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, quite common process. The effects on transport, however, are difficult to assess because of the complexity of the phenomenon, which often happens in a dynamic way. In this work we investigate a basic way in which transport may be altered due to the presence of magnetic islands: the modification of the ambipolar radial electric field Er due to the distortion of the flux surfaces and the presence of a separatrix. For the latter, we modify the flux-surface averaged metric coefficients of an ideal magnetic configuration consisting of nested tori. The new radial coordinate excludes the island region. Once the geometry is modified, we also perturb the original Er with a neoclassical scaling taken from Shaing's theory [Phys. Plasmas 9 3470 (2002)]. Figure 1 shows an example application to TJ-II plasmas in a configuration-scan discharge where the 5/3 low-order rational of the rotational transform moves through the plasma minor radius. From the experimental data, the variations of electron temperature with respect to the mean indicate an increment around the resonant location (left), while the model indicates a decrement (right). It is suggested that a more profound modification of transport properties is necessary in order to explain the experimental results.