In order to increase the reactor advantages of the reversed field pinch (RFP) concept, the possibility of limiting the plasma region by means of a magnetic separatrix has been discussed. However, in experiments on this type of configuration a large pressure gradient, and an associated diamagnetic current, is observed in the separatrix region. The present paper investigates the effect of such a pressure gradient on the MHD stability of the plasma. It is shown both analytically and numerically that, due to the low value of Bz near the plasma boundary, such a system is susceptible to a short-wavelength, edge-localized kink instability, driven by the pressure gradient. These instabilities are akin to the edge-localized modes encountered in high-beta, H-mode tokamak plasmas. Furthermore, while the long-wavelength branch of the m=1 mode is effectively stabilized by a close-fitting conducting wall, the edge-localized kink mode is found to be more or less unaffected by the wall, and persists even when the wall distance shrinks to zero.
An explanation for hard MHD stability limits in low-q95 diverted tokamak plasmas