Tuesday, November 27, 2018
Fast-camera imaging in the visible range has been used to study the motion of high-density filaments 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 topologies of stellarators. The MIT fast camera on the W7-X stellarator, which records a full plasma cross section at up to 30,000 frames per second, observed a quasi-coherent 1-2 kHz fluctuation in light from the divertor area. The fluctuation occurs during steady-state operation in hundreds of shots with the standard magnetic configuration for W7-X, but is absent from other configurations. Langmuir probes in the far scrape-off layer and Mirnov coils observe the fluctuation, indicating that it is an electromagnetic one. The temperature of the divertor tiles is also observed to fluctuate at 1-2 kHz. Studying this fluctuation could lead to a better understanding of edge particle and energy transport and divertor heat loads in stellarators.