Modular Coil Design for the Ultra-low Aspect Ratio Quasi-axially Symmetric Stellarator MHH2

A family of two field-period quasi-axisymmetric stellarators generally known as MHH2 with aspect ratios of only ∼2.5 was found. These configurations have low field ripples and excellent confinement of α particles. This discovery raises the hope that a compact stellarator reactor may eventually be designed with the property of tokamak transport and stellarator stability. In this paper we demonstrate that smooth modular coils may be designed for this family of configurations that not only yield plasmas with good physics properties but also possess engineering properties desirable for compact power producing reactors. We show designs featuring 16 modular coils with ratios of major radius to minimum coil-plasma separation ∼5.5, major radius to minimum coil-coil separation ∼10 and the maximum field in coil bodies to the field on axis ∼2 for 0.2 m2 conductors. These coils is expected to allow plasmas operated at 5% β with α energy loss < 10% for a reactor of major radius <9 m at 5 T.