Localized fast magnetoacoustic eigenmodes in tokamak plasmas

An equation is derived for fast magnetoacoustic eigenmodes (FMEs) in a tokamak fusion plasma with an elongated cross-section. It is shown that edge localized fast magnetoacoustic waves (FMWs) with both positive and negative poloidal wavenumbers are possible, provided that the product a 2 exceeds a certain critical magnitude N cr , where is the plasma density at a point near the plasma edge and a is the plasma radius. Fulfilment of this condition in various devices is analysed, which is important in explaining the fine structure of the spectrum of suprathermal ICE from tokamak plasmas. It is found that a small population of high energy ions with a non-equilibrium distribution function can significantly affect the FMEs.