Influence of charge-exchange processes on x-ray spectra in TEXTOR tokamak plasmas: experimental and theoretical investigation

Time-resolved high-resolution soft x-ray spectra from gas-puff injected Ar impurity ions have been investigated for neutral beam heated and ohmically heated discharges in the TEXTOR tokamak. The experimental spectra show syst ematic deviations from corona model calculations for the line intensities o f the forbidden He-like lines x, y, z and the Li- and Be-like dielectronic satellite spectra: theoretical corona model calculations predict intensitie s significantly too low. High-intensity Li-like inner-shell excited satelli tes correlate with the neutral beam injection. The discrepancies could also be observed in the stationary phase of an inductively heated discharge; In the heating phase the discrepancies are even larger We propose charge-exch ange processes between the neutral atoms and the impurity ions as an explan ation of the experimental findings. Good agreement with the experimental ob servations can then be obtained without the need for invoking large (anomal ous) diffusion coefficients. A self-consistent coupling of the population k inetics of the neutrals and the impurity ions, also taking into account cha rge-exchange processes from excited states of hydrogen/deuterium permit the determination of the neutral fraction and of the electron lifetime on the sole basis of impurity spectra analysis. Independent Monte Carlo simulation s of neutral gas transport also provides the ionization degree in the centr e and the electron lifetime. These calculations are also in good agreement with the spectroscopic results.