Characteristics of Edge MHD Modes and ELM Activity Observed in Large Helical Device Plasmas

Abstract

AbstractIn the Large Helical Device (LHD), nonlinear evolution of strongly destabilized edge MHD modes, which are thought to be resistive interchange modes, induce repetitive bursts of magnetic fluctuations, soft X‐ray (SX) fluctuations, and extreme ultra‐violet (XUV) fluctuations, and generates a train of sharp spikes in Hα emission signals. These Hα spikes exhibit the character of edge localized mode (ELM). The ELM activities are also observed in high beta or high density L‐mode plasmas having steep pressure gradient at plasma edge as well as H‐mode plasmas with the L‐H transition. The responsible instabilities for ELMs in LHD plasmas are thought to be resistive interchange modes, and are clearly different from those in a tokamak where peeling‐ballooning modes are the candidates. The repetition frequency of ELMs (fELM) increases with the increase of external heating power, of which the character is similar to that of type I ELMs in tokamaks. The product of fELM and the relative ELM amplitude (δHα/Hα) is roughly in proportion to the input heating power, where the ELM induced loss energy (ΔWp) increases with δHα/Hα (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)