Equilibrium and Stability Properties of \textsc{Pegasus} Edge Plasmas

ELM-like filamentary edge instabilities are observed under conditions of high ${j_\parallel } \mathord{\left/ {\vphantom {{j_\parallel } B}} \right. \kern-\nulldelimiterspace} B$ ($\ge $ 1 MA/m$^{2}$T) in \textsc{Pegasus}. Their properties include: a high-$m$, low-$n$ (1--5) electromagnetic signature, consistent with $m \mathord{\left/ {\vphantom {m n}} \right. \kern-\nulldelimiterspace} n\simeq q_a $; characteristic frequencies $<$ 100 kHz; high poloidal coherence; rotation; and, explosive filament detachment followed by accelerating outboard radial propagation. Presently, these modes' dependence on the peeling instability parameter ${j_\parallel } \mathord{\left/ {\vphantom {{j_\parallel } B}} \right. \kern-\nulldelimiterspace} B$ is being systematically studied through variation of ${\partial I_p } \mathord{\left/ {\vphantom {{\partial I_p } {\partial t}}} \right. \kern-\nulldelimiterspace} {\partial t}$ and I$_{TF}$. To date, all data indicate these instabilities lie in the peeling regime. The modest edge $T_e $ and short pulse lengths of \textsc{Pegasus} afford direct diagnostic access to the edge via internal magnetic and Langmuir probe measurements. A novel edge probe utilizing a radial array of Hall-effect sensors\footnote{ M.W. Bongard \textit{et al.,} accepted for pub. in Rev. Sci. Instrum. (2010)} measures $B_z (R,t)$ with high spatial and $\sim $50 $\mu s$ temporal resolution, and provides strong experimental constraint on $j(\psi )$ in equilibrium reconstructions on ELM-relevant timescales. Initial magnetic equilibrium reconstructions and ideal stability analysis with DCON imply instability when edge filamentation occurs.