Speaker
Glen Anthony Wurden
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P5.1077.pdf
Quasi-continuous low frequency edge fluctuations in the W7-X stellarator
G. A. Wurden1, S. Bozhenkov2, C. Brandt2, B. Buttenschoen2, M. Endler2, S. Freundt2, K.
Hammond2, M. Hirsch2, U. Hoefel2, G. Kocsis3, P. Kornejew2, M. Krychowiak2, S.
Lazerson4, K. Rahbarnia2, L. Rudischhauser2, T. Szepesi3, V. Winters5, and the W7-X Team
1
Los Alamos National Laboratory, Los Alamos, USA
2
Max Planck Institute for Plasma Physics, Greifswald, Germany
3
Wigner Research Centre for Physics, Budapest, Hungary
4
Princeton Plasma Physics Laboratory, Princeton, USA
5
University of Wisconsin, Madison, USA
We have observed quasi-continuous low frequency (150-400 Hz) n=0 edge oscillations via
multiple diagnostics in Wendelstein 7-X for some magnetic configurations. These events
appear to originate at mid-radius, losing energy outwards to the walls, while a weak cold
wave propagates towards the core. They are characterized by easily observable decreases in
plasma kinetic energy (via diamagnetic loops) and simultaneous large (∆I/I = 300%)
transient plasma current increases, albeit on a small net plasma current (<1kA). Core and
edge responses occurs after the initial (~1-3%) global energy drop on each event. Fast
cameras and segmented Rogowski diagnostics show an overall m=0 edge brightening, while
electron cyclotron emission show an electron temperature fluctuation inversion point at
mid-to-outer-radii. Using fiber filterscopes at multiple toroidal locations, the n=0 nature of
the edge response (H-alpha, Carbon-III emission) is determined. Langmuir probes in the
divertor show an edge density increase with each burst. These events are especially visible in
so-called “high iota” discharges, when iota-bar is nearly 1 in the core, rising to 5/4 at the
edge. Their magnitude is larger with higher input power, and their frequency is increased at
higher plasma density. Their associated sawtooth-like energy loss, integrated over one
energy confinement time, accounts for ~30% of the total energy loss.
