Published online by Cambridge University Press: 20 June 2018
Kinetic ballooning modes in magnetically confined toroidal plasmas are investigated putting emphasis on specific stellarator features. In particular, we propose a Mercier criterion which is purposely designed to allow for direct comparison with local flux-tube gyrokinetics simulations. We investigate the influence on the marginal frequency of the mode of a magnetic curvature which is inhomogeneous on the magnetic flux surface due to the fieldline-label dependence. This is a typical (surface) global effect present in non-axisymmetry. Finally, we propose an artificial equilibrium model that explicitly retains the fieldline-label dependence in the magnetic drift, and analyse the stability of the system by introducing a representation of the perturbations similar to the flux-bundle model of Sugama et al. (Plasma Fusion Res., vol. 7, 2012, 2403094). The coupling of flux bundles is shown to have a stabilising effect on the most unstable local flux-tube mode.
 $\unicode[STIX]{x1D6FD}$
                   on drift-wave stability in the national compact stellarator experiment. Phys. Plasmas 
               19 (12), 122306.Google Scholar
$\unicode[STIX]{x1D6FD}$
                   on drift-wave stability in the national compact stellarator experiment. Phys. Plasmas 
               19 (12), 122306.Google Scholar $n$
                   modes and Mercier modes in stellarators. Phys. Fluids B 
               4 (6), 1401–1411.Google Scholar
$n$
                   modes and Mercier modes in stellarators. Phys. Fluids B 
               4 (6), 1401–1411.Google Scholar $V^{\prime \prime }$
                   criterion. Plasma Phys. 
               9 (5), 611.CrossRefGoogle Scholar
$V^{\prime \prime }$
                   criterion. Plasma Phys. 
               9 (5), 611.CrossRefGoogle Scholar