Enhanced performance of high current discharges in JET produced by ICRF heating during the current rise

The performance of high current discharges can be improved by applying central ICRF heating before or shortly after the onset of sawtooth activity in the plasma current rise phase. Long sawtooth-free periods have been obtained which result in a transiently-enhanced discharge performance. High Te(0)=9-10.5 keV with peaked profiles Te(0)/(Te)=3-4 were obtained giving values of Ne(0)Te(0) up to 6*1020 (keV m-3). Improvements in Ti(0) and neutron production are observed. A best value of nD(0)Ti(0) tau E=1.65*1020 (m-3 keV s) was achieved. Local transport simulation shows that the electron and ion thermal diffusivities do not differ substantially in the two cases of current-rise (CR) and flat-top (FT) heating, the performance of the central plasma region being enhanced, in the case of current-rise, entirely by the elimination of the sawtooth instability. The maximum D-D reaction rate is enhanced by a factor of 2 compared to the flat-top value. An appreciable part of the reaction rate is attributed to 2nd harmonic deuterium (2 omega CD) heating. In all current-rise discharges radiation amounts to 25-50% of total power and Zeff remains roughly constant.