Origin of superthermal ion cyclotron emission in tokamaks

It is shown that the key mechanism in the origin of superthermal ion cyclotron emission (ICE) appearing in tokamak fusion experiments is the instability of fast magnetoacoustic waves having very small longitudinal wavenumbers and being excited through toroidicity affected cyclotron resonance with an energetic ion population. This new type of cyclotron resonance leads to an instability growth rate well exceeding the inverse bounce/transit period of fast ions even for a small number of high energy ions. The proposed theory explains the main features of the ICE spectrum observed in JET and the main conclusions are obtained without any assumptions about the explicit form of the fast ion distribution function.