Assessment of Transport in NCSX
Authors: D.R. Mikkelsen, H. Maassberg, M.C. Zarnstorff, C.D. Beidler, W.A. Houlberg, W. Kernbichler, H. Mynick, D.A. Spong, P. Strand, and V. Tribaldos
We explore whether the energy confinement and planned heating in NCSX are sufficient to test MHD stability limits, and whether the configuration is sufficiently quasi-axisymmetric to reduce the neoclassical ripple transport to low levels, thereby allowing tokamak-like transport. A 0-D model with fixed profile shapes is related to global energy confinement scalings for stellarators and tokamaks; neoclassical transport properties are assessed with the DKES, NEO, and NCLASS codes; and a power balance code is used to predict temperature profiles. Reaching the NCSX goal of < beta >=4% at low collisionality will require H_ISS-95=3, which is higher than the best achieved in present stellarators. However, this level of confinement is actually ~10% lower than that predicted by the ITER-97P tokamak L-mode scaling. By operating near the stellarator density limit, the required H_ISS-95 is reduced by 35%. The high degree of quasiaxisymmetry of the configuration and the self-consistent 'ambipolar' electric field reduce the neoclassical ripple transport to a small fraction of the neoclassical axisymmetric transport. A combination of neoclassical and anomalous transport models produces pressure profile shapes that are within the range of those used to study the MHD stability of NCSX. We find that < beta >=4% plasmas are 'neoclassically accessible', and are compatible with large levels of anomalous transport in the plasma periphery.
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*NOTICE: PPPL-4205 is a preprint version of work that has been published in Fusion Science and Technology. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in:
Fusion Science and Technology 51:2 (February 2007) 166–180
Download PPPL-4205 Preprint December 2006 (pdf 1.1 MB)