H-mode transition in the TJ-II stellarator plasmas

Abstract

Since the first H-mode transitions were observed in TJ-II plasmas in 2008, an extensive experimental effort has been done aiming at better physics understanding of confinement transitions. In this article, an overview of the main findings related to the L-H transition in TJ-II is presented including how the radial electric field is driven, what are the possible mechanisms for turbulence suppression and what are the related temporal and spatial scales that can impact the transition. The trigger of the L-H transition in TJ-II plasmas is found to be more correlated with the development of fluctuating E × B flows than with steady-state E r effects, pointing to the role played by zonal flows in mediating the transition. Experimental evidence supporting the predator–prey relationship between turbulence and flows as the basis for the L-H transition, found for the first time in TJ-II, reinforces this conclusion. Besides, the reduction in the turbulent transport at the transition is detected at the barrier region but also in a wider radial range with weak or even zero E × B flow shear, which points to other mechanisms beyond the turbulence suppression by local sheared flows. This article is part of a discussion meeting issue ‘H-mode transition and pedestal studies in fusion plasmas’.

H-mode transition and pedestal studies

“…Teaca et al [ 12 ] investigate the self-organization nature of a sheared plasma flow as transitions between (turbulent and laminar) states in a reduced fluid (interchange) model. The theme issue closes with a comprehensive analysis of H-mode transitions on the TJII Stellarator by Estrada et al [ 13 ]…”

Importance of the Rotational Transform for L–H Transitions in the TJ-II Stellarator

“…In TJ-II, spontaneous L-H transitions are achieved [7] under NBI heating conditions. As in other low-magnetic-shear stellarators, the L-H transition is achieved at constant heating power by increasing the plasma density, and, in general, the higher the heating power, the higher the density needed to trigger the transition.…”

“…Confinement transitions in high-density plasmas, or L-H transitions, have been studied intensely at the low-shear stellarator TJ-II in the past. These spontaneous confinement transitions often occur in plasmas heated using neutral beam injection (NBI) [7] and should be distinguished from the low-density (electron to ion root) transitions typical of electron cyclotron-heated (ECRH) plasmas [8,9]. The L-H transitions are characterized by a moderate increase in the energy confinement time (typically about 30%), an increase in the line average electron density, a drop in the edge H α emissions, the development of steep edge density gradients, and a reduction in the edge turbulence level, occurring within a few ms.…”