In fast ignition research, the transport dynamics of fast electrons is one of the critical issues. Fast electrons generated by an intense laser pulse show a highly anisotropic velocity distribution. To gain insight into the anisotropy of the velocity distribution of fast electrons, polarized x‐ray spectroscopy has been proposed. The polarization spectroscopy of Cl Heα radiation was experimentally demonstrated at 1017W/cm2 (∼100 mJ in 130 fs), and a new time‐dependent atomic population kinetics code was also developed. It predicts that the high polarization arises only in a low‐density region of the target plasma. Additional x‐ray polarization measurements were done at 101717–18W/cm2 (∼10 J in ∼1 ps). Polarization was measured as a function of the overcoat thickness of a target. The polarization is negative in the shallow region near the target surface, and becomes near zero at the laser intensity of ∼1018W/cm2. At ∼1017W/cm2, the polarization varies from negative to positive, and finally zero along with an increase in the overcoat thickness.

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