Stability properties of stellarators with a net toroidal current

The influence of the toroidal current profile on the stability has been shown by previous theoretical and experimental works. A question that arises is the non uniform behavior when different magnetic configurations are compared. In the Wendelstein VII-AS, the bootstrap current always increases the rotational transform and the stability properties are worse, the confinement time is shorter than in a current free regime. Theoretical calculations of external global modes in the ATF device, show that if the current reduces the externally applied rotational transform, the stability properties are deteriorated, and vice versa. Mercier stability calculations for the LHD show a contrary effect of the current; the decreasing of the rotational transform improves the stability while increasing it, deteriorates the stability properties. Previous work on the TJ-II heliac has shown that a shearless configuration is stabilized by locally introducing a shear, and this effect is stronger when the rotational transform decreases. We are trying to elucidate these discrepancies by taking advantage of the flexibility of the TJ-II. We consider a sequence of equilibria with a given net toroidal current and then we study its stability properties. Bootstrap currents are very low in the TJ-II (preliminary calculations showed it is of some kA). So, this current could be attainable by EC Current Drive for instance. (author) 11 refs., 4 figs