Edge and core impurity transport study with spectroscopic instruments in LHD

Impurity transport was investigated at both edge and core regions in large helical device (LHD) with developed spectroscopic instruments which can measure one- and two-dimensional distributions of impurities. The edge impurity behavior was studied recently using four carbon resonant transitions in different ionization stages of CIII (977Å), CIV (1548Å), CV (40.3Å) and CVI (33.7Å). When the line-averaged electron density, n_e, is increased from 1 to 6 × 10¹³ cm^-3, the ratio of (CIII+CIV)/n_e increases while the ratio of (CV+CVI)/n_e decreases. Here, CIII+CIV (CV+CVI) expresses the sum of CIII (CV) and CIV (CVI) intensities. The CIII+CIV indicates the carbon influx and the CV+CVI indicates the emissions through the transport in the ergodic layer. The result thus gives experimental evidence on the impurity screening by the ergodic layer in LHD, which is also supported by a three-dimensional edge particle simulation. The core impurity behavior is also studied in high-density discharges (n_e ≤ 1× 10¹⁵ cm^-3) with multi H₂-pellets injection. It is found that the ratio of V/D (V: convection velocity, D: diffusion coefficient) decreases after pellet injection and Z_eff profile shows a flat one at values of 1.1∼1.2. These results confirm no impurity accumulation occurs in high-density discharges. As a result, the iron density, n_Fe, is analyzed to be 6 × 10^-7 ( n _Fe/n_e) of which the amount can be negligible as radiation source even in such high-density discharges. One- and two-dimensional impurity distributions from space-resolved VUV and EUV spectrometers newly developed for further impurity transport study are also presented with their preliminary results.