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.