Edge density and temperature measurements on the Pegasus toroidal experiment using Langmuir probe arrays

Langmuir probes are being deployed on the Pegasus Toroidal Experiment to measure the edge electron density and temperature in both Ohmic and plasma gun-driven plasmas. Typical edge temperatures in the region of interest are expected to be less than 50 eV, and edge densities are estimated to be on the order of $10^{19}~{\rm m}^{-3}$. Each probe has four 1 mm diameter tungsten tips, extending several millimeters into the plasma, and is protected by a graphite housing. This first setup can be employed as a swept probe or a triple probe. Future efforts will focus on deploying an array of such probes. The first probe is mounted on a sliding shaft 10 cm below the outboard midplane, enabling the probe tips to be positioned several centimeters within the scrape-off layer. The probe provides a high time resolution measurement of the edge plasma parameters, including fluctuations, up to the megahertz scale. In Ohmic discharges, the Langmuir probe measurements will support edge stability studies. In helicity-driven plasmas, the Langmuir probes will be used to characterize the plasma edge in unrelaxed plasma streams, gun-driven edge current sheets, and in relaxed tokamak-like equilibria.