Low-m Tearing Mode Studies on \textsc{Pegasus}

Large scale, low-m/n=1 tearing activity is commonly observed in ohmic discharges on \textsc{Pegasus}. Accentuated by broad regions of low magnetic shear, this tearing activity can limit performance in high TF utilization regimes (${I_p } \mathord{\left/ {\vphantom {{I_p } {I_{TF} }}} \right. \kern-\nulldelimiterspace} {I_{TF} }\approx$ 1) where $q_0$ is a low-order rational. In addition to increasing TF to raise $q_0 $, tearing activity can be mitigated by: TF ramp-downs; electrostatic preioniziation to obtain OH breakdown at lower TF; and, DC helicity injection to provide strong edge current drive. Systematic scans of TF, $I_p$, and ${\partial I_p } \mathord{\left/ {\vphantom {{\partial I_p } {\partial t}}} \right. \kern-\nulldelimiterspace} {\partial t}$ are presently being conducted to characterize these modes under a range of operational conditions. Measurements of magnetic activity via internal Mirnov probes are complicated by electrostatic noise arising from \textsc{Pegasus'} high-frequency switching power supplies. This noise can be mitigated somewhat by shielding and signal processing, but can be eliminated altogether by freewheeling all power supplies for a brief period.