In the Wendelstein 7-X stellarator (W7-X), two related global low-frequency electromagnetic oscillations are observed on many diagnostic systems (e.g.plasma stored energy, Mirnov coils, Langmuir probes, etc).
The activity is related to the presence of magnetic islands at certain radii.However, in contrast to initial analyses in earlier works, the mode activity is not localized at the magnetic islands, but rather of an m = 1 type in the plasma outer-core.We observe a strong cross-correlation between electron temperature fluctuations in the confined plasma region, measured with electron cyclotron emission radiometer, and scrape-off layer fluctuations measured with electric probes.
Two limiting cases of the mode activity can be observed based on magnetic configuration.A quasi-continuous (QC) mode case, with frequencies of typically some $100 , extrm{Hz}$ , is found in configurations that have Pizza Oven Thermostat a $5/5$ magnetic island chain at the last closed flux surface (LCFS).The mode frequency is found to increase linearly with the core electron temperature and decrease with increasing electron density.
In contrast, for magnetic configurations with the $5/5$ magnetic island chain just inside the LCFS, the mode activity Oil becomes bursty and intermittent, exhibiting a crash-event like signature.Although these intermittent events are less frequent, more plasma stored energy is lost per event compared to a period of the QC oscillations.The intermittent oscillations are observable as a broadband modulation in turbulence spectra on most diagnostic systems.
The presence of these intermittent bursts goes along with an improved energy confinement in the plasma, superficially resembling H-mode and Edge Localized Modes in tokamaks.A self-limiting process is proposed as an explanation of the intermittent oscillations based on observed ejection of particles during each crash event, which leads to steepening of density profiles and temporary suppression of ion temperature gradient turbulence and in turn suppression of turbulence driven m = 1 mode causing the crash.