Stability analysis of improved confinement discharges: internal transport barriers in Tore Supra and radiative improved mode in TEXTOR

Abstract

Results of stability analysis are presented for two types ofplasma with good confinement: internal transport barriers (ITBs) onTore Supra and the radiative improved (RI) mode on TEXTOR. The stabilityanalysis has been performed with an electrostatic linear gyrokineticcode, evaluating the growth rates of microinstabilities. The codedeveloped, KINEZERO, is aimed at systematic microstability analysis.Therefore the trade-off between having perfect quantitative agreementand minimizing computation time is made in favour of the latter. In theplasmas analysed, it is found that the onset of the confinementimprovement involves a trigger. For the ITB discharges, negativemagnetic shear is involved, whereas for the RI discharges, thetriggering role is played by the increase of the impurity concentration.Once the improved confinement is triggered, the simultaneous increasesof temperature and density gradients imply an increase in both thegrowth rate and the rotation shearing rate. The rotation shear is foundto be high enough to maintain an improved confinement through thestabilization of the large scale modes.