Transport Optimization in Stellarators

Stellarators have much in common with tokamaks, and some attractive features relative to them -- disruption-free performance, and no requirement for current drive to produce a rotational transform. However, a major drawback has been elevated transport levels due to their non-axisymmetry. Since the early 1980s, stellarator research has addressed this deficiency, developing a range of approaches for mitigating transport, both neoclassical and, more recently, also anomalous. Several of these are now being implemented in a new generation of experiments in the US and abroad. This talk will present the fundamental physics of transport reduction:\newline (1) Basic stellarator neoclassical theory, including\par $\quad$(a) the various transport ``branches'' contributing in a 3D toroidal system,\par $\quad$(b) multiple roots of the ambipolarity constraint,\par $\quad$(c) constraints on plasma flows, and\par $\quad$(d) confinement of energetic versus thermal particles. \newline \newline (2) The various transport reduction concepts, including \par $\quad$(a) quasi-symmetric,\par $\quad$(b) quasi-omnigenous/quasi-isodynamic,\par $\quad$(c) use of the multiple ambipolarity roots. \newline \newline (3) The reduction of transport in the presence of microturbulence.\\ \newline Work supported by U.S.Department of Energy Contract No.DE-AC02- 76-CHO3073.