Theoretical prediction of bootstrap current in the large helical device with unbalanced helical coil currents

The Large Helical Device (LHD), which is a heliotron/torsatron device with two helical coils, is designed so that the current in each helical coil can be controlled independently. Unbalancing these currents leads to spatial axis configurations. The bootstrap current is found to be strongly affected by any imbalance between these currents. When the ratio of the currents in the two helical coils is small enough, a bootstrap current flows in a direction so as to decrease the rotational transform because of the enhancement of the bumpiness component of the magnetic field as well as of the spatial axis component. This leads to improved stability. (author). 17 refs, 9 figs