Description
In stellarators and tokamaks, transport of energy and particles in the plasma core critically determines device performance. Therefore, it is essential to identify advanced operational regimes that yield high-performance conditions and understand the mechanisms governing such regimes. In the TJ-II stellarator several routes to improved confinement have been identified, including L–H transitions driven by turbulence suppression via sheared flows and more recently, enhanced confinement induced by pellet injection, resembling pellet-fueled high-performance plasmas in W7-X. A better confinement is achieved already after the injection of a single pellet as well as after multiple pellets. In order to understand the mechanisms behind this confinement improvement a series of gyrokinetic simulations (using stella and EUTERPE) complemented by neoclassical simulations (using SFINCS) have been carried out and compared with power balance analysis estimates and source modeling using HPI2 and ASCOT. The results show that in TJ-II the physical mechanisms behind the improved confinement do not seem to be so tightly linked to a reduction in turbulent transport due to an increase in the density gradient as demonstrated in W7-X. This work contributes to the interpretation and validation of TJ-II high-performance scenarios in the broader context of stellarator research.