Ballooning Beta Limits of J* Optimized Small Aspect Ratio Tokamak/Stellarator Hybrid Devices

Low aspect ratio, modular stellarators offer the attractive features of compact steady-state fusion power systems, high volume utilization, axisymmetric diverted regions, and absence of low order resonances near the plasma edge. Recent explorations of confinement in this region of design space have indicated the need for further transport optimization. We have developed a new form of optimization using the adiabatic invariant J* to minimize the deviation of drift orbits from flux surfaces. Our method varies the shape of the outer flux surface within a Levenberg-Marquardt optimization loop and uses the VMEC 3D equilibrium code at each iteration to recalculate the interior magnetic fields. Coupled with this, we also monitor ballooning stability limits and find that a reasonable window of ballooning stability < β><=sssim 6 % can be maintained in conjunction with significant improvements in thermal and fast ion transport. The resulting nearly omnigenous configurations are neither quasi-toroidal nor quasi-helical.