Confinement properties of small aspect ratio tokamak/stellarator hybrid devices

We have examined the confinement properties for a range of small aspect ratio tokamak/stellarator hybrid devices. Vacuum rotational transform is produced in these configurations using relatively simple tilted/modulated toroidal magnetic field coils, which can then be supplemented with driven plasma current. The iota profile is tokamak-like with a maximum in the center and decreasing towards the plasma edge. Evaluation of confinement characteristics is carried out on several levels including B_min, B_max, contours, longitudinal adiabatic invariant J* contours, guiding-center trajectories, and Monte Carlo simulations starting with a random ensemble of particles. We find that confinement generally improves with increasing driven current, lower ripple, increasing magnetic field, and increasing electric field. Another avenue for confinement improvement is through optimization of the magnetic geometry. The tools mentioned above for evaluating confinement suggest several simple criteria as targets for the magnetic optimizations.