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