TOTAL AND PARTIAL RADIATIVE POWER LOSS AT HIGH IMPURITY RECYCLING STUDIED IN THE EXTRAP-T1 REVERSED-FIELD PINCH EXPERIMENT

Measurements of the radiative power loss are performed on the Extrap-T1 reversed-field pinch [Phys. Scr. 44, 358 (1991)] using an absolutely calibrated vacuum ultraviolet spectrometer in the wavelength range between 100 and 1700 angstrom. The contributions to the radiative power loss from deuterium, carbon, nitrogen, oxygen, and chlorine are measured. These partial radiative power losses are further divided into contributions from different ionization stages of these impurities. The additional contribution to the cooling rate due to ionization processes is estimated. Impurities are found to be a main contributor to the power loss in the initial phase of the discharge. Toward the end of the discharge, radiative power loss amounts to less than 10% of the input power, due to the decrease of the electron density. Because of the short particle confinement time and high recycling of impurities, specifically oxygen, the rate coefficient for radiative power loss per ion is only slightly reduced by the increase of the temperature beyond the temperature of maximum radiative power loss per ion.