25G05
Absolute Kα line spectroscopy for direct observation of laser-plasma coupling
Zhe Zhang1, Hiroaki Nishimura1, Takuya Namimoto1, Yasunobu Arikawa1, Shinsuke Fujioka1,Atsushi Sunahara2, Tomoyuki Johzaki1, Hong Bo Cai3, Mayuko Koga1, Hirokazu Hosoda1,
Hiroyuki Shiraga1, Hiroshi Azech1, Masaharu Nishikino4, Tetsuya Kawachi4, Alex S. Pirozhkov4,Akito Sagisaka4, Satoshi Orimo4, Koichi Ogura4, Akifumi Yogo4, Yasuaki Okano5, ShinsukeOhshima1, Hiromitsu Kiriyama4, Kiminori Kondo4, Takuya Shimomura4 and Shuhei Kanazawa4
Zhe Zhang1, 西村博明1, 波元拓哉1, 有川安信1, 藤岡慎介1, 砂原淳2, 城﨑知至1, Hong BoCai3, 古賀麻由子1, 細田裕計1, 白神宏之1, 疇地宏1, 錦野将元4, 河内哲哉4, Alex S.
Pirozhkov4, 匂坂明人4, 織茂聡4, 小倉浩一4, 余語覚文4, 岡野泰彬5, 大島慎介1, 桐山博光4,近藤公伯4,下村拓也4, 金沢修平41

Institute of Laser Engineering, Osaka University, 2-6 Yamada-oka, Suita, Osaka 565- 0871, Japan
2
Institute for Laser Technology, 2-6 Yamada-oka, Suita, Osaka 565-0871, Japan3
Institute of Applied Physics and Computational Mathematics, P.O. Box 8009, Beijing 100088, China
4
Quan. Beam Sci. Directorate, Kansai Photon Sci. Institute, JAEA, Kyoto 619-0215, Japan
5
Laser Research Center for Molecular Science, Institute for Molecular Science, National Institute ofNatural Science, 38 Nishigo-Naka, Myodaiji, Okazaki 444-8585, Japan1
大阪大学レーザーエネルギー学研究センター 〒565-0871	  大阪府吹田市山田丘 2-6
2
レーザー技術総合研究所	  〒565-0871 大阪府吹田市山田丘2-6
3Institute of Applied Physics and Computational Mathematics, P.O. Box 8009, Beijing 100088, China
4
原子力研究開発機構	  関西光科学研究所	  〒619-0215	  京都府木津川市梅美台8丁目1番地75
自然科学研究機構	  分子科学研究所	  〒444-8585 愛知県岡崎市明大寺町字西郷中38
Absolute Ka line spectroscopy is proposed to study laser-plasma interactions. The Kα line emission fromMo was experimentally and theoretically studied using clean, ultrahigh-intensity femtosecond laser pulses.
The absolute yields of Kα x-rays at 17 keV from Mo was measured as a function of the laser pulse contrast
ratio and irradiation intensity. Significantly enhanced Kα yields were obtained by employing high contrast
ratio at the optimum irradiance. Conversion efficiencies of 4.28×10−5 /sr, the highest values obtained todate, was demonstrated with contrast ratios in the range of 10−10 to 10−11. By analyzing the Kα yield withsimulations, we can find the laser absorption mechanism and quantitative information about the transfer
efficiency of laser energy to hot electrons.

1. Introduction
Kα emission, as a main production of laser
plasma interaction, has been widely used as
diagnostics in the field of high-energy-density
physics. It has long been expected to provide morequantitative information about the hot electrongeneration and transport in laser plasma interaction.
In this way, we propose an absolute Kα line
spectroscopy
dedicatedforquantitative
measurement of hot electron generation andtransport in the high-Z target. This method provides
local information about the hot electrons
propagating through specific materials.
In the present study, hard x-rays at 17 keV wasgenerated from Mo targets. The target wasirradiated by an intense laser pulse with anextremely suppressed leakage level. A two-staged
preplasma was formed and several absorption
mechanisms happens. The multi-mechanism
absorption not only leads to a high transfer
efficiency (TE) of laser energy, but also produce a
multi-temperature hot electron energy distribution.
As a result, the highest Kα conversion efficiency(CE) of 4.28×10−5 /sr was achieved [1].
2. Experiments and results(a)Fig.1. The conversion efficiency as a function of laser
contrast and intensity.

Experiments were performed at J-KAREN laser
facility at Japan Atomic Energy Agency, Kansai
Photon Science Institute. This system delivered
laser pulses at a wavelength of 800 nm. The
effective pulse duration was 58 fs, which provided
the peak power of 30 TW for a pulse energy of 1.8 J.
With optical parametric chirped pulse amplifier(OPCPA) technology, the contrast ratio of this laser

ηT E nA ωKα EKα4πTh� d∞x)
λx,mf p cos(θ)(1)

Transfer Efficiency:TE (%)

100
Beg's
Kruer'sPondero.

80
60
40
20

10

1710ne (nc)

10–2
10–30

12x ( 0)345(b)

dEσKα (E)×where ηKα is the CE of Kα, which is experimentallymeasured. ηTE is the laser energy transfer efficiency.
Assuming refluxing of hot electrons [5], the energy
transfer efficiency can be derived from Eqn. (1). Asshown in Fig. 2, by applying different power
scaling laws for Th, including Kruer’s and Beg’s[6,7], the transfer efficiency is estimated as afunction of laser intensity with contrast of 10-10.
According to Beg’s law, the TE in this experimentis about 20%.

0161010–10dx fh (E0 , x)exp(−010018101910202intensity (W/cm )Fig. 2. Transfer efficiency.

3. PIC simulation
The preplama profile is estimated with aradiation hydrodynamic code, as shown in Fig. 3(a).
The laser plasma interaction simulation isperformed with a kinetic PIC code LPIC++ [8].
Figure 3(b) shows the time evolution of electrondensity. We can see, in the early stage, the front oflaser pulse reaches the low density plasma and theelectrons are pulled inside the high density region
with ponderomotive force. Later, when the laser
pulse propagates into the high density region, a partof the electrons are expelled into vacuum and gain
energy from the electron field. This is so calledvacuum heating (VH) process. Meanwhile, thebehavior of J×B heating can be seen. The
combination of 3 absorption mechanisms results inan enhancement of TE and consequently the Kαyield.

10ηKα =(a)101

03. Transfer efficiency
According to D. Salzmann’s model [4], the CE isa function of energy transfer efficiency and hotelectron temperature Th: �102t/τ0pulse could be enhanced to 10-10 and 10-11 [2,3]. The
absolute Kα yield was measured by a calibrated
CCD running in single photon counting mode. The
conversion efficiency of Mo Kα as a function oflaser contrast and intensity is shown in Fig. 1.

01x ( 0)2Fig. 3. Simulation profile: (a) Initial density profile;(b) time evolution of the electron density.

4. Conclusion
With quantitative Kα spectroscopy measurement,
the behavior of laser plasma interaction with laser
absorption and transfer have been experimentallyand theoretically studied. The TE of laser energy tohot electron is estimated to be ~20%. This highvalue is due to a two stage sharp preplasma formedby the high contrast, high irradiance intensity laserpulse. Beside Mo, the Kα from Au and TE ofcone-guide target have also been investigated inLFEX facility, the details will be given in thepresentation.
Acknowledgments
The authors are indebted to the computer group
at ILE and Cyber Media Center, Osaka University;and gratefully acknowledge the support of theGEKKO XII and LFEX group in ILE. This work
was partly supported by the Common-Use Facility
Program of JAEA.
References[1] Z. Zhang, M. Nishikino, H. Nishimura, T. Kawachi,
A. S. Pirozhkov, et. al., Opt. Exp. 19 (2011) 4560.
[2] H. Kiriyama, M. Michiaki, Y. Nakai, T. Shimomura,H. Sasao, et. al., Appl. Opt. 49, (2010) 2105.
[3] H. Kiriyama, M. Mori, Y. Nakai, T. Shimomura, H.
Sasao, et. al., Opt. Lett. 35, (2010) 1497.
[4] D. Salzmann, C. Reich, I. Uschmann, E. Forster, andP. Gibbon, Phys. Rev. E 65 (2002) 036402.
[5] H. Park, D. Chambers, H. Chung, R. Clarke, R.
Eagleton, et. al., Phys. Plasmas, 13 (2006) 056309.
[6] W. L. Kruer: The Physics of Laser Plasma
Interactions. (Addison-Wesley, 1988).
[7] F. Beg, A. Bell, A. Dangor, C. Danson, A. Fews, et.
al., Phys. Plasmas, 4 (1997) 447.
[8] R. Lichters, J. Meyer-ter-Vehn, and A. Pukhov, Phys.
Plasmas 3 (1996) 3425.