Co-counter asymmetry in fast wave heating and current drive

Different plasma responses to neutral beam injection in the directions co a nd counter to the plasma current have long been accepted as well understood in neutral beam heating of tokamak plasmas. Differences can also occur in fast wave heating and current drive in the ion cyclotron range of frequenci es (ICRF) when antenna arrays are phased to drive current co and counter to the plasma current. The source of this asymmetry can be easily seen in the cold plasma wave equation with an applied magnetic field in the z directio n and the parallel ICRF electric field set to zero (small electron mass lim it). In the absence of absorption, the wave equation displays perfect up-do wn symmetry. However, when absorption is introduced, the up-down symmetry i s destroyed by Hall terms, which depend on density and magnetic field gradi ents. This is confirmed by simple numerical solutions of the cold plasma wa ve equation with and without collisions. The same up-down asymmetry appears in three dimensional (3-D) antenna coupling calculations with outgoing bou ndary conditions. These show a natural poloidal shift in the antenna's radi ated power spectrum even when no poloidal magnetic field is present. When a poloidal magnetic field is introduced, the up-down asymmetry acquires a to roidal component. This leads to differences in electron heating and current drive depending on the direction that fast waves are launched relative to the plasma current. Such differences are clearly seen in full wave modellin g calculations of heating and current drive in NSTX, where poloidal and tor oidal magnetic fields are comparable in magnitude near the antenna. When de nsity gradients are forced to zero, both up-down and co-counter asymmetries disappear.