05aE61P
超高強度レーザー生成プラズマ放射硬X線の分光観測
Characterization of Relativistic Electron Beams Generated by Intense LaserPulses with High Energy X-ray Spectrometer
池之内 孝仁、波元 拓哉、西村 博明、藤岡 慎介、小島 完興、有川 安信、
白神 宏之、坂田 匠平、Zhang Zhe、疇地 宏
T. Ikenouchi, T. Namimoto, H. Nishimura, S. Fujioka, S. Kojima, Y. Arikawa,
H. Shiraga, S. Sakata, Zhang Zhe, and H. Azechi
大阪大学レーザーエネルギー学研究センター
Institute of Laser Engineering, Osaka university, Japan
Fast ignition is a way to directly heat a dense fuelwith an ultra-short high intensity laser pulse. In thisscheme, laser energy is converted to a few MeV
electron beam at the laser-plasma interaction region,and the beam heats the fuel at its maximum
compression. Efficient heating highly demands
controlling energy distribution function anddivergence angle of electron beams. Electronspectrometer (ESM) has been widely used tomeasure energy spectrum of relativistic electrons
emanating from a laser-irradiated target. However,measured hot electron spectrum is substantiallyaffected by a plasma potential so that ESM will notprovide direct information on electrons at theinteraction and propagation regions. High energy
x-ray spectrometer (HEXS) was therefore
developed to characterize energetic electrons inside
the target via bremsstrahlung x rays emitted fromtargets.
Twelve pairs of absorption filter and imaging plate(IP) are piled in HEXS as shown in Fig. 1 [1]. The
filters are selected to cover x-ray energy range from
10keV to 700keV. Geant4 code [2], which is aMonte Carlo code to calculate energy deposition
and transport of particle through matter, is used toestimate x-ray spectrum from IP signals.

Absolute sensitivity of HEXS to energetic x-ray
photons was calibrated with radioisotopes andLINAC. In the calibrations, the spectrometer wasexposed to γ-rays from 137Cs and 60Co. HEXS wasalso calibrated with bremsstrahlung from a Pb plateirradiated by an electron beam from LINAC. The
calibrations were conducted to confirm the Geant4
predictions. Good agreements in the cases wereobtained between the experiment and thecalculation.
HEXS was introduced to the fast ignition
experiment with Gekko XII and LFEX lasers. The
target was a tantalum cube attached with a goldcone and a CD hemi-sphere. The CD hemi-sphere
was driven with Gekko XII, and LFEX laser
irradiated the inside of the gold cone at themaximum compression of the sphere. Two HEXSs
were used. One was set at 20.9 degrees from LFEX
axis, corresponding to the cone axis, and the otherwas at 69.1 degrees. Geant4 code simulated theexperiment. In this calculation, a single Maxwellianelectron and plausible angular distribution ofelectrons are supposed to propagate from the cone
tip to the tantalum cube. The best fit between theexperiment and the calculation was obtained when
2.3 MeV slope temperature and Gaussian
divergence of σ=30 degrees [3] are assumed. Theenergy conversion efficiency from laser to hotelectrons in this case was estimated to be 41 %.
[1]
[2]
[3]

Fig.1 Schematic diagram of high energy x-ray spectrometer(HEXS). The major part of HEXS is composed of twelve
pairs of x-ray absorption filters and imaging plates (IP).

C. D. Chen et al., Rev. Sci. Instrum. 79,
10E305 (2008)
Geant4 code, http://geant4.cern.ch/
F. Perez et al., Phys. Plasmas 17, 113106(2010)