Observation of a new energy channel from energetic particles to bulk ions through geodesic acoustic mode

The energetic-particle driven geodesic acoustic mode (GAM) has been identified on the basis of the spatial structures and frequency measured by heavy ion beam probe (HIBP) and Mirnov coil array. The frequency of the mode is higher than the GAM frequency estimated using parameters of bulk plasmas, and the high frequency GAM can be derived by taking into account the energy distribution function of the energetic particles. The temporal behaviors of energy distribution functions of ions have been measured by neutral particle analyzer (NPA). During the burst of the GAM, the ions lose the energy in the frequency range higher than the GAM frequency, and gain the energy in the frequency range below the GAM frequency. The variation in the energy distribution function indicates the energy transfer through the resonance between the energetic particles and the GAM. The energy transfer through the GAM can become an energy channel from the energetic ions to the bulk ions.