Berlin 2001 – scientific programme

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P: Plasmaphysik

P 1: Magnetic Confinement and Plasma Wall Interaction

P 1.4: Talk

Monday, April 2, 2001, 17:15–17:30, 2053

Determination of Z_eff from central bremsstrahlung in ASDEX Upgrade discharges — •Denis Bolshukhin, Rudolf Neu, Ralph Dux, and Arne Kallenbach — Max-Planck-Institut für Plasmaphysik EURATOM Assoziation

The pulse height analysis (PHA) system of ASDEX Upgrade is based on a new Silicon Drift Detector (SDD). The recently installed SDD offers major advantages over analogue Si(Li) systems. It has a high quantum efficiency (≥ 50 % ) and accepts about 10 times higher countrates as conventional detectors (above 10⁵ Hz) without effects of saturation. Besides, no liquid nitrogen cooling and suppression of the microphony effects are needed. Soft X ray spectra were measured with the sampling rate of 10 Hz in several H-mode discharges. Absolutely measured spectra in the energy range between 1 keV and 24 keV are dominated mainly by bremsstrahlung which is overlapped with few characteristical emission lines of impurities (mainly Si, Ar and Fe). Strong emission lines were approximated with gauss functions and subtracted from the spectra. This yields almost pure bremsstrahlung spectra which are proportional to Z_eff, because the recombination radiation under usual plasma conditions is negligible. Reconstructions of the bremsstrahlung using the independently measured electron density and temperature profiles show that the spectrum observed by SDD is dominated by the central plasma radiation (ρ < 0.4). Hence the found Z_eff values of 1.5 - 2.5 characterise mainly the central plasma. Comparison with Z_eff data obtained from the visible spectral range show good agreement with the data obtained from the central soft X-ray radiation in standard H mode plasmas.