On the possibility of low-threshold anomalous absorption in tokamak 2nd-harmonic electron cyclotron resonance heating experiments

Abstract

The parametric decay of an extraordinary electron cyclotron pump wave to electron Bernstein (EB) wave and ion Bernstein wave leading to anomalous absorption in 2nd-harmonic electron cyclotron resonance heating (ECRH) experiments is analyzed. It is shown that in an axisymmetric system, like tokamak, excitation of the low-threshold absolute parametric decay instability (PDI) is possible in the case of non-monotonic behaviour of plasma density in radial direction and due to poloidal magnetic-field inhomogeneity. The threshold of the absolute PDI instability determined by linear dissipation of the EB wave is shown to be substantially exceeded in nowadays 2nd-harmonic ECRH experiments in tokamaks that provide an explanation for the mysterious ion acceleration effect observed there.