Sheared plasma flow generation - a new measure for stellarator optimization

Recent stellarator optimization efforts have primarily targeted transport measures such as quasi-symmetry, effective ripple and alignment of particle guiding center orbits with flux surfaces. For the three forms of quasi-symmetry (helical/toroidal/poloidal), as well as for a variety of nearly-omnigenous systems, this has led to significant reductions in neoclassical losses so that, at least for near-term experiments, the neoclassical transport of particles and energy can be made insignificant compared to anomalous transport. However, momentum transport properties provide an additional dimension for characterizing optimized stellarators. The momentum and flow damping features of optimized stellarators can vary widely, depending on their magnetic structure, ranging from systems with near tokamak-like properties where toroidal flows dominate to those in which poloidal flows dominate and toroidal flows are suppressed. We have developed a set of tools for self-consistently evaluating the flow characteristics of different types of stellarators based on using a moments method coupled with the DKES model; applications to a variety of stellarators will be discussed. The understanding of momentum transport in three-dimensional systems is of importance due to its relevance to transport barrier generation, enhanced confinement regime access, impurity transport, bootstrap current prediction, and magnetic island suppression. Although it is too early to define what constitutes the optimal form of momentum transport characteristics, the wide variation of possibilities in present and planned stellarator experiments provides an attractively diverse environment both for answering this question and developing improved scientific understanding. Comparisons across devices can aid in unfolding the interplay between anomalous and neoclassical damping effects as well as the impact of momentum transport properties on related plasma phenomena. (author)