A quasi-isodynamic magnetic field generated by helical coils

Abstract

This paper presents a set of continuous helical coils that generate a magnetic field showing characteristics of quasi-isodynamic magnetic configuration. The magnetic field generated by the coil system is similar to the quasi-isodynamic configurations of advanced stellarators with regard to the shape pattern of the flux surface and the field strength in Boozer coordinates. The ratio between the mirror, helicity and toroidicity components of field strength of the found configuration is quantitatively similar to that of the Wendelstein 7-X quasi-isodynamic stellarator, although the relative amplitudes of these modes to uniform field components are larger. The contours of longitudinal adiabatic invariant, , are well closed poloidally and aligned to the flux surface for some of the trapped orbits, indicating the realization of approximate omnigeneity. The major advantages of this configuration are the absence of short-wavelength toroidal coil ripples and the existence of multiple divertor legs whose footprints on the vacuum wall are well localized.