X-ray polarization spectroscopy of Heα line emission for diagnosis of the anisotropy of hot electrons

X-ray polarization spectroscopy is proposed as an advanced diagnostic tool to measure the anisotropic velocity distribution of hot electrons generated by an ultra-high intensity laser pulse. The relationship between the anisotropy of the hot electron velocity distribution and the polarization degree was investigated for various tracer materials. It is shown that the polarization degrees of Heα lines are a function of electron energy normalized by the excitation threshold. Depolarization is caused by isotropic bulk electrons in high temperature plasma. To diagnose the anisotropy of the hot electron velocity distribution without depolarization, it is essential to select a tracer material whose energy of Heα line is 10 times higher than the bulk electron temperature. (author)