Study of High-energy Ion Tail Formation with Second Harmonic ICRF Heating and NBI in LHD

"Ion cyclotron range of frequencies (ICRF) heating is a good tool for the alpha-particle simulation of a future fusion reactor. In the Large Helical Device (LHD), enhancement of the high-energy ion tail formed by second harmonic ICRF heating by reducing the magnetic field strength was observed in the presence of energetic particles having a large Larmor radius supplied by a perpendicular NBI. Moreover, it was clarified by a pellet charge exchange (PCX) measurement that high-energy deeply trapped particles exist near the ion cyclotron resonance layer. The power modulation experiments clarified the large particle loss due to the large Larmor radius normalized by the plasma minor radius rho_1*. The spectrum of the high-energy ion tail was consistent with the calculation by the GNET code in the rho_1* region lower than 0.065. This experimental result supports the GNET simulation that alpha particles will be well confined in the heliotron reactor.