Modeling of the nonlinear mode coupling of lower hybrid waves in tokamak plasmas

A nonlinear wave equation is derived to describe the nonlinear mode coupling of lower hybrid (LH) waves launched in tokamak plasmas, driven by low-frequency (LF) (10 MHz) ion-sound evanescent modes (quasi-modes). The spectrum of the LF fluctuations is calculated considering the beating of the LH wave at the radiofrequency (RF) operating line frequency (pump wave) with the noisy background of the RF power generator. This spectrum is calculated in the frame of the kinetic theory, following a perturbative approach. Numerical solutions of the nonlinear LH wave equation show the evolution of the nonlinear mode coupling in condition of a finite depletion of the pump power. Under operating conditions generally met in the experiments, the sidebands, excited by the noise of RF power generators and amplified by the nonlinearity, broaden the launched antenna spectrum, and determine the LH propagation and deposition behavior.