Optimization of the Magnetic Gradient Scale Length Improves Coil Plasma Distances in Stellarators

The distance between the last closed flux surface and the nearest electromagnetic coils is a dominating factor in the cost and construction of stellarators. The magnetic gradient scale length has been shown to be a good proxy for minimum plasma-coil separation in winding surface coil optimizations such as REGCOIL. However it has not yet been shown as comprehensively that the same is true for filament coils, or that the magnetic gradient scale length is an effective objective function in optimization. In this talk, we explore examples in which L∇B is correlated with the minimum plasma-coil separation for filament coils. In the first example, we compare L∇B to the minimum plasma coil separation with filament coils optimized using a continuation method for a randomized set of finite beta equilibria. In the second example, we optimize a family of NFP 4 QH equilibria with improved minimum L∇B, and also optimize coils via a continuation method. For all datasets, when using these methods, we find that improving L∇B generally improves both the minimum plasma-coil separation and minimum coil-coil distance at sufficient total coil length.