Effects of island chains on transport through changes in the radial electric field (TJ-II stellarator)

The opening of island chains in magnetically confined fusion plasma is a natural and common process. The effects on transport, however, are difficult to assess because of the complexity of the phenomenon, which often happens dynamically. In this work we investigate one basic way in which transport may be altered due to the presence of magnetic islands: the modification of the ambipolar radial electric field E-r due to the distortion of the flux surfaces and the presence of a separatrix. For the latter, we first adapt the metric coefficients to a new configuration that excludes the island region; then we also perturb the original E-r with a neoclassical scaling taken from Shaing's formulation [Phys. Plasmas 9 3470 (2002)]. Some examples are shown for TJ-II plasmas with dynamic configuration scans, i.e., with some loworder rational of the rotational transform moving through the plasma minor radius during one discharge. It is suggested that a modification of transport properties other than neoclassical is necessary in order to explain the experimental results.