Polarized Heα Radiation by Anisotropic Fast Electron Transport in Ultra-Intense Laser Produced Plasmas.

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 He_α radiation was experimentally demonstrated at 10¹⁷ W/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 10^1717–18 W/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 ∼10¹⁸ W/cm². At ∼10¹⁷ W/cm², the polarization varies from negative to positive, and finally zero along with an increase in the overcoat thickness. [ABSTRACT FROM AUTHOR]