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 alpha radiation was experimentally demonstrated at 10(17) W/cm(2) (similar to 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(17-18) W/cm(2) (similar to 10 J in similar to 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 10(18) W/cm(2). At similar to 10(17) W/cm(2), the polarization varies from negative to positive, and finally zero along with an increase in the overcoat thickness.