Mode conversion at the higher ion cyclotron harmonics

It has been demonstrated that mode conversion of fast waves to ion Bernstein waves can be calculated from a reduced second order differential equation for the wave fields rather than the 4th order equations used in earlier studies near the ion-ion hybrid resonance and the second harmonic resonance. Here the underlying justification of the method is discussed. It is shown that the method works for high harmonic resonances and an analytical formula for the tunneling coefficient is derived. The result is a generalization of a previous result obtained by Ngan and Swanson and is applicable when κ/sub /perpendicular///rho//sub i/ is large. Recently, there is interest in using fast waves for current drive at high ion cyclotron harmonics frequencies in tokamaks. Generally, the fast wave will encounter ion cyclotron within the plasma cross-section. For efficient current drive, the minimization of the mode conversion processes sets restrictions to the choice of frequencies and magnetic fields. This is discussed using the derived formula. 14 refs., 1 fig