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 (3He) ion velocity distribution that excites 3He ICE [ICE(3He)] 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 3He ions. Then, the fast 3He ion distributions are evaluated with the OFMC code using the birth distributions and compared between cases when the ICE(3He) is observed and not. The evaluated fast 3He 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(3He) 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 3He ion distribution makes a key contribution to the excitation of the ICE(3He) on JT-60U.
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