Measurement of distribution of x-ray polarization degrees caused by anisotropic hot electrons in ultra-high intensity laser produced plasma

In fast-ignition, investigation of the velocity distribution function (VDF) of hot electrons is critical for clarifying energy transport in ultra-high intensity laser-produced plasmas. X-ray polarization spectroscopy is a useful diagnostic tool for measuring the VDF of electrons inside plasma [1, 2]. A new polarization measurement was performed using a laser pulse (10 J in $\sim$1 ps). Chlorinated triple-layer targets were irradiated, and polarization degrees of Cl-He$\alpha$ line were measured. Obtained distribution of polarization degrees indicates that the VDF is pancake-like shape at the target surface and cigar-like in deep region. Moreover, depolarizations due to isotropic excitation by bulk electrons and elastic collision of bulk electrons, which were predicted by a model calculation [3] and a time-dependent atomic kinetic code [4], was observed. [1] J. C. Kieffer, et al., Phys. Rev. Lett. $\textbf{68}$, 480 (1992), [2] Y. Inubushi, et al., JQSRT $\textbf{99}$, 305 (2006). [3] Y. Inubushi, et al., J. Plasma Fusion Res. $\textbf{2}$, 0013 (2007). [4] T. Kawamura, et al., submitted.