High Frequency Instabilities Induced by Energetic Ions Inside Magnetic Islands

Energetic ions, which are used for plasma heating in modern tokamaks, as well as fusion alphas, which will sustain fusion burn in future reactors, have a gyro-radius comparable with the half-width of magnetic islands, which are often present in discharges with high plasma pressure. When such ions are injected inside the island, they encircle the magnetic axis of the island, which is formed by magnetic field lines with the island O-points. As a result, a population of particles with strong azimuthal flow around the axis of the island is formed. In the present work it is shown that such population excites high-frequency modes, which are localized inside and move together with the island. The mode frequency (growth rate) can considerably exceed the frequency of the lower hybrid resonance (fast ion gyro-frequency). The possible relation of these modes with observed anomalous scattering of the gyrotron beams off magnetic islands is discussed.