A two-dimensional magnetohydrodynamic stability model for helicity-injected devices with open flux

Models of the ideal magnetohydrodynamic (MHD) stability of spheromaks and spherical tokamaks are presented, including the effects of current on the open flux which plays a key role in helicity-injected current drive. The stability of spheromak equilibria with both open and closed flux and realistic current profiles representative of helicity-injected state is investigated, where a kink instability in the open flux is shown to dominate a tilt mode in the closed flux as the open flux current density is increased. A previous one-dimensional model is extended to more realistic two-dimensional equilibria which properly incorporate a region of closed magnetic flux as well as open flux penetrating the boundaries at electrodes. A new stability code SCOTS has been developed and benchmarked which can determine the growth rates of ideal MHD modes in this geometry. The coordinate system for this code has been developed such that it extends smoothly across the separatrix between closed and open flux, thus not imposing...