Published by Fusion Energy Division , Oak Ridge National Laboratory

All opinions expressed herein are those of the authors and should not be reproduced, quoted in publications, transmitted, or used as a reference without the author's consent. The problem of cross-field plasma transport in the presence of stochastic magnetic fields has received considerable attention in the literature because of its importance in providing a potential explanation of the anomalous electron thermal conductivity χ e in magnetic confinement devices such as tokamaks and stellarators. In terms of the magnetic fluctuation spectrum, which is assumed to be known, analytic formulas have been given for χ e in essentially all parameter regimes of possible physical relevance [1]. However, confirmation of the theoretical models has not been done experimentally. On the Compact Auburn Torsatron (CAT), a " stellarator diode " system [2] was adopted as a tool to provide a steady-state electron source. A set of helical coils was installed inside the CAT vacuum vessel to create resonant magnetic perturbation fields. The main components of the perturbation magnetic field spectrum are n/m = 1/3 and n/m = 1/4 modes, which will create magnetic islands on the rational surfaces of rotational transform at ι =1/3 and 1/4. The vacuum magnetic field configuration was chosen so that the average minor radius for the n/m = 1/4 surface is at about 5 cm, and the average minor radius of the n/m = 1/3 surface is at about 8 cm. With sufficient externally applied resonant fields, the islands will overlap and create stochastic regions in the magnetic fields which enable study of the stochastic transport of the electrons. The subsequent motion of the electrons leaving the space charge sheath near the hot filament is diffusive along and across the magnetic field, mainly involving collisions of electrons with neutral particles of residual gas. In cylindrical approximation, the electron flux across the magnetic surfaces is given by