Drift kinetic theory of neoclassical tearing modes in a low collisionality tokamak plasma: magnetic island threshold physics

Abstract A new drift kinetic theory for the plasma response to neoclassical tearing mode (NTM) magnetic perturbation is presented. Small islands of width, w ? a ( a tokamak minor radius) are assumed, retaining limit w ? ? bi ion banana orbit width) include finite width effects. When collisions small, ions/electrons follow streamlines in phase space; passing particles, these lie surfaces that reproduce island structure but have radial shift by an amount, proportional $\rho_{\vartheta i/e}$?> ? ? i / e , where ion/electron poloidal Larmor radius. This associated with curvature and ? B drifts found be opposite directions $V_\parallel \lessgtr 0$?> V ? ? 0 $V_\parallel$?> component velocity parallel field. The particle distribution function then flattened across shifted or rather than island. results pressure gradient being sustained $w \sim \rho_{\vartheta i}$?> ? hence reduces drive NTMs when small. provides physics basis NTM threshold, which quantified. In Imada et al (2019 Nucl. Fusion 59 046016, references therein), 4D non-linear code has been applied describe modes. present paper, formalism employed. Valid at low collisionality, it allows dimensionality reduction 3D problem, simplifying numerical task efficiently resolving collisional boundary layer trapped-passing boundary. An improved model adopted frequency. decreases compared shown making quantitative agreement experimental observations, $w_c = 0.45 c = 0.45 c threshold half-width, 2.85 full predicted our equilibrium.