Confinement improvement beyond the usual L-mode proceeds in most cases throughthe formation of localized transport barriers in the edge (H-mode) or in theinterior regions of the plasma (ITBs). The standard interpretation of this isthe quenching of the usually prevailing turbulence by the formation of layersof sheared E×B rotation. Regarding interior transport barriers, thisphenomenon has been observed in two distinctively different situations, withpredominant ion or electron heating, and Te either significantlysmaller or larger than Ti. The reactor relevant regime is, however,one with approximately equal Te and Ti. The availability ofECRH (and, to a lesser degree, also that of LH or FW heating) has allowed thestudy of the effect of the Te/Ti ratio in a controlled way. Thestandard transport model of ion-temperature-gradient andtrapped-electron-mode-driven turbulence predicts (for situations withsignificant ion heating) a negative effect of an increase in this ratio.Experimental results show, however, that this confinement deterioration can beavoided in certain situations. These observations can be reconciled withtheoretical expectations, if the simultaneous increase in Shafranov shift,which can reverse the theoretical predictions, is taken into account.
Electron thermal internal transport barriers triggered by the effect of ion shielding