Experimental determination of the dispersion relation of plasma radiation at frequencies of the ion cyclotron resonance and its harmonics in tokamak

A study of the spectral properties of electromagnetic radiation in the ion-cyclotron range from the plasma of the TUMAN-3M tokamak in the NBI heating mode has been carried out. The spectrum of this radiation consists of several (up to four) narrow lines corresponding to different harmonics of the ion-cyclotron resonance frequency of injected fast ions in the center of the plasma. Wave vectors corresponding to individual spectral lines are determined from the signal delays between spatially separated probes. It is shown that under the assumption that all the measured frequencies and wave vectors follow the same dispersion relation, the observed radiation can be explained by the appearance in the plasma of an unstable fast magneto-acoustic wave propagating almost normally to the direction of the magnetic field.