Impurity accumulation spectroscopically observed during ion cyclotron resonance frequency heating experiments in the JIPP T-II-U tokamak.

Abstract

Spectroscopic investigations of iron-ion line radiation (Fe x--xxiv) in the extreme-ultraviolet region have been carried out during ion cyclotron resonance frequency (ICRF) heating experiments in the JIPP T-II-U tokamak, at the Japanese Institute of Plasma Physics (JIPP), Nagoya University, Nagoya. The central-chord radiances of the iron-ion resonance lines were observed to increase during ICRF heating. After the radiance of the highest ionization stage attained the peak toward the end of ICRF heating, the radiances of the lower stages were observed to attain their peaks at successively later times. The rf-power injection was accompanied by strong gas puffing and additional current rise, which raised the electron density and concurrently lowered the central electron-temperature. The electron-temperature decrease in the central region, which was further promoted by radiative losses due to metallic impurities, continued after ICRF heating. The spectroscopic observations are interpreted as showing that the ICRF heating causes a large additional influx of iron into the plasma, and that the iron accumulates preferentially in the plasma interior with long confinement times. The sequential peaking of the radiation from the highest to the lowest ionization stages reflects the decrease in the electron temperature and the concurrent recombination into successively lower stages. These interpretations are supported by the soft-x-ray and bolometric measurements, and confirmed by the modeling calculations that reproduce the iron-ion line radiances.