Ballooning stability optimization of low-aspect-ratio stellarators

The implementation of ideal ballooning stability within an optimization cod e is used to determine stable, moderate-beta compact stellarator configurat ions. Due to the large computational requirements of existing ballooning co des, such calculations within the optimization process were previously impr actical. The recently developed COBRA code can efficiently compute ideal ba llooning growth rates on various magnetic surfaces, using the VMEC code to supply equilibrium data. The optimization code has been used to minimize th ese growth rates, giving rise to new stellarator configurations at low aspe ct ratios with good ballooning stability properties, which also maintain pr eviously determined desirable physics properties. This particular implement ation is robust due to the enhanced convergence features included in COBRA, while incurring only a small overhead on the total computational time.