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Recent optimized stellarator designs

Author: Donald A. Spong
Requested Type: Poster Only
Submitted: 2012-12-07 08:33:19

Co-authors: J. H. Harris, W. T. Reiersen, A. Clark, A. Nielsen, F. Volpe, A. Ware

Contact Info:
Oak Ridge National Laboratory
1 Bethel Valley Road
Oak Ridge, TN   37831-6
U. S. A.

Abstract Text:
Stellarators offer a number of attractive features such as reduced current drive requirements, lowered risk of damage from current-driven instabilities (disruptions, ELMs) and greater flexibility for physics optimization that motivate their continued development. Since finding globally optimized states in high-dimensional parameter spaces remains difficult, optimization efforts are ongoing. Two new types of stellarators have recently been studied: (a) the tilted planar coil torus and (b) the high aspect ratio quasi-poloidal stellarator.
Tilted circular/planar coil systems are one of several simple 3D design evolutionary progressions away from the tokamak and have been analyzed over a range of field periods (2 - 18) and aspect ratios. The coil locations and tilt angles are chosen so that the inner legs of the coils are interlinked (allowing lower aspect ratio) and align with a cylindrical center stack. The coil centers follow a planar toroidal path rather than the helical path that would characterize a heliac. A set of vertical field coils and finite levels of plasma current are generally required for flux surface formation, but the plasma current drive requirements are reduced from those of conventional non-tilted coil tokamaks. These systems also have lowered levels of TF ripple from those of conventional tokamaks featuring the same number of coils. A small exploratory 6-field period device of this kind is under construction at Columbia University.
Quasi-poloidal (QP) configurations offer strong bootstrap current suppression, reduced levels of non-symmetric ripple, poloidal flow shearing and access to high ballooning stability limits. The latter two effects arise from the reduced redirection of diamagnetic flows and currents (by incompressibility/Pfirsch-Schluter effect) into the toroidal direction in comparison to other forms of symmetry. Such effects allow stronger cross-field flows and currents to persist in QP systems; this facilitates turbulence suppression by flow shearing and diamagnetic well-digging effects that improve ballooning stability. The suppression of toroidal currents also reduces the drive for kinks and tearing instabilities. Recent QP designs at higher aspect ratios (6 - 20) have resulted in improved levels of QP symmetry and lead to potentially simplified coil geometries and regions of the torus that have nearly cylindrical geometry. Such higher aspect ratio configurations can ease blanket design and offer a pathway for high density, high beta stellarator reactors with more tolerable wall heat fluxes than would be possible in more compact systems.

Characterization: 1.2,1.4

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