ICRF heating in JET high-performance discharges: Analysis of the effects on the plasma performance

Abstract

In JET high-performance discharges (ELM-free hot-ion H-mode plasmas and optimised shear discharges) ion cyclotron resonance frequency (ICRF) heating is used routinely in combination with neutral beam injection (NBI). Although NBI is the dominating heating mechanism, ICRF heating has been found to play an important role in obtaining good performance. The impact of various ICRF techniques on the performance has been analyzed using numerical simulations. In the hydrogen minority heating scheme second harmonic deuteron absorption is found to improve central bulk ion heating. By using multiple ICRF frequencies the tails in the distribution functions of the resonating ions can be tailored to enhance the bulk ion heating. Bulk ion heating fractions of up to 50% of the total ICRF power have been obtained in this way. Analysis of 3He minority heating in first optimised shear discharges at 4 T is presented. The effects of different antenna phasings on the performance have been found to be consistent with an ICRF induced pinch of resonating ions predicted by theory. In optimised shear discharges +90° phasing is found to give the best performance, while in hot-ion H-modes smaller differences between various phasings are observed.