Analysis of Fe XXI Spectral Lines Measured in LHD Plasmas

Abstract

We measured extreme ultraviolet spectra of plasmas in the Large Helical Device after an Fe pellet was injected and analyzed the Fe XXI spectral lines λ 12.12 nm (2s22p2 3P2-2s2p3 3P2) and λ 12.875 nm (2s22p2 3P0-2s2p3 3D1). By constructing a collisional-radiative model for Fe XXI, we obtained the electron density dependence of the line intensity ratio, which is the result of the excitation process among the fine-structure levels of the ground state, 2s22p2 3PJ by electron and proton impact. Using the obtained electron density dependence, we estimated the electron density from the measured intensity ratio. By comparing the estimated density with the electron density distribution measured by a far infrared interferometer, we could estimate the location of the Fe XXI emitting region. We found that Fe XXI appeared first around the density peak formed by the pellet and later expanded toward a low-density region. The corresponding electron temperature of the location is lower than that for ionization equilibrium, and the Fe XXI ions are expected to be in a non-equilibrium ionization state.