Observation of ion-cyclotron-wave instability caused by perpendicular neutral beam injection in the JT-60 tokamak.

A microinstability at a frequency equal to the second-harmonic ion cyclotron frequency was observed in the JT-60 tokamak with the injection of high-power perpendicular neutral beams. This instability appeared in a limiter configuration having relatively large clearance between the plasma surface and the wall of the vacuum vessel, but not in an outside divertor configuration. The instability was accompanied by a high-energy ion tail in the almost perpendicular direction. These experimental observations suggest that the strong anisotropy of the ion velocity distribution may be brought about by an ion banana motion localized at the outer region of the major radius, and that strong perpendicular neutral beam injection may suffer from insufficient thermalization of injected beams.