Abrupt in time and wide in space variations of heat diffusivity during ITB formation in JT-60U reverse shear discharges

Abstract

New features of the space-time evolution of the internal transport barrier (ITB) were highlighted during recent JT-60U reverse shear (RS) experiments. An ITB evolution in RS plasmas is often a combination of the fast time-scale processes and the gradual ones. Fast time-scale processes are the common intrinsic features of JT-60U RS plasmas and are seen as the simultaneous (within a few milliseconds) rise and decay of electron temperature (Te) on two zones separated by a region without variation of Te. The region without variation of Te is located near the position of the minimum safety factor profile (qmin) for many fast processes. The present robust result is that the region of fast-time-scale improvements of electron heat diffusivity is wide in space (around 0.3 of the minor radius) and well extended to the zone of Te decay.