Peeling Instability in the Pegasus ST

Ohmic plasmas in \textsc{Pegasus} are often initially unstable to peeling modes, an instability underlying deleterious edge localized mode (ELM) activity in fusion-grade plasmas. These edge-localized instabilities are observed under conditions of high parallel edge current density ($J_\parallel \sim 0.1$ MA/m$^{2})$ and low magnetic field ($B\sim 0.1$ T) present at near-unity aspect ratio, corresponding to high peeling instability drive ($\propto J_\parallel /B)$. They generate electromagnetic MHD activity with low toroidal mode numbers $n\le 3$ and ELM-like, field-aligned edge filaments with high poloidal coherence that detach from the plasma and propagate outward. The modest edge temperatures and short pulse lengths of \textsc{Pegasus} discharges permit time-resolved measurements of the edge current density profile $J_{edge} $ using an insertable Hall probe. Peeling MHD fluctuation amplitudes scale strongly with measured $J_\parallel /B$, consistent with theory. Ideal stability analysis of Hall-constrained equilibrium reconstructions with DCON finds instability to peeling modes. Filaments form from an initial $J_{edge} $ ``current-hole'' perturbation and carry currents $\sim $100-250 A. Their radial trajectories feature transient acceleration due to magnetostatic repulsion followed by constant-velocity motion, consistent with models of ELM dynamics.