The effect of plasma beta on high-n ballooning stability at low magnetic shear

An explanation of the observed improvement in H-mode pedestal characteristics with increasing core plasma pressure or poloidal beta, βpol, as observed in MAST and JET, is sought in terms of the impact of the Shafranov shift, Δ′, on ideal ballooning MHD stability. To illustrate this succinctly, a self-consistent treatment of the low magnetic shear region of the ‘s−α’ stability diagram is presented using the large aspect ratio Shafranov equilibrium, but enhancing both α and Δ′ so that they compete with each other. The method of averaging, valid at low s, is used to simplify the calculation and demonstrates how α, Δ′, plasma shaping and ‘average favourable curvature’ all contribute to stability.