Study on Radial Position of Impurity Ions in Core and Edge Plasma of LHD Using Space-Resolved EUV Spectrometer

Abstract

Radial profiles of impurity ions of carbon, neon and iron were measured for high-temperature plasmas in large helical device (LHD) using a space-resolved extreme ultraviolet (EUV) spectrometer in the wavelength range of 60 Å to 400 Å. The radial positions of the impurity ions obtained are compared with the local ionization energies, Ei of these impurity ions and the electron temperatures TeZ there. The impurity ions with 0.3 keV ≤ Ei ≤ 1.0 keV are always located in outer region of plasma, i.e., 0.7 ≤ ρ ≤ 1.0, and those with Ei ≤ 0.3 keV are located in the ergodic layer, i.e., 1.0 ≤ ρ ≤ 1.1, with a sharp peak edge, where ρ is the normalized radial position. It is newly found that TeZ is approximately equal to Ei for the impurity ions with Ei ≤ 0.3 keV, whereas roughly half the value of Ei for the impurity ions with 0.3 keV ≤ Ei ≤ 1.0 keV. It is known that TeZ is considerably lower than Ei in the plasma edge and approaches to Ei in the plasma core. Therefore, this result seems to originate from the difference in the transverse transport between the plasma edge at ρ ≤ 1.0 and the ergodic layer at ρ ≥ 1.0. The transverse transport is studied with an impurity transport simulation code. The result revealed that the difference appearing in the impurity radial positions can be qualitatively explained by the different values of diffusion coefficient, e.g., D= 0.2 m2/s and 1.0 m2/s, which can be taken as a typical index of the transverse transport.