Dependence of the internal inductance on the radial distance between the primary and secondary X-point surfaces in the EAST tokamak

In view of their advantage in long-pulse high-performance operations, low internal inductance scenarios are being developed and tested for the EAST and other tokamaks. Dependence of the internal inductance on several main plasma configuration parameters are statistically analyzed. It is found that the internal inductance of the plasma is closely related to the radial distance dR_sep between the primary and secondary X-point surfaces on the outer mid-plane as well as the poloidal field of the last closed flux surface. Moreover, dR_sep is also related to the distribution of the fast electrons driven by the injected lower-hybrid waves, which is partially responsible for the observed relation between dR_sep and the internal inductance. The results here should therefore be helpful as a guide for future experiments on internal inductance control and long-pulse operations of the EAST and other tokamaks, as well as for detailed theoretical study of the underlying physics.