Radio frequency emissions driven by energetic ions from neutral beam in KSTAR low confinement mode plasma

Abstract

The tangential neutral beam injection in KSTAR low confinement mode plasma is rapidly accompanied by the electromagnetic emissions in radio frequency (RF) range (0.1–1 GHz). The RF emission is initially onset within 1 ms from the beam injection, at discrete frequencies with steadily increasing intensity. The frequency spacing for these discrete emission lines corresponds to the deuteron cyclotron frequency, at a location midway between the magnetic axis and the edge. Further, the observed discrete frequencies lie in the lower hybrid frequency (f LH) range in a broad region on the low field side (LFS). As the initial RF emission becomes saturated, there is another onset of intense RF bursts occurring at discrete frequencies, broadening the emission frequency range further, either at higher or lower frequencies. In some cases, the time interval of the intense RF bursts at the dominant frequency is comparable with the toroidal rotation period at the radial location where f LH ~ dominant frequency. The rapid rise and saturation of RF emission intensity in a broad frequency range indicate that a small population of fast ions is sufficient for the growth of energetic particle driven instabilities on the LFS. The multiple onsets in RF emission and the intense RF burst repetition frequency comparable with toroidal rotation frequency indicate the possibility that reorganization in the anisotropic fast ion population results in localized growth of the above instabilities. A gradual decay of RF emission intensity over few tens of milliseconds indicate that enhanced population of fast ions has damping effect on these instabilities.