Magnetic resonances and electric fields in the TJ-II Heliac

TJ-II is a medium-sized Heliac-type stellarator operating at low magnetic shear. Low-order rational values of the rotational transform ι-bar = 1/q (magnetic resonances, for brevity) can be introduced anywhere in its plasmas causing modifications in the electric potential and, consequently, radial structures in the radial electric fields that can be used to alter transport and stability in an externally controllable way. The ability of the Heliac to perform dynamic configuration scans has been used to illustrate these aspects and find practical realizations, such as exerting control on the L–H transition.