Characteristics of fast 3He ion velocity distribution exciting ion cyclotron emission on JT-60U

Ion cyclotron emission (ICE) is probably excited by non-thermal ion velocity distribution. Identifying characteristics of the ion velocity distribution that excites the ICE can contribute to understanding its emission mechanism. The characteristics of fast helium-3 (He-3) ion velocity distribution that excites He-3 ICE [ICE(He-3)] on JT-60U are investigated. First, fast deuterium ion distributions, which mainly contribute to deuterium-deuterium fusion reactions, are calculated with a fast ion orbit following Monte-Carlo code (OFMC code) under realistic conditions to evaluate birth spatial and velocity distributions of the He-3 ions. Then, the fast He-3 ion distributions are evaluated with the OFMC code using the birth distributions and compared between cases when the ICE(He-3) is observed and not. The evaluated fast He-3 ion distributions at the midplane edge of the plasma on the low field side have strong pitch-angle anisotropy in both cases. In the case with the ICE(He-3) excitation, a relatively peaked bump-on tail structure in the energy direction is formed in the distributions. The formation of the relatively peaked bump-on tail structure is localized at the plasma edge on the low field side. On the other hand, the distributions at the plasma edge on the low field side have a broader structure in the energy direction in the case without the excitation. The comparison results show that the formation of the relatively peaked bump-on tail structure in the fast He-3 ion distribution makes a key contribution to the excitation of the ICE(He-3) on JT-60U.