The 1-2 kHz Divertor-Area Fluctuation in the W7-X Stellarator

Passive fast-camera imaging has been used to study the characteristics and motion of filamentary fluctuations in tokamak edge plasmas, shedding light on their role in boundary layer transport. Fast cameras may also help to understand transport in the unique magnetic topology of stellarators. A camera viewing a full plasma cross section of Wendelstein 7-X was used to record integrated visible light at up to 30,000 frames per second during the OP 1.2a campaign, featuring a passively cooled divertor. Videos from the camera reveal a quasi-coherent fluctuation in divertor-area emissions, with a steady peak frequency varying between 1-2 kHz from shot to shot. The fluctuation occurs during steady-state operation over hundreds of helium and hydrogen shots with the standard magnetic configuration but is absent from other magnetic configurations such as the high iota and high mirror configurations. Langmuir probes in the far scrape-off layer also observe the fluctuation with a close match in peak frequency. Videos of OP 1.2b campaign shots filtered for Dα, He I and C III line emission are also analyzed. We attempt to understand the physics underlying this fluctuation and its impact on the plasma and machine.