Study of TJ-II configurations with net toroidal current using the PIES code

Introduction The TJ-II stellarator is very well suited for investigating confinement properties in different magnetic configurations, as it covers an ample range of rotational transform profiles. In addition to this, a set of coils designed to induce toroidal plasma currents, Ip, can be used to change the rotational transform profile during a discharge. This has been used in past experiments to study the confinement in TJ-II under positive and negative induced plasma currents [1], using the magnetic configuration called 100_44_64. The main result found in those experiments was that Ip< 0 causes an increase in density, probably due to an improvement of confinement, in a very reproducible way. The equilibria in these experiments were analyzed with the VMEC code, which showed the appearance of lowand medium-order resonances in the discharges [2]. However, it has been suggested that magnetic islands can have an impact on the confinement [2,3], and VMEC assumes that the configuration is formed only by nested flux surfaces. The PIES code [4], which does not suffer from this limitation, is able to study configurations with islands and stochastic regions and has already been used in the past to study TJ-II equilibria with large islands [5]. The present report presents the first, preliminary free boundary results obtained with the PIES code for TJ-II configurations with net toroidal current corresponding to the mentioned experiments.