Formation of radially elongated flow leading to onset of type-III edge localized modes in toroidal plasmas

The dynamic features of triggering of type-III edge localized modes (ELMs) by macro-scale radially elongated flow (REF) in magnetically confined toroidal plasmas are presented. The REF, localized near the mid-plane and with a long radial correlation length at the low field side, was observed prior to the collapse of pedestal in series. It is demonstrated that the REF gets energy from and modulates ambient turbulence via nonlinear three wave interaction. The formation of REF leads to the onset of type-III ELMs, where the collapse of plasma energy occurs in the outer region ~0.3 of the plasma column within a few tens of microseconds without global MHD instabilities. The path of the onset is identified and a clue to the trigger mechanism of violent events in high temperature plasmas is discussed. (paper)