The causes and properties of resistive MHD modes and unstable spectra in advanced tokamak regimes

Advanced tokamak regimes, with extended regions of low magnetic shear, are promising candidates for future fusion reactors (record breaking DT pulses at JET in hybrid mode) but are also more prone to specific MHD instabilities. The proximity to a rational surface in a very low shear region weakens field line bending stabilisation and amplifies the effects of toroidal coupling between modes, leading to the emergence of long-wavelength ideal or resistive infernal modes. These can grow collectively as a discrete spectrum, forming a cascade of different perturbations for single mode numbers (m, n). Subdominant modes with lower growth rates have distinct radial structures, oscillating more (they can be characterized by Bessel functions). These spectra of fast-growing modes are significant for stable scenario design, and for the understanding of global reconnection events like sawteeth, motivating a deeper investigation into their fundamental physics.

Deriving new analytic solutions and extending a modular resistive MHD solver¹, we investigate how resistivity, compressibility, toroidal effects, and shaping influence stability in both monotonic and reversed shear q profiles. In particular, we generalise the ideal interchange dispersion relation to non-monotonic q profiles, and derive modified Mercier criteria for stability in advanced regimes. We also find that the combination of negative triangularity and resistivity can trigger an internal kink spectrum with substantial growth rates².