MODE CONVERSION OF FAST ALFVEN WAVES AT THE ION-ION HYBRID-RESONANCE

Substantial radio-frequency power in the ion-cyclotron range of freque ncies can be effectively coupled to a tokamak plasma from poloidal cur rent strap antennas at the plasma edge. If there exists an ion-ion hyb rid resonance inside the plasma, then some of the power from the anten na, delivered into the plasma by fast Alfven waves, can be mode conver ted to ion-Bernstein waves. In tokamak confinement fields the mode-con verted ion-Bernstein waves can damp effectively and locally on electro ns [A. K. Ram and A. Bers, Phys. Fluids B 3, 1059 (1991)]. The usual m ode-conversion analysis that studies the propagation of fast Alfven wa ves in the immediate vicinity of the ion-ion hybrid resonance is exten ded to include the propagation and reflection of the fast Alfven waves on the high magnetic-field side of the ion-ion hybrid resonance. It i s shown that there exist plasma conditions for which the entire fast A lfven wave power incident on the ion-ion hybrid resonance can be conve rted to ion-Bernstein waves. In this extended analysis of the mode con version process, the fast Alfven waves can be envisioned as being coup led to an internal plasma resonator. This resonator extends from the l ow magnetic-field cutoff near the ion-ion hybrid resonance to the high magnetic-held cutoff. The condition for 100% mode conversion correspo nds to a critical coupling of the fast Alfven waves to this internal r esonator. As an example, the appropriate plasma conditions for 100% mo de conversion are determined for the Tokamak Fusion Test Reactor (TFTR ) [R. Majeski et al., Proceedings of the 11th Topical Conference on RF Power in Plasmas, Palm Springs (American Institute of Physics, New Yo rk, 1995), Vol. 355, p. 63] experimental parameters. (C) 1996 American Institute of Physics.