The VH-mode at JET

The nature and behaviour of the VH-mode, as observed in the JET tokamak, have been investigated; this is a regime of enhanced energy confinement which is often attained during the H-mode phase, following the disappearance of the ELMs. As a result of the high pressure gradients and the high current densities, which develop in an extended region near the separatrix during the VH-phase, a large fraction of the total plasma volume becomes unconditionally stable against ideal and resistive ballooning modes. This effect has been identified as being relevant to the attainment of the VH-mode, but a causal link has not been established. Furthermore, the processes which are responsible for the termination of the VH-mode are not understood.