
adjusted for Cl Kα and Cl Heβ lines.

In fast-ignitor research, efficient energy transfer to anddeposition in the dense plasma is one of the critical issues[1]. A new spectroscopic method providing time- and spaceresolved information has been under development to provide
more quantitative understanding of the energy deposition thanthose provided by particle measurements [2,3]. Because of
the relatively low temperature available with current lasersand the high density-radius product, ρR, of the imploded coreexceeding 0.1 g/cm2, conventional K-shell line spectroscopyusing near-fully ionized plasma is not suitable. To K-shell ion
in a low-temperature plasma one must use a low-Z seedmaterial; but, the implosion core would be too opaque for Kshell lines. Shifted Kα lines are group of Kα lines emitted
from partially ionized ion. In case that high Z material is used
as a tracer, we can use high energy photon even if electron
temperature is comparatively low.
Numerous works have been done to derive the fastelectron spectrum and energy transfer efficiency by observing
Kα lines from double-layer targets [4,5]. X-ray spectra
observed have shown that the line intensities decrease
dramatically with increase in over-layer thickness of the
double-layer targets at the laser intensity of 1017–1018 W/cm2[6-8], indicating that the depth of hot region would be muchless than those predicted by the fast electron temperatures
measured or estimated from the laser-intensity scaling [9].
However, at the irradiance over 1019 W/cm2 relativistic self

2. Experimental setup
Experiment was performed using the PW laser systemat the Institute of Laser Engineering (ILE), Osaka University.
This system provided a laser pulse of 130 J/700 fs at thewavelength of 1053 nm. The contrast ratio of the laser pulse
was better than 10–6. The P-polarized pulse was focused with
an off-axis parabolic mirror of f /7.5 at the incident angle of25.9°. The laser spot size was 30 µm containing 70 % of laser©2004 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchCorresponding author’s e-mail: yinubusi@ile.osaka-u.ac.jp1337

Inubushi Y. et al., X-Ray Spectroscopic Measurements of Energy Transport in Ultra-Intense Laser Produced Plasmaenergy to yield irradiance of 2 × 1019 w/cm2.
The target was a triple-layer planar target: Substrate wasa 2 mm CH plate. Middle layer consisted of a 1 µm C8H7Cl
film. The third layer, in the side of laser incidence, was madeof CH whose thickness was varied from 0 µm to 5 µm.
We used two spectrographs and two monochromaticimagers. One of the spectrogtraphs had a RbAP flat crystal(100) covering the energy range from 2.6 keV to 3.3 keV forCl Heα to Heβ lines. The other had a troidally bent quartz
crystal (10–11) covering the energy range from 2.6 keV to2.85 keV for the shifted Kα lines.
One of the x-ray imagers, called x-ray monochromaticcamera (XMC) [10], had a troidally bent silicon crystal (220)adjusted for Cl-Heβ line (3.27 keV). Experimentallyconfirmed spatial resolution was better than 10 µm. The
second imager was an x-ray pinhole camera (XPHC) having
a 20 µm saran foil involving Cl as a K-edge filter. Overall
spectral range was 1.8–2.82 keV, covering Cl Kα to Cl Heαlines. As will be shown below, this pinhole camera is identicalto a Cl Kα monochromatic imager when the over-coating was
thick. Spatial resolution was better than 15 µm.

size. Figure 3(d)-(f) show the images obtained with XPHC.
For Fig. 3(d) and (e), contribution of Heα line emission
dominants over the results as is seen in Figs. 1(a) and (b). On
the contrary, Fig. 3(f) provides the image of Kα line emission.

3. Experimental results and discussion
Figure 1 shows the spectra obtained with the x-ray flat
crystal spectrometer. With increase in the over-coating
thickness, the lines become much weaker except for the Kαline. The Cl Lyα, Heα, and Heβ lines, as good signatures ofhot plasma formation, could be observed only for 0–0.2 µm
over-coating. On the contrary, Kα line, a good signature of
hot electron propagation in a cold material, was observed evenfor 5 µm coating. From the line intensity ratio of Lyα to Heα,one can derive electron temperature by using the spectrum
analysis code FLY [11]. For the case of no-coating the ratio
is 0.07 ± 0.003 then corresponding electron temperature is650 ± 30 eV assuming that plasma concerning consists of a
mixture of C and Cl (in the ratio of 8 : 1 and the total ion
density is 9 × 1022 cm–3). For the case of 0.2 µm coating, theratio is below 0.01 including experimental error, therefore theelectron temperature is 400 eV or less.
Figure 2 shows the spectra obtained with the bent crystal
spectrometer. Between Kα and Kβ lines, inner-shell transitionlines corresponding to KαCl9+ to KαCl14+, Heα line and its
unresolved Li-like satellite lines are weakly seen. We obtained
electron temperature using intensity ratios of these lines incomparison with calculations by an atomic kinetic code
specified for the shifted-Kα lines [12]. The calculation wasmade assuming plasma conditions that ion density is 9 × 1022
cm–3, hot electrons temperature is 500 keV, and fractional hot
electrons with respect to the total (bulk plus hot) electrons is5 %. From comparison between the experiment and the model
calculations, electron temperatures were obtained as 119 eV,
111 eV, 113 eV respectively for the case of 0.2, 0.5, 1.26 µm
coating.
Figures 3(a)-(c) show monochromatic Heb line images
for various coating. There is no large difference among themand lateral extension is within a few times of the laser spot
2338

Fig. 1

X-ray spectra obtained with the flat crystal
spectrograph. For various over-coating thickness: (a) 0µm, (b) 0.2 µm, (c) 0.5 µm, (d) 1.26 µm, (e) 2 µm, and (f)5 µm.

Fig. 2

X-ray spectra obtained with the x-ray bent crystal
spectrograph. Thicknesses of the over-coating are (a)0.2 µm, (b) 0.5 µm, and (c) 1.26 µm.

Inubushi Y. et al., X-Ray Spectroscopic Measurements of Energy Transport in Ultra-Intense Laser Produced Plasma

Fig. 3Fig. 4

Monochromatic images of Heb line for over-coating thickness of (a) 0 µm, (b) 0.2 µm, and (c) 0.5 µm. And x-ray images fromXPHC for over-coating thickness of (d) 0 µm, (e) 0.2 µm, and (f) 0.5 µm.

the depth of target, electron temperature is around 100 eV.
Temperature of hot electrons, considered as a primary energy
carrier for the target heating, was predicted by particle-in-cell(PIC) code calculation [13]. For the experimental conditionof our case, the hot electrons consist of two components
whose slope temperatures are 500 keV and 2 MeV. It has beenshown that the former 500 keV component has an importantrole in heating the compressed core on the basis of classicaltransport [13]. According to this result, we can predict depthof heat transport over 200 µm in solid CH [14], contrary to
the result of this experiment.
Typical absorption depth for x-ray photons of 2.6 keV
in solid CH is 100 µm, therefore possibility of x-ray
absorption by the surface coating can be excluded. Overall
ionization energy, consumed in generation of experimental
plasma of 300 eV in temperature, 9 × 1022 cm–3 in density,and 1 µm in depth, was calculated to be about 0.4 J. This
corresponds to only 0.3 % of incident laser energy. Thenionization loss can be neglected. Another possibility is due toinhibition of heat transport attributed to space-charge effectin the depth of target. However, as is evidenced in the studyof ref. [8], this possibility is also excluded as far as electron
conductivity between metallic material and fully ionizedplasma are nearly the same. The most plausible mechanism
will be anomalous resistivity caused by electromagnetic fields
self-induced in surface plasma by hot electrons [15]. The
electric currents carried by hot electrons in the vacuumplasma interface induced self-organized magnetic-fields but
the current values are well above the Alfvén limit [16]. The
induced magnetic fields prevent their penetration in overdense
region. Furthermore, the cold bulk electrons in the return flow
are scattered by the magnetic fields. As a result, the plasma

Electron temperature profile obtained by the intensity
ratios of the resonance lines (closed circles) and theshifted Ka lines (open circles).

Again lateral size is within a few times of the laser spot,
indicating that there is no largely diverging or converging
propagation of hot electrons for the over-coating investigatedin this experiment.
By combining the results from the spectroscopic
measurements, we could derive temperature profile in thedepth of target as shown in Fig. 4. In the thin surface region,
electron temperature are around several hundreds eV. But in3339

Inubushi Y. et al., X-Ray Spectroscopic Measurements of Energy Transport in Ultra-Intense Laser Produced Plasma

becomes resistive and longitudinal electric fields are createdand maintain global current neutralization. In this experiment,
the scale length of the magnetic fields is calculated to be 0.4µm [17], corresponding to the depth of heated region by PWlaser.

822 (1998).
[5] D. Batani, J.R. Davies et al., Phys. Rev. E 61, 5725
(2000).
[6] H. Chen and B. Soom, Phys. Rev. Lett. 70, 3431 (1993).
[7] U. Teubner, I. Uschmann et al., Phys. Rev. E 54, 4167
(1996).
[8] H. Nishimura, T. Kawamura et al., JQSRT 81, 327(2003).
[9] S.C. Wilks and W.L. Kruer, IEEE J. Quantum Electron.
33, 1954 (1997), and references therein.
[10] M. Vollbrecht, I. Ushmann et al., JQSRT 58, 965 (1997).
[11] R.W. Lee, J.T. Larsen et al., JQSRT 56, 535 (1996).
[12] T. Kawamura, H. Nishimura et al., Phys. Rev. E 66,016402 (2002).
[13] T. Johzaki, K. Mima et al., Ann. Prog. Rep. of ILE Osaka
U. 95 (2002): Y. Sentoku and K. Mima, submitted toPhys. Plasmas.
[14] W. Bambynek, B. Crasemann et al., Rev. of Mod. Phys.
44, 716 (1972).
[15] Y. Sentoku, Ann. Prog. Rep. of ILE Osaka U. 1343
(2001).
[16] H. Alfvén, Phys. Rev. 25, 425 (1939).
[17] A.R. Bell and J.R. Davies, Phys. Rev. E 58, 2471 (1998).

4. Conclusion
Energy transport in ultra-high intensity laser produced
plasma has been studied by x-ray spectroscopic
measurements. The depth of heated region is much less thanthat expected from the hot electrons generated by laser. The
lateral extent of the heated region is closed to the laser spot,
consistent with this hot region. This might be attributed tothe anomalous resistiivity by self-induced field formation.

References[1] R. Kodama et al., Nature 418, 933 (2002): ibid. 390, 490(2001).
[2] G. Guethlein, M.E. Food and D. Price, Phys. Rev. Lett.
77, 1055 (1996).
[3] Y. Kitagawa, Y. Sentoku et al., Phys. Plasmas. 9, 2202
(2002).
[4] K.B. Wharton, S.P. Hatchett et al., Phys. Rev. Lett. 81,4340