Resistive X-point modes in tokamak boundary plasmas

It is shown that the boundary (edge and scrape-off-layer) plasma in a typical low (L) mode diverted tokamak discharge is unstable to a new class of modes called resistive X-point (RX) modes. The RX mode is a type of resistive ballooning mode that exploits a synergism between resistivity and the magnetic geometry of the X-point region. The RX modes are shown to give robust instabilities at moderate mode numbers, and therefore are expected to be the dominant contributors to turbulent diffusion in the boundary plasma of a diverted tokamak.