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Plasma Flow Characteristics and Enhanced Confinement Regimes in Stellarators
| Author: | Lyon J.F. |
| Coauthor: | D.A. Spong |
| Institution : | Oak Ridge National Laboratory |
| Abstract text: | Stellarators, due to their three-dimensional magnetic field structure, provide a qualitatively different coupling between plasma flows, momentum sources, radial electric fields, and neoclassical transport than is the case for toroidally-symmetric devices. In tokamaks and STs, sheared radial electric fields can be driven by parallel momentum input, enhanced fast ion loss, or internally generated by mechanisms such as the Reynolds stress. For stellarators, the same mechanisms apply, but in addition neoclassical transport requires a finite electric field to achieve ambipolarity; also static magnetic islands can induce flow shear. The viscous flow damping characteristics of stellarators differ from those of tokamaks due to the different variation of |B| along a field line; the flow damping characteristics depend on the type of quasi-symmetry, i.e., quasi-helical (HSX), quasi-poloidal (QPS), or quasi-toroidal (NCSX). The connection between electric fields and flows depends on the flow damping mechanism. For example, for exact symmetry the radial electric field is (Btoroidal/Bpoloidal)2 higher in a poloidally symmetric system than in a toroidally symmetric system for the same v||.
Recent developments in stellarator neoclassical theory allow improved quantitative evaluations of these effects. Stellarators provide important sources of data on enhanced confinement regimes that augment and extend the physics that can be addressed in tokamaks and STs. On the fundamental level, ion and electron neoclassical transport is modified by E x B drifts affecting guiding center orbits. For anomalous transport, sheared poloidal flows should be more effective than sheared toroidal flows in shredding turbulent eddies. Some unique aspects of enhanced confinement in existing experiments include: the HDH mode in W7-AS, which provides improved confinement and expels impurities; simultaneous formation of ion and electron temperature gradient barriers in CHS; influence of stationary magnetic islands on sheared electric fields and barrier formation (TJ-II) and measurements of the electric field with two HIBPs (CHS).
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