On possibility of strong anomalous damping of an ordinary pump wave in the electron cyclotron resonance heating experiments due to the low-power-threshold parametric decay instability

Abstract

We analyze the low power-threshold parametric decay instability of an ordinary wave in the first electron cyclotron harmonic frequency range, which leads to the excitation of a localized upper hybrid (UH) wave and a lower hybrid wave. We investigate its transition into the saturation mode due to both the pump wave depletion and the cascade of successive secondary decays. A set of equations describing this nonlinear frequency down-conversion of the primary UH wave is derived and then solved both analytically and numerically under the conditions typical of the off-axis O1-mode electron cyclotron resonance heating experiments in the FTU tokamak. The possibility of a strong anomalous absorption of the pump power in the case of even-step cascade of secondary decays is discovered.