Abstract
Accurate equilibrium reconstruction and detailed stability analysis of a strongly shaped, double-null, βT=11% discharge shows that the plasma core is in the second stable regime to ideal ballooning modes. The equilibrium reconstruction using all the available data (coil currents, poloidal magnetic loops, motional Stark effect data, the kinetic pressure profile, the magnetic axis location, and the location of the two q=1 surfaces) shows a region of negative magnetic shear near the magnetic axis, an outer positive shear region, and a low shear region connecting the two. The inner negative shear region allows a large positive shear region near the boundary, even at low q (q95=2.6), permitting a large outer region pressure gradient to be first regime stable. The inner region is in the second stable regime, consistent with the observed axial beta [βT(0)=44%]. In the low shear region p′ vanishes, consistent with Mercier stability. This is one way to extend the ballooning limit in shaped plasmas while maintaining stability against external kinks. The n=1 analysis shows that the plasma is unstable to an ideal internal mode, consistent with the experimental observations of a saturated internal m/n=1/1 mode. The core plasma pressure, not being limited by ballooning stability, appears to be reaching a local equilibrium limit at the magnetic axis.
Highlights
Improved axisymmetric control on DIII-Di has allowed operation near the axisymmetric stability limit, enabling an increase in the achieved normalized current, IN, at moderate q and leading to a record level volume average toroidal beta, fir, of 11%
From analysis of the plasma equilibria we infer the magnetic shear profile and the plasma pressure profile. These profiles indicate a region of negative magnetic shear and high pressure near the magnetic axis, which is in the second stable regime to ideal ballooning modes, as is expected under these circumstances.’
The constraints on this equilibrium are quite severe; the plasma pressure is at the ballooning limit and the convergence of the kink and axisymmetric stability limits excludes the possibility of any significant change in the current profile, in that a narrowing will destabilize axisymmetric modes, while a broadening, or an increase in fl, will destabilize the external kink.”
Summary
Improved axisymmetric control on DIII-Di has allowed operation near the axisymmetric stability limit, enabling an increase in the achieved normalized current, IN, at moderate q and leading to a record level volume average toroidal beta, fir, of 11%. We have achieved beta values in excess of 9% in the full-radius DIII-D double-null divertor configuration’ (DND), where K- 2.1 with triangularity S-0.9 In this configuration, at low B the plasma is very near the marginal point for axisymmetric stability, at high beta (p-8%) the current profile (characterized by the internal inductance) becomes quite broad (l,-0.8), and the plasma is no longer near the axisymmetric limit. From analysis of the plasma equilibria we infer the magnetic shear profile and the plasma pressure profile These profiles indicate a region of negative magnetic shear and high pressure near the magnetic axis, which is in the second stable regime to ideal ballooning modes, as is expected under these circumstances.’.
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