New Results May Prolong Fusion Reactor Components' Life
count: [2018-11-29]

In the high-confinement mode (H-mode) plasmas, a narrow outer region is characterized by a spontaneously generated steep pressure gradient and a large self-driven plasma current, referred to as the ‘pedestal’. The large pressure gradient and plasma current in the pedestal can provide free energies for a variety of instabilities, and thereby cause either the coupled peeling-ballooning modes, i.e. edge-localized modes (ELMs) or kinds of coherent or quasi-coherent modes. 

Crash of large ELMs, which produces intolerable divertor heat and particle loads, has been foreseen to result in a critical lifetime problem of plasma-facing components (PFCs) in the next-generation fusion devices, such as the international tokamak experimental reactor (ITER). 

Therefore, pedestal coherent or quasi-coherent modes have attracted a lot of interest in the fusion research community due to their considerable contribution to regulating pedestal structure. 

Recently, scientists in the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP) made an experimental analysis on the low-n magnetic coherent mode (MCM). 

Firstly, radial distribution and poloidal propagation of the MCM are investigated. Secondly, temporal evolution of MCM amplitude during large ELM crashes is evaluated in detail, which suggests that the mode is closely correlated with pedestal buildup. In addition, dedicated experiments reveal the possible correlations of MCM’s frequencies with edge line-averaged density and edge safety factor. Finally, effect of the MCM on edge particle transport is explored. Corresponding results suggest that the MCM is likely to primarily result in a notable poloidal redistribution of the divertor particle flux, rather than a considerable net increase of the total flux. 

This new exploration on high constraint mode base magnetic coherent mode will provide valuable reference for the extension of fusion reactor PFCs’ lifetime. (ZHANG Heng reports)

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In a typical EAST high-constraint mode discharge, MCM is found in the magnetic fluctuations signal and the electrostatic fluctuations signal in the pedestal region.