Comparing Linear Microinstability of the National Compact Stellarator Expriment and a Shaped Tokamak
Authors: J.A. Baumgaertel, G.W. Hammett and D.R. Mikkelsen
Abstract:
One metric for comparing con nement properties of di erent magnetic fusion energy con gurations is the
linear critical gradient of drift wave modes. The critical gradient scale length determines the ratio of the core
to pedestal temperature when a plasma is limited to marginal stability in the plasma core. The gyrokinetic
turbulence code GS2 was used to calculate critical temperature gradients for the linear, collisionless ion tem-
perature gradient (ITG) mode in the National Compact Stellarator Experiment (NCSX) and a prototypical
shaped tokamak, based on the pro les of a JET H-mode shot and the stronger shaping of ARIES-AT. While a
concern was that the narrow cross section of NCSX at some toroidal locations would result in steep gradients
that drive instabilities more easily, it is found that other stabilizing e ects of the stellarator con guration
o set this so that the normalized critical gradients for NCSX are competitive with or even better than for
the tokamak. For the adiabatic ITG mode, NCSX and the tokamak had similar critical gradients, though
beyond marginal stability, NCSX had larger growth rates. However, for the kinetic ITG mode, NCSX had a
higher critical gradient and lower growth rates until a/LT ≈ 1:5 a/LT;crit, when it surpassed the tokamak's.
A discussion of the results presented with respect to a/LT vs R/LT is included.
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Submitted to: Physics of Plasmas (October 2012)
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Download PPPL-4832 (pdf 275 KB 9 pp)
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