Nonlinear mode mixing in high beta PBX-M discharges

A variety of MHD phenomena have been shown to occur during β saturation and collapse in PBX high βpol discharges at large values of the Troyon parameter (≥4). It has been shown that some of them contribute heavily to the β saturation and collapse. To stabilize the ballooning and the external kink modes, which are thought to be a hindrance on the path to higher betas, PBX has been upgraded to PBX-M by enabling it to go to higher indentation and covering a large part of the plasma surface with a close-fitting conducting shell. This resulted in somewhat different MHD behavior in PBX-M discharges. A new feature of these PBX-M discharges with a high βpol and a large Troyon parameter, not observed in the corresponding PBX discharges, is nonlinear (weakly turbulent) mode mixing. This paper will concentrate primarily on this issue. (author) 5 refs., 4 figs