Ballooning stability optimization of low-aspect-ratio stellarators

The implementation of ideal ballooning stability within an optimization code is used to determine stable, moderate-β compact stellarator configurations. Due to the large computational requirements of existing ballooning codes, such calculations within the optimization process were previously impractical. The recently developed COBRA code can efficiently compute ideal ballooning growth rates on various magnetic surfaces, using the VMEC code to supply equilibrium data. The optimization code has been used to minimize these growth rates, giving rise to new stellarator configurations at low aspect ratios with good ballooning stability properties, which also maintain previously determined desirable physics properties. This particular implementation is robust due to the enhanced convergence features included in COBRA, while incurring only a small overhead on the total computational time.