Wall-Actuated Scanning Probe(WASP) For High-Field Side Plasma Measurements On Alcator C-Mod

A new, high-field side scanning probe has been added to Alcator C-Mod's complement of edge diagnostics. The WASP is designed to provide all the benefits of a linear plunge, multi-electrode scanning probe while working from the confined space of the inner tokamak wall. The drive mechanism is an embedded coil which produces a torque with the ambient toroidal magnetic field when energized, thus allowing the probe to plunge to different pre-programmed depths at different times during a plasma discharge. The probe tip is designed for easy replacement and is presently configured to operate as a modified, high heat-flux 'Gundestrup'-type probe with four tungsten electrodes. The probe has demonstrated the ability to obtain cross-field profiles for electron temperature, density , floating potential and plasma flow information (parallel and perpendicular to B) up to a depth of a few millimiters inside the last-closed flux surface in standard C-Mod discharges. The tungsten-tipped probe has proved very robust and shows little or no damage though it routinely handles surface heat fluxes on the order of 100 MW/m 2 at peak insertion.