A fast helium beam diagnostic at the island divertor of W7-X for the investigation of plasma edge dynamics

Understanding and quantifying particle and energy transport at the plasma edge region is crucial for magnetic confinement fusion. For this purpose, a new polychromator system with 1 µs time resolution was installed for the thermal helium beam diagnostic in the island divertor of the stellarator Wendelstein 7-X (W7-X). The time resolution is four orders of magnitude faster than the spectrometer-based system. This allows for the first time access to turbulence-relevant time scales, enabling the investigation of plasma edge dynamics such as modes and instabilities. Their connection to averaged plasma profiles is likewise accessible with this diagnostic. Furthermore, the diagnostic system measures in two magnetically connected divertors, which enables the study of long-range correlation of fluctuations. Utilising a collisional-radiative model, the diagnostic can reconstruct fast electron density and temperature variations with an effective resolution of 10 kHz, associated with plasma modes and bursts. Additionally, it provides high temporal resolution measurements of the detachment process in the divertor. Since it simultaneously measures at an upper and lower divertor, it is the first edge diagnostic at W7-X capable of measuring the up–down asymmetry of turbulence. ... mehrThis article presents the design and implementation of the diagnostic as well as an analysis of the signal quality. It also looks at the influence of diagnostic gas puffs on the plasma in detail. To present the potential of the diagnostic, this work briefly shows a long-range correlation analysis of an edge mode between two divertors that are magnetically connected as well as the electron temperature and density calculation for this mode. Furthermore, a detachment process measured with high time resolution is presented.