Experimental Observation of Neoclassical Currents

The parallel and perpendicular equilibrium currents have been experimentally studied in the Levitated Octupole, with toroidal field. Observations of the spatial structure, collisionality and toroidal field dependence of the complete current and the ion portion of the current are presented. Experimentally measured currents are compared to neoclassical current distributions calculated for the actual machine geometry. Measurements of the total plasma parallel current for absolute-MHD-stable flux surfaces (inside the separatrix) agree well with theory, showing the bootstrap and Pfirsch-Schluter currents as predicted by neoclassical transport theory. On average -MHD-stable flux surfaces (outside the separatrix) there is more parallel current than predicted by theory, though the spatial structure is correct. Separate measurements of the ion parallel and perpendicular currents agree with theory, showing the lack of measurable ion poloidal rotation in all situations.