Particle Surfaces for High-Energy Electrons in a Stellarator

Abstract

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.