MHD characteristics and edge plasma stability during periods of ELM activity in PBX-M

A common repetitive feature of H-mode plasmas is the so-called Edge Localized Mode (ELM), which is identified by a rapid increase in the Dα emission, denoting an increase in edge recycling, and is associated with a loss of plasma energy and density. In this sense, the term ELM is a misnomer, as the energy loss can extend into the core of the plasma, and, in some cases, can be coupled to internal activity. The mechanism causing ELMs is still not well understood. It was initially suggested that ELMs in DIII-D could be due to microtearing modes, but later stability analysis of detailed profiles near the plasma edge indicated that 'giant' ELMs may be triggered by the ideal ballooning instability. In this work, we will show the existence of a high frequency magnetic precursor to ELMs, and we will present the results of stability analyses that indicate the pressure-driven ideal kink to be a likely candidate for driving these oscillations and, therefore, perhaps the ELM itself. (author) 5 refs., 3 figs