Ion-temperature-gradient turbulence from finite to weak magnetic shear regime

Abstract

The tokamak, a toroidal magnetic device confining a hot plasma, is one of the most advanced approaches to fusion energy. A central obstacle is turbulent transport driven by the ion-temperature-gradient (ITG) mode. Operation scenarios that rely on a weak or zero magnetic shear core, such as the ITER hybrid scenario, have achieved markedly improved confinement, yet the underlying mechanism has remained unclear. Here we show that the radial width of ITG poloidal harmonics is constrained not only by the familiar parallel Landau damping, but also by the isotropy of micro-turbulence in the plane perpendicular to the magnetic field. In the weak-shear limit, micro-isotropy dominates and constrains the radial width at one poloidal wavelength, yielding a critical magnetic shear $s_{\text{crit}} \approx 1/(2π)$. Above this threshold, the extended Type II ballooning modes prevail; below it, the localized Type I modes emerge, comprising only two or three harmonics. Global gyrokinetic simulations spanning DIII-D, JET~and ITER parameters confirm this geometric criterion. Nonlinear simulations with sustained heating show that weak-shear plasmas spontaneously form internal transport barriers with the turbulence suppressed in the radial region determined by $|s| < s_{\text{crit}}$. This geometric criterion defines the weak-magnetic-shear regime and offers a fresh perspective on turbulent transport in fusion plasmas.