Production and confinement characteristics of ICRF-accelerated energetic ions in JT-60U negative-shear plasmas

Characteristics of energetic ion tail formation during the second-harmonic ion cyclotron range of frequency (2 omega(CH) ICRF) heating and confinement of tail ions were investigated in negative magnetic shear plasmas on JT-60U. Charge-exchange neutral particle measurements show significantly lower tail temperature in negative-shear plasmas than those in positive-shear plasmas. The tail temperature in negative-shear plasmas increases with plasma current and shows improvement of energetic ion confinement. Charge-exchange neutral fluxes are also lower in negative-shear plasmas than in positive-shear plasmas. Meanwhile, tail ion stored energy is comparable between negative- and positive-shear plasmas. In order to understand these experimental results, simple modelled calculations were carried out with an orbit-following Monte Carlo (OFMC) code. The model calculations suggest that the ICRF power absorption is dominant near the 2 omega(CH) resonance in the negative-shear plasma and in the low-field side away from the 2 omega(CH) resonance in the positive-shear plasma. A larger scale length of pressure profile in the negative-shear plasma compensates for a lower density of tail ions than that in the positive-shear plasma. Accordingly, negative-shear plasmas can contain comparable stored energy of energetic tail ions to positive-shear plasmas.