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

Motoshi Goto,Nilam Ramaiya

doi:10.3390/sym13020297

Abstract

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.

Highlights

In a magnetically confined fusion plasma, the velocity distribution function (VDF) of electrons and ions is thought to be more or less anisotropic
Energetic ions are unidirectionally introduced by the neutral beam, and the cyclotron motions of ions and electrons are selectively accelerated by the electron cyclotron resonance heating (ECRH) and the ion cyclotron resonance heating (ICRH), respectively
In the Large Helical Device (LHD), the so-called density clamping observed with strong ECRH is thought to be attributed to the difference in the confinement characteristics of the trapped and passing electrons [3]

Summary

Introduction

In a magnetically confined fusion plasma, the velocity distribution function (VDF) of electrons and ions is thought to be more or less anisotropic. The anisotropy is regarded as an important subject for characterizing the plasma confinement as seen above, no reliable measurement method for the anisotropic VDF of electrons and ions has been established to date. Under such circumstances, polarization spectroscopy has been proposed as a technique to address the problem of anisotropy in the EVDF [5]. Polarization spectroscopy has been proposed as a technique to address the problem of anisotropy in the EVDF [5] This novel diagnostic method consists of two issues, i.e., the measurement of polarization in line emission from the plasma and the construction of an atomic model for analyzing the observation data. An analysis has been made of the LHD experimental data and derivation of an anisotropic EVDF is attempted

Polarization Formation of the Lyman-α Line

Polarization Due to Anisotropic Electron Collisions

Results and Discussion