A linear empirical threshold condition ωE×B/γITG>0.68s-0.095 has been found for the onset of theion internal transport barriers in the JET optimised sheardatabase. Here, s is the magnetic shear, ωE×Bthe flow shearing rate and γITG is an approximate of thelinear growth rate of the ion temperature gradient instability. Thepresent empirical threshold condition for the ITB formation willprovide a first clear indication of the strong correlation of s andωE×B at the ITB transition. The empiricalanalysis consists of ITB discharges from a wide plasma parameterrange; the toroidal magnetic field varies between 1.8-4.0 T, theauxiliary heating power between 10-30 MW and the diamagnetic energybetween 3-12 MJ. The predictive simulations of several ITB dischargeswith the empirical ITB threshold condition reproduce the experimentswith time averaged prediction errors of the order of 10-25% inTi and Te profiles and 10-15% in neprofiles as well as the toroidal flow velocity with errors of theorder of 10-20%. The simulated times of the onset of the ITBcompared to the experimental ones are typically within 0.4 s and thesimulated ITB widths within 0.1 in r/a throughout the wholesimulations.
Internal transport barrier in tokamak and helical plasmas