Ballooning mode calculations in stellarators

A magnetohydrodynamic (MHD) ballooning mode formalism and calculation is developed to show how a field‐line‐following code can be used to study MHD stability. The asymptotic analysis of the ballooning equation yields the Mercier condition. It is shown that first‐order equilibrium effects on the vacuum fields from finite pressure cancel the intrinsically destabilizing term of the Mercier condition. A ballooning unstable solution is found in a Heliac configuration that has a magnetic well at zero beta and a rotational transform that increases radially outward.