Local stability in heliac type stellarators

A study of the stability limits for ideal ballooning modes in helical axis stellarators (heliac type stellarators) has been performed. Specific configurations similar to those of the TJ-II heliac have been taken, having a deep magnetic well and almost zero shear, which is characteristic for a heliac. The stability limits are those imposed by the destabilization of ideal ballooning and local interchange modes. The number of periods is varied in order to make a progressive approximation to helical symmetry as the aspect ratio increases. The very high stability limits for straight heliacs are recovered as the aspect ratio tends to infinity.