Tearing mode stability studies near ideal stability boundaries in DIII-D

Abstract

For high β, highly shaped plasmas in the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)], the value of the tearing stability index Δ′ calculated at a rational surface can be especially sensitive to the pressure and current profiles. Near marginal stability for a global ideal mode, a pole in Δ′ exists in equilibrium parameter space, as predicted by analytic theory. The proximity of an equilibrium reconstruction to this pole in parameter space strongly decreases the accuracy of the Δ′ calculations. Tearing stability calculations on kinetic equilibrium reconstructions of a series of times in three DIII-D discharges are presented, which indicate that the tearing modes in these discharges are classically unstable at the time of onset. The onset mechanism of two of these discharges (which are in H-mode) is related to the approach of ideal stability boundaries and the occurrence of poles in Δ′. Several ideal modes (sawteeth, edge localized modes, and resistive wall modes) are thought to seed neoclassical tearing modes (NTMs) through forced reconnection, after the ideal mode is unstable. However, tearing modes often appear suddenly and grow quickly without an obvious ideal mode causing a seed island through forced reconnection, which could be explained by this mechanism. This is proposed as an alternative mechanism for the onset of NTMs in tokamaks, which is not incompatible with forced reconnection.

Beta-limiting MHD Instabilities in Improved-performance NSTX Spherical Torus Plasmas

“…However, the role of neoclassical drive in the mode growth is not yet clear, as many such discharges are near the beta limit for internal pressure driven kink modes. In particular, the classical tearing drive can be naturally strong due to positive tearing index ∆ [30] computed near ideal limits. As seen in Figure 11, it is possible for the mode amplitude to decrease with increasing β suggesting that neoclassical tearing drive may not be the dominant term determining mode growth.…”

The impact of 3D fields on tearing mode stability of H-modes

“…β N may influence this process in two ways: increasing β N raises the plasma ideal response, amplifying the field, and therefore the local field at q = 2, so potentially increasing the tearing response. But increasing β N is also known to lower stability [16], thereby potentially further increasing plasma resistive response in the situation of weak shielding. From the data, it is hard to see whether and which of these specific β related effects are part of the process-thus we explore them further with modelling in the next section.…”

Theory and observations of magnetic islands

“…[19] An important property of this parameter is that it grows without bounds as the plasma approaches ideal marginal stability (as a result of rising β, for example). [16] This is a manifestation of the approach of an equilibrium bifurcation. An ongoing effort to replace PEST-III by a code that accounts for the geometry of the coils and uses more advanced numerical techniques is making steady progress.…”

“…[13,14,15,16,17] Due to the high conductivity of fusion plasmas, however, island widths generally evolve very slowly compared to the Alfvén time. Long simulation times are required for this reason as well as to address the question of the role of changes in the profiles.…”