Measurement of the ion flux onto a target plate in the presence of a strong induced radial electric field in Tokamak de Varennes

During negative unipolar divertor plate biasing in TdeV, a radial electric field is expected to be established across the transition region between grounded and biased magnetic flux surfaces, Measurements by Langmuir probes that are flush-mounted into one of the outboard neutralizer tiles reveal a layer of high poloidal current density, and a strong radial gradient of floating potential localized on those same magnetic flux surfaces. Spectroscopic measurements of D-alpha and D-beta emissivities made by a tangentially viewing TV camera show a forked strike point in the outboard divertor, the inner prong of which corresponds to the usual flux maximum near the magnetic separatrix, and the outer prong of which corresponds to the immobile bias-induced current layer measured by the probes. Infrared camera images of the tile show the same double pronged structure manifested as twin strips of power deposition. A large fraction of the injected biasing power is contained in this thin "death ray".