Plasma Properties in the \textsc{Pegasus} Edge Region

The plasma edge region in the \textsc{Pegasus} ST exhibits: peeling modes at high ${j_\parallel } \mathord{\left/ {\vphantom {{j_\parallel } B}} \right. \kern-\nulldelimiterspace} B$; broadband electrostatic turbulence; and strong deformation due to large-scale internal tearing modes. In addition, local helicity injection sources in the edge give rise to large current densities in the plasma scrapeoff, and potentially to a large non-axisymmetric edge current distribution. To address these issues, electrostatic and magnetic probes are deployed to measure the edge characteristics. A pair of scanning Langmuir probes measure edge $n_e $, $T_e $, and $p_e $ in ohmic discharges. For an ohmically driven plasma with $I_p =125$ kA, $B_T =0.15$ T, time-average profiles indicate $n_e =9$ eV, $T_e =$ 7x10$^{18}$ m$^{-3}$ at the radial location of the plasma limiter. A new 3-axis magnetic probe array is under consideration to measure ${j_\parallel } \mathord{\left/ {\vphantom {{j_\parallel } B}} \right. \kern-\nulldelimiterspace} B$ in the edge of ohmic and helicity-driven discharges. This $j_\parallel (R,t)$ is of special interest for quantifying helicity-driven Taylor relaxation states.