Impact of internal magnetic islands on electron temperature profile gradients from ECE measurements in Wendelstein 7-X

A magnetic configuration scan was performed at Wendelstein7-X by varying the rotational transform between 5/4 and 5/6 at the plasma boundary for analyzing the impact on plasma confinement [1]. For the continuous measurement of electron temperature, T_e, an electron cyclotron emission (ECE) diagnostic consisting of 32 channel heterodyne radiometer is deployed, which measures the X2-mode from 120 to 160 GHz covering the plasma width with a temporal resolution in the order of µs, which is sufficient to resolve the fast magnetohydrodynamic events, temperature crashes, and gradient changes [2]. An advanced data analysis scheme utilizing the Bayesian forward modeling of the ECE diagnostic is adopted to infer the T_e profile [3]. For the magnetic configurations, where the 5/5 island chain was moved inside the last closed flux surface, the ECE measurement showed that a T_e pedestal develops during the plasma buildup phase, and the corresponding transport barrier is broken by subsequent fast crashes which have characteristics like tokamak edge localized modes [4]. The impact of the transport barrier on plasma confinement can be seen through the increased core T_e profile which could be responsible for the overall increase in the stored diamagnetic energy by approximately 10% for these configurations.

The work investigates the transport barrier for different magnetic configurations, focusing on understanding the interplay between the internal magnetic islands and the occurrence of the transport barrier.