Effects of low particle fuelling and electron heating on the internal transport barrier in ICRF heated reversed magnetic shear plasmas of JT-60U

Abstract

Plasmas with an internal transport barrier (ITB) were heated with a second-harmonic ion cyclotron range of frequency (ICRF) minority heating during the reversed magnetic shear (RS) experiments in JT-60U. In order to control the central particle fuelling, the input power of the neutral beam injection (NBI) heating during ICRF heating was scanned shot-by-shot. The electron temperature gradient around the ITB increased with the total input power. Steeper gradients were obtained with additional ICRF heating by avoiding collapses induced by the steep pressure gradient. The difference of the time derivative at the ICRF injection timing indicated a fast ion loss of about 40% during the RS operation. The density gradient was increased with the central fuelling by the NBI heating and the shoulder position of the slope at the ITB moved outward with the particle fuelling. The particle confinement time at the ITB increased with the central particle fuelling while the particle confinement time of the low-fuelling discharges were similar to that of the normal shear L-mode discharge.