On the Possibility of Parametric Excitation of Two-Dimensional Eigenmodes of Ion Bernstein Wave in ECRH Experiments at the TCV Tokamak

For typical conditions of the ECRH experiments at the TCV tokamak, a new scenario of low-threshold decay of the extraordinary wave, the frequency of which corresponds to the second harmonic of electron cyclotron frequency at the axis of the plasma column, is considered. Within the scenario, the parametric decay of the pump wave is accompanied by the excitation of a two-dimensional eigenmode of the ion Bernstein (IB) wave and a traveling (non-localized) electron Bernstein wave. It is shown that the saturation of this parametric decay instability occurs at a relatively low level, as a result of stochastic amplitude-dependent damping of the IB wave. It was discovered that there is a “second threshold” in the pump wave power, above which the stochastic damping only reduces the growth rate of primary instability, and the saturation occurs at considerably higher level, as a result of a cascade of secondary decays of the primary IB wave, accompanied by the excitation of eigenmodes of the plasma waveguide of the IB wave.