Theoretical predictions for the polarization of X-ray laser lines in the presence of a directed beam of hot electrons

The 19.6 nm laser line due to a 3p - 3s, J=0-1 transition in Ne-like Ge ions was found to be polarized parallel to the target plane with a degree of polarization of -53 % by Kawachi et al. (1995). They intepret it as a consequence of a an unbalanced population of the 3s magnetic sublevels. Kieffer et al. (1993) have shown that, during plasma heating, some electrons could be accelerated preferentially in the same direction as the pump laser. In the present work, we shall show that such anisotropic electrons can give a small difference in populations of the 3s sublevels. The 3s level spontaneous emission to the ground level is linearly polarized and can be reabsorbed by the Ne-like ion ground state which amplifies the population of the 3s levels, as well as it increases the different sublevel populations. The 3p-3s X-ray laser amplification depending on the 3s level population becomes polarized but with an opposite sign to the absorbed radiation from the ground level.