Roles of aspect ratio, absolute B and effective Z of the H-mode power threshold in tokamaks of the ITPA database

An analysis of the international tokamak physics activity (ITPA) H-mode power threshold database is advanced to study the roles of the aspect ratio, A, the absolute magnetic field and the effective charge number, Z eff . A new scaling expression for the power threshold, P thr , is presented. The absolute B at the outer surface is used instead of B t in the scaling, and the I p dependence of Pair in low-A tokamaks is described without the explicit use of the Ip variable. A clear and strong dependence on Z eff is found; P thr Z 0.7 eff. Both the scattering nature in the data fitting and the 'low-density anomaly' can be reduced. A nonlinear A dependence is also suggested. The P thr in ITER is predicted on the basis of the present scaling.