Light impurities in JET plasmas: Transport mechanisms and effects on thermal transport

A series of experiments was carried out in JET ILW L-mode plasmas in order to study the transport of light impurities and their effects on core thermal transport. These discharges feature the presence of 3He, Be, C, N, whose profiles are all measured by active Charge Exchange, although with different degrees of accuracy. The range of variation of Zeff obtained by different levels of N injection is between 1.5 and 2.2., with concentrations relative to ne: C~0.1% Be~0.6%, 3He~6-8%, N~ 0.3-0.8%. To study the effects on ion heat transport, ICRH power was deposited on- and off-axis mainly to ions in (3He)-D minority scheme, in order to have a scan of the ion heat flux versus R/LTi, and also modulated for ion heat wave propagation. The discharges feature minimal NBI power in order to minimize the effects of rotation and fast ion stabilization and ease the detection of the effects linked to impurities. For comparison, one discharge with high NBI power at the highest Zeff value was also performed. Different types of profiles are measured for the various impurities, typically Be and 3He profiles tend to be peaked whilst N tends to be flat and C hollow. This trend at the moment could not be recovered by gyro-kinetic modeling using GKW, and further investigations are ongoing. Interestingly, a peaked C profile is observed in the shot with strong NBI. The fact that N has a flat profile for a wide radial range (0.35< r/a <0.7), on the other hand, could explain why no ITG stabilization was observed in the core when increasing the N concentration, contrary to original expectations. Linear and nonlinear gyrokinetic simulations were carried out both before the experiment to estimate the effects to be expected and afterwards with actual plasma parameters. The effects on ITGs are due to main ion dilution and related to the value of R/Ln,imp. A stabilization effect is expected for ion heat transport in presence of light impurities with peaked profiles (positive R/Ln,imp) while the opposite effect is expected with hollow profiles (negative R/Ln,imp). The main effect foreseen is on the ITG threshold, although a small decrease of the ion stiffness with peaked impurity profiles is predicted by the theory. The nitrogen is found in simulations to have the strongest stabilizing effects but it is predicted to be peaked in our discharges. The experimental observation of a flat N profile could instead explain why a small effect (and if anything of destabilization) is observed when increasing N, with higher RLTi values at lower Zeff for similar other parameters.