Dynamics and interplay of L-H-L transitions and ITB events in reversed shear plasmas with internal barriers in JT-60U

Abstract

Abrupt in time (correlated within a few ms with L-H and H-L transitions) and wide in space (from the edge up to ~0.3 of minor radius in the extreme case) variations of electron heat diffusivity δχe were found in reversed shear (RS) JT-60U tokamak plasmas with internal transport barrier (ITB) at various profiles of safety factor q. The δχe profile follows the position of q minimum (qmin ) and penetrates into the RS region deeper for the weak ITB than for the strong one, suggesting the presence of `global' edge-core connection throughout qmin in a ms timescale at L-H and H-L confinement bifurcations. The δχe value at the position of qmin is small for the strong ITB (~0.05 m2 s-1) and increases by an order of magnitude for the weak ITB. The ITB event is another non-local transport bifurcation (improvement or degradation) inside and around ITB. The case of strong ITB formation in H-mode via a series of ITB events in ELM-induced L-mode is described. ITB event degradation in the positive shear zone causes L-H transition.