Bursty fluctuation regimes in the core and edge plasma of the Wendelstein 7-X stellarator

Wendelstein 7-X (W7-X) is a neoclassically optimized stellarator for which turbulence is the dominant contributor to particle and heat transport in most heating and fueling schemes. A major project focus is therefore the characterization of instabilities and turbulence regimes in order to develop turbulence-optimized scenarios. This contribution explores a parameter window at low ratios of heating power over density (typ. 1-3 MW / 8e19 m^-2) which is characterized either by short dropouts (typ. <10 ms) or spikes (typ. <5 ms) of broadband core turbulent density fluctuations measured by the phase contrast imaging (PCI) diagnostic. These phenomena are highly correlated with equally fast changes of scrape-off layer light and fluctuations measured by the gas puff imaging (GPI) diagnostic, indicating rapid changes in edge profiles, as well as fluctuations of the electron cyclotron emission in the plasma edge. In controlled power step-down experiments, both regimes are crossed with heating power thresholds that vary with magnetic configuration. Within a narrower range, these events are periodic with a repetition frequency of 40-120 Hz, reminiscent but distinctly different from ELMs, especially in their temporal dynamics and lack of strong effect on confinement.