Presheath profiles in simulated tokamak edge plasmas

Abstract

Steady state magnetized plasmas produced by the PISCES experiment are used to study plasma-wall interaction phenomena relevant to confinement devices such as tokamaks. An experimental investigation of the presheath region that extends from a wall surface into “simulated tokamak” edge plasmas along magnetic field lines is reported. Diagnostics especially developed for this work include a fast-scanning multiple Langmuir/Emissive/Mach probe system and a CID camera imaging system. Measurements of density, electron temperature, floating potential, space potential, and bulk plasma flow velocities have been obtained in plasmas with densities ranging from 10 12 to 10 13 cm −3, electron temperatures from 5 to 15 eV. and axial magnetic fields of 0.2 to 1.4 kG. Plasma density profiles along the magnetic field typically show a characteristic factor of 2 decrease towards the wall surface. A plasma potential variation in the near presheath zone of order 0.5 T e is measured, consistent with the bulk plasma flow approaching the ion sound speed near the wall surface, as inferred from a simple “free fall” model. A Boltzmann model for the presheath density profile accuracy tracks the density profile measured both by the Mach probe and by spectroscopic means. Flow profiles are used as a consistency check on various magnetized Mach probe theories. Results suggest that cross-field transport of parallel momentum through viscosity is relatively unimportant in PISCES plasmas and thus may be unimportant in tokamak boundary layer plasmas. Discharges with non-thermal electrons display axial profiles of space potential and floating potential which indicate a “hotter” electron distribution function near the wall surface, consistent with “colder” electrons being reflected by the presheath potential drop.