Some theoretical advances for easing stellarator power plant engineering challenges

Two challenges for stellarator power plant designs are the small coil-to-plasma distance typically required to produce stellarator fields, requiring scale-up of designs to fit a blanket and neutron shielding, and the large forces on high-field strongly shaped magnets. Here we present some recent theoretical advances to help address these challenges. The limit on coil-to-plasma distance can be understood from the scale length of the magnetic field of the target plasma: coils cannot be much farther from the plasma than this scale length. This correspondence is demonstrated by the strong correlation between the scale length and coil-to-plasma distance across a diverse database of plasma configurations, spanning an order of magnitude in these quantities compared to minor radius. To minimize coil forces in design optimization, or to minimize magnetic energy or maximize critical current, these quantities need to be computed efficiently. It does not initially appear possible to compute these quantities within the typical 1D filament model used for coil optimization, due to division by zero where the source and evaluation points coincide. However, here we show how calculation of these quantities within a reduced 1D model is possible, by introducing regularization terms to rigorously match finite-thickness calculations. Excellent agreement between the reduced model and high-fidelity calculations for a stellarator coil are demonstrated.