PPPL-5382
High-β equilibrium and ballooning stability of the low aspect
ratio CNT stellarator
Author: S. A. Lazerson
Abstract: The existence and ballooning-stability of low
aspect ratio stellarator equilibria is predicted for the Columbia
Neutral Torus (CNT) with the aid of 3D numerical tools. In addition
to having a low aspect ratio, CNT is characterized by a low magnetic
field and small plasma volume. Also, highly overdense plasmas were
recently heated in CNT by means of microwaves. These characteristics
suggest that CNT might attain relatively high values of plasma beta
and thus be of use in the experimental study of stellarator
stability to high-beta instabilities such as ballooning modes. As a
first step in that direction, here the ballooning stability limit is
found numerically. Depending on the particular magnetic configuration
we expect volume-averaged β limits in the range 0.9-3.0%, and
possibly higher, and observe indications of a second region of
ballooning stability. As the aspect ratio is reduced, stability is
found to increase in some configurations and decrease in others.
Energy-balance estimates using stellarator scaling laws indicate
that the lower β limit may be attainable with overdense heating at
powers of of 40 to 100 kW. The present study serves the additional
purpose of testing VMEC and other stellarator codes at high values
of β and at low aspect ratios. For this reason, the study was
carried out both for free boundary, for maximum fidelity to
experiment, as well as with a fixed boundary, as a numerical test.
Submitted to: Physics of Plasmas
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Download PPPL-53821.9 MB (11 pp)
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