Macroscopic stability of high β MAST plasmas

Abstract

Abstract. The high-beta capability of the spherical tokamak, coupled with a suite of world-leading diagnostics on MAST, has facilitated significant improvements in the understanding of performance-limiting core instabilities in high performance plasmas. For instance, the newly installed Motional Stark Effect (MSE) diagnostic, with radial resolution < 25mm, has enabled detailed study of saturated long-lived modes in hybrid scenarios. Similarly, the upgraded Thomson Scattering (TS) system, with radial resolution < 10mm and the possibility of temporal resolution of 1µs, has allowed detailed analysis of the density and temperature profiles during transient activity in the plasma, such as at a sawtooth crash. High resolution Charge Exchange Recombination Spectroscopy (CXRS) provided measurement of rotation braking induced by both applied magnetic fields and by magnetohydrodynamic (MHD) instabilities, allowing tests of neoclassical toroidal viscosity theory predictions. Finally, MAST is also equipped with internal and external coils that allow non-axisymmetric fields to be applied for active MHD spectroscopy of instabilities near the no-wall beta limit. Such resonant field amplification measurements suggest that MAST has been able to operate above the no-wall limit. In order to access such high pressures, the resistive wall mode must be damped, and so numerical modelling has focussed on assessing the kinetic damping of the mode and its nonlinear interaction with other instabilities. The enhanced understanding of the physical mechanisms driving deleterious MHD activity given by these leading-edge capabilities has provided guidance to optimise operating scenarios for improved plasma performance. Macroscopic Stability of High β MAST plasmas2 Figure 1. Virtuous cycle when high β N plasmas are accessed; giving rise to enhanced bootstrap fraction, allowing lower internal inductance and increased shaping which further enhances the non-inductively driven fraction, allowing the safety factor on axis to be raised, in turn permitting higher pressure plasmas.

Disruptions, Disruptivity, and Safer Operating Windows in the High-β Spherical Torus NSTX

“…At these low q min values, it appears that the ideal MHD onset mechanisms for the m/n = 1/1 modes discussed in Refs. [160,163,164] are likely at play. For the few triggerless cases at higher q min it is possible that there is some trigger mechanism not observable in the data.…”

Exploration of the Equilibrium Operating Space For NSTX-Upgrade

“…Rather, reducing the density at fixed H tends to rapidly lower the central safety factor, as the central neutral beam current drive drives down q min . As noted above, maintaining q min >1 is critical for the avoidance of n=1 kink and coupled core/kink tearing modes, as documented in Refs [34,41,42,44,[106][107][108][109]. Hence, this trend in q min provides a low-density limit for scenarios with fully relaxed current profiles.…”

“…The change in safety factor is less numerically dramatic, but perhaps more significant, in the B T =0.55 T case, where q min >1 is only maintained for the largest outer gap. The maintenance of q min >1 is critical for the ST, so as to avoid the onset of non-resonant m/n=1/1 kink modes, often coupled to 2/1 islands [34,41,42,44,[106][107][108][109].…”

“…As discussed above, an additional requirement is that the fully evolved current profile yield q min >1 [34,41,43,44,[107][108][109]. For the present device, this condition implies that the density of the centrally-peaked Ohmic current not become too large, and that the NBCD be configured to drive current off-axis.…”

Study of energetic particle physics with advanced ECEI system on the HL-2A tokamak

“…The SKs have been observed and investigated on MAST [12], NSTX [13] and HL-2A [14,15]. When these modes appear, there are strong damping of the toroidal rotation, degradation of stored energy and significant losses of fast particles.…”