Particle Surfaces for High-Energy Electrons in a Stellarator

A simple theory of high-energy (``runaway'') electron surfaces in a stellarator is presented. X-ray intensity measurements qualitatively confirm the prediction of the simple theory, namely that, in a machine whose figure-eight twist is negative (i.e., each U bend rotated counterclockwise as seen from beyond that U bend) runaway surfaces drift toward the center of curvature (inside) of the stellarator loop for accelerating field E parallel to confining field B and toward the outside for E antiparallel to B. The time behavior of x-ray intensity indicates that the actual runaway surfaces do not have the predicted circular cross section but have, superposed on this, perturbations of m-fold azimuthal symmetry at values of plasma current corresponding to the predicted ranges for various modes of the ``kink'' instability.