A new approach to transport improvement in a helical magnetic axis stellarator is proposed. First of all, the proposal is presented for the $L=1\mathrm{}$ system. The effective toroidal curvature term ${\epsilon }_T$, defined as the sum of the usual toroidal curvature and one of the nearest satellite harmonics of the helical field, determines confinement conditions of localized trapped particles. There exists a certain correlation between the smallness of ${\epsilon }_T$ and the omnigeneity. This approach would give rise to the possibility of a stellarator design study in a wider parameter domain than quasisymmetry approaches.

• Received 29 July 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.2638