Design of modular coils for a quasi-axisymmetric stellarator with a flexible control of the magnetic field configuration

A design of the modular coil system for CHS-qa, of which aspect ration is 3.2, has been made. The magnetic field strength and the major radius are 1.5 T and 1.5 m, respectively. The normal component of magnetic field produced by the modular coils is minimized on the plasma boundary to obtain the optimum coil design. In addition, the mirror component of magnetic field is evaluated to reproduce the configuration more accurately. Engineering constraints are imposed on the distance between adjacent modular coils and the radius of coil curvature. The dependence of the residual normal component of the field on these conditions is examined, and the realistic values for them are selected. Additional coils to control various properties of the magnetic field configuration, such as the rotational transform, the magnetic well depth, have been designed and flexibility of the magnetic field configuration is realized.