Periodic ejection of fast ions from a tokamak

A phenomenological system of two equations is proposed to describe the behavior of the fast-ion pressure and of the electromagnetic noise level in the plasma during the onset of an instability in a tokamak with intense injection of neutral atoms. It is assumed that the instability arises from a resonant interaction between particles and waves. This new model yields a picture of the oscillations in the amplitude of the electromagnetic field and of the fast-ion pressure which agrees not only qualitatively but also quantitatively with the results observed in the PDX tokamak. A general analysis has been carried out to determine the necessary conditions for the appearance of fishbone waves and to find the conditions under which steady-state turbulence is established in the plasma.