Toroidal field effects on the stability of a Heliotron configuration

The addition of a small toroidal field to the Heliotron E configuration [Plasma Phys. 13, 243 (1981)] improves the stability of the n -- 1 mode and increases the value of the stability beta critical. Total stabilization of this mode can be achieved with added toroidal fields between 5% and 15% of the total field. In this situation, the plasma can have direct access to the second stability regime. For the Heliotron E configuration, the self-stabilization effect is due to the shear, not to the magnetic well. The toroidal field threshold value for stability depends strongly on the pressure profile and the aspect ratio of the plasma