Parametric scattering of extraordinary waves in various electron cyclotron resonance heating schemes for tokamak plasmas

Threshold intensities for parametric side-scattering of extraordinary heating waves off ion cyclotron waves are evaluated for various tokamak heating schemes near the electron cyclotron frequency. It is found that at local electron temperatures T_e ~ 15-500 eV in large tokamaks (a gtrsim 0.5 m) the obtained intensity threholds may be of the same order as the intensities presently employed (1-10 kW/cm²) in the experiments. Especially those heating schemes where oblique launching is used may suffer from scattering near the plasma edge or near the turning point of the heating wave. Furthermore, it is found that the parametric scattering studied here has a lower threshold than the parametric decay into an ion acoustic wave and a magnetized Langmuir wave which can make the nonlinear heating method based on the latter process questionable.