Omnigenity and quasihelicity in helical plasma confinement systems

Abstract

Omnigenous stellarators, those with bounce averaged drifts lying within the magnetic surfaces, form a much larger class than quasihelical stellarators (i.e., with magnetic-field strength depending on only a single linear combination of the toroidal flux angles) while nevertheless having very good transport properties. It is shown by construction that exactly omnigenous magnetic strengths can be very far from quasihelical, but that all such magnetic strengths are nonanalytic. However, by truncation of the Fourier representation one can obtain analytic magnetic strengths that are very close to being omnigenous while still very far from quasihelical. This indicates that the condition of quasihelicity can be significantly relaxed and yet retain good neoclassical transport. Finally, it is conjectured that reasonably good transport can be obtained by requiring omnigenity for only the deeply trapped and marginally trapped particles. Such fields will have no transition particles and, hence, no separatrix crossing chaos. These conditions reduce to the requirements that the field line maxima and minima of the magnetic field and the separatrix action be constant on a magnetic surface.