Electromagnetic interchange instability induced by sheath resistivity in the SOL

A linear analysis of MHD instabilities driven by the combined effects of unfavourable curvature and the pressure gradient is performed in the scrape-off layer of a limiter configuration. The dispersion relation of fast growing (electromagnetic) modes is derived by means of both analytical and numerical techniques. The growth rate dependence on the sheath resistivity and the magnetic shear is investigated. Perturbations are not completely stabilized by conducting end walls due to finite sheath conductivity. Modes develop into ideal ballooning modes as the plasma beta increases. The comparison of their behaviour to resistive ballooning modes shows that in most cases of interest the plasma stability is determined by the electromagnetic modes. As a consequence, a new dependence of the instability growth rate on the plasma parameters is found, where the key parameter is the density close to the end plates. Results given here can be applied to a single-null divertor configuration. (Author)