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

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 60A to 400 A. The radial positions of the impurity ions obtained are compared with the local ionization energies, El of these impurity ions and the electron temperatures T,z there. The impurity ions with 0.3 keV 〈_ Ei ≤ 1.0 keV are always located in outer region of plasma, i.e., 0.7 ≤ p ≤ 1.0, and those with Ei ≤ 0.3 keV are located in the ergodic layer, i.e., 1.0 ≤ p≤ 1.1, with a sharp peak edge, where p 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 El 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 p 〈1.0 and the ergodic layer at p ≥ 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.

Observation of the Two-Dimensional Distribution of Impurity Line Emissions Using a Space-Resolved EUV Spectrometer in LHD

A space-resolved EUV spectrometer for measuring the one-dimensional distribution of impurity line emissions in Large Helical Device （LHD） has been upgraded to measure two- dimensional distributions of impurity line emissions with an extension of working wavelength range to 30-650% The two-dimensional measurement is performed by scanning the observation chord horizontally. A rectangular plasma region of 520 × 700 mm2 in vertical and horizontal sizes can be observed during a single horizontal scan. The horizontal scan requires a time duration of 5 s at least. The spatial resolution is 10 mm in the vertical direction when a spatial-resolution slit of 0.2 mm in width is adopted. Although a spatial resolution in the toroidal direction is 75 mm, it is a function of CCD exposure time and horizontal scanning speed. Two-dimensional distribution of EUV line emissions from several impurities has been successfully observed for the first time from steady discharges in LHD. In this paper two-dimensional distributions of He II （303.78A）, C V （40.27A）, C VI （33.73A） and Fe XX （132.85A） located at different radial positions are presented with simple analysis on the magnetic field structure of LHD.

Observation of Impurity Accumulation After Hydrogen Multi-Pellet Injection in Large Helical Device

Impurity accumulation is studied for neutral beam-heated discharges after hydrogen multi-pellet injection in Large Helical Device （LHD）. Iron density profiles are derived from radial profiles of EUV line emissions of FeXV-XXIV with the help of the collisional-radiative model. A peaked density profile of Fe2a＋ is simulated by using one-dimensional impurity transport code. The result indicates a large inward velocity of -6 m/s at the impurity accumulation phase. However, the discharge is not entirely affected by the impurity accumulation, since the concentration of iron impurity, estimated to be 3.3x10-5 to the electron density, is considerably small. On the other hand, a flat profile is observed for the carbon density of C6＋, which is derived from the Zeff profile, indicating a small inward velocity of -1 m/s. These results suggest atomic number dependence in the impurity accumulation of LHD, which is similar to the tokamak result.