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

Takizuka, Tomonori; ITPA H-mode Threshold Database Working Group*

To evaluate the H-mode power threshold in future tokamak reactors, quantitative scaling formulas have been developed by using the ITPA threshold database. Recently data from spherical tokamaks (MAST and NSTX) were supplied to the ITPA database. A new scaling expression, which includes the absolute magnetic field B instead of the toroidal magnetic field, subsumes the plasma current dependence at low aspect ratio and is consistent with the previous data without any explicit current dependence. Other possible influence of the low aspect ratio to increase the power threshold is also investigated. It is cralified for the whole data set that the effective charge number Z raises the power threshold. By adding Z as a parameter of the scaling expression, both the scattering nature and the low-density anomaly found in the experimental data can be reduced.