Speaker
Jeffrey Levesque
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.1058.pdf
Asymmetric scrape-off layer currents during MHD and disruptions*
J.P. Levesque1, J. Bialek1, J.W. Brooks1, S. DeSanto1, C.J. Hansen2, M.E. Mauel1,
G.A. Navratil1, and I.G. Stewart1
1
Columbia University, New York, New York, USA
2
University of Washington, Seattle, Washington, USA
During disruptions, large asymmetric currents arise in the first wall and vacuum vessel that
have the potential to damage tokamaks, and understanding this behaviour is important for
mitigating disruption loads. In this presentation, we report asymmetric scrape-off-layer (SOL)
and vessel currents measured during MHD activity and disruptions in the HBT-EP tokamak.
Low-field-side diagnostic tiles measure currents to the vessel with poloidal resolution in three
toroidal locations. Tiles and the associated wall segments can be moved radially between shots
to investigate radial SOL current structure and dependence on wall geometry. Additionally, a
biased electrode in the SOL influences MHD dynamics measured by magnetic sensors and the
SOL tiles (see Figure). Measurements reveal poloidal and toroidal structure of currents to the
wall that correlate with rotating MHD activity during the main discharge and disruption. Tile
currents exceed the ion saturation current during transient events. Electrically-isolated regions
of the vacuum vessel detect toroidal vessel currents oscillating between co- and counter-Ip
directions when the sections are connected via diagnosed jumpers. Asymmetric vessel currents
correlate with rotating kink modes, and reach ~5% of the pre-disruption plasma current during
the current quench [1]. Relative increases in local
plasma current measured by segmented Ip
Rogowski coils coincide with nearby counter-Ip
vessel currents. Measurements are interpreted in
the context of Wall Touching Kink Mode
(WTKM) [2] and Asymmetric Toroidal Eddy
Current (ATEC) [3] models, which give contrary
predictions for the sign of asymmetric toroidal
vessel current during disruptions. Both models
are needed to explain HBT-EP disruption data.
Magnetic sensors and SOL current tiles pictured
*Supported by U.S. Department of Energy, Office of
in (a) measure MHD activity with poloidal and
Fusion Energy Science, Grant DE-FG02-86ER53222. toroidal resolution. Sensor layout and SOL flux
[1] J.P. Levesque et al., Nucl. Fusion 57 086035 (2017) surfaces for a discharge with SOL biasing are
shown in (b). Tile current (c) and magnetic (d)
[2] L.E. Zakharov et al., Phys. Plasmas 19 055703 (2012) fluctuations versus poloidal angle show rotating
[3] R. Roccella et al., Nucl. Fusion 56 106010 (2016) MHD responding to applied SOL current (e).
