Polarization of Lyman-α Line Due to the Anisotropy of Electron Collisions in a Plasma

We have developed an atomic model for calculating the polarization state of the Lyman-α line in plasma caused by anisotropic electron collision excitations. The model assumes a nonequilibrium state of the electron temperature between the directions parallel (T‖) and perpendicular (T⊥) to the magnetic field. A simplified assumption on the formation of an excited state population in the model is justified by detailed analysis of population flows regarding the upper state of the Lyman-α transition with the help of collisional-radiative model calculations. Calculation results give the polarization degree of several percent under typical conditions in the edge region of a magnetically confined fusion plasma. It is also found that the relaxation of polarization due to collisional averaging among the magnetic sublevels is effective in the electron density region considered. An analysis of the experimental data measured in the Large Helical Device gives T⊥/T‖=7.6 at the expected Lyman-α emission location outside the confined region. The result is derived with the absolute polarization degree of 0.033, and T⊥=32 eV and ne=9.6×1018m−3 measured by the Thomson scattering diagnostic system.