Radial localization of electron temperature pedestal and ELM-like events using ECE measurements at Wendelstein 7-X

¹ Max Planck Institute for Plasma Physics, Wendelsteinstrasse-1, 17489, Greifswald
² Los Alamos National Laboratory, Los Alamos, NM, USA

^* e-mail: neha.chaudhary@ipp.mpg.de
^** See Thomas Sunn Pedersen et al 2022 Nucl. Fusion 62 042022 for the W7-X Team

Published online: 23 February 2023

Abstract

A magnetic configuration scan was performed at Wendelstein 7-X stellarator by varying the rotational transform to analyze the plasma confinement for magnetic configurations with different edge magnetic island locations and sizes. For the magnetic configurations, where the 5/5 island chain was moved inside the last closed flux surface, it was observed with electron cyclotron emission measurements that an electron temperature, T_e, pedestal develops in the plasma buildup phase and followed by the edge localized mode (ELM)-like crashes. From the mapping of the island to the plasma radius from HINT equilibrium, it was found that the T_e pedestal is formed at the island location on the high field side of the plasma. The ELM-like crashes occur at the location of the pedestal and the transport barrier is broken typically with an energy loss of 3-4% during a single ELM-like event. The frequency and the amplitude of the ELM-like crashes were observed to be changing with island size, plasma heating power and density. Additionally during the plasma decay, after the heating was switched-off, a transition to degraded plasma confinement state was observed with changed T_e profile gradients, faster decay rate of diamagnetic energy, and increased H-alpha levels.