Lower hybrid current drive and ion Bernstein wave heating experiments on PBX-M

This paper presents an overview of the experiments conducted on PBX-M to evaluate the feasibility and effect of current profile and pressure profile control on the plasma stability. Utilizing the inaccessibility of the lower hybrid waves, it has been possible to obtain a certain degree of power deposition localization and off-axis current drive. The effect of fast electron diffusion has been studied and found not to be a limiting factor; consequently, the current profile has been modified in a non-transient manner. More serious is the destabilization of global MHD modes, due to the change of the current profile, which can lead to disruption or to a rapid radial redistribution of the fast electron population. Experiments with ion Bernstein wave heating have shown that power can be deposited off-axis and that the ion temperature can be modified locally. Application of IBW into a strongly neutral beam (NBI) heated H-mode plasma causes a substantial increase of thermal and particle confinement in the core of the plasma: this produces a localized bootstrap current sufficient to significantly raise the value of q(0). We propose to refer to this condition as the CH-mode (or core high-confinement mode).