Abstract
The power threshold for low (L) to high (H) confinement mode transition achieved by radio-frequency (RF) heating and lithium-wall coating is investigated experimentally on EAST for two sets of walls: an all carbon wall (C) and molybdenum chamber and a carbon divertor (Mo/C). For both sets of walls, a minimum power threshold Pthr of ~0.6 MW was found when the EAST operates in a double null (DN) divertor configuration with intensive lithium-wall coating. When operating in upper single null (USN) or lower single null (LSN), the power threshold depends on the ion ∇B drift direction. The low density dependence of the L–H power threshold, namely an increase below a minimum density, was identified in the Mo/C wall for the first time. For the C wall only the single-step L–H transition with limited injection power is observed whereas also the so-called dithering L–H transition is observed in the Mo/C wall. The dithering behaves distinctively in a USN, DN and LSN configuration, suggesting the divertor pumping capability is an important ingredient in this transition since the internal cryopump is located underneath the lower divertor. Depending on the chosen divertor configuration, the power across the separatrix Ploss increases with neutral density near the lower X-point in EAST with the Mo/C wall, consistent with previous results in the C wall (Xu et al 2011 Nucl. Fusion 51 072001). These findings suggest that the edge neutral density, the ion ∇B drift as well as the divertor pumping capability play important roles in the L–H power threshold and transition behaviour.
Highlights
The high-confinement mode, i.e. H-mode, is the baseline operational scenario for ITER [1]
The L–H power threshold and transition behaviors have been studied with respect to divertor configuration and lithiumwall coating on EAST for two sets of walls: the C and molybdenum chamber and a carbon divertor (Mo/C) wall
For the ion ∇ B drift towards the upper divertor, dithering L–H transitions were normally obtained in the double null (DN) configuration and almost all single-step L–H transitions were obtained in the lower single null (LSN) configuration
Summary
The high-confinement mode, i.e. H-mode, is the baseline operational scenario for ITER [1]. In order to demonstrate high performance, long pulse plasma and in preparation for ITER, the carbon wall is being replaced by a metal wall step by step. The full carbon wall in the 2010 campaign (C wall) was replaced by molybdenum in the main chamber and carbon in the divertor in the 2012 campaign (Mo/C wall). EAST has ITER-like heating schemes, i.e. dominated by electron heating from a lower hybrid current drive (LHCD) and ion cyclotron resonance heating (ICRH). These enabled capabilities make EAST a unique platform for the low/high (L–H) power threshold studies
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