Speaker
Eric Edlund
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P4.1052.pdf
Observations of electron-driven Alfvén eigenmodes in Wendelstein 7-X
E. M. Edlund1, M. Porkolab2, and the W7-X Team3
1
SUNY Cortland, Cortland, NY, USA
2
Massachusetts Institute of Technology, Cambridge, MA, USA
3
Max Planck Institut für Plasmaphysik, Greifswald, Germany
Experiments conducted in the Wendelstein 7-X OP1.2a campaign regularly achieved
electron temperatures in the range of 1-4 keV with densities of 1-4´1019 m-3 with H and
He gas puffing, though pellet fueling was able to substantially increase the density
toward the X2 cutoff around 8´1019 m-3. Despite the absence of energetic ions, clear
signatures of Alfvénic modes were observed in many experiments covering a wide
range of conditions. Calculations from the CKA-EUTERPE code suggested that Alfvénic
modes identified during the W7-X OP1.1 experimental campaign may have been driven
by the gradient of the thermal electron pressure. In this work, we summarize the
observations and present the trends compiled over and exhaustive search of all
experiments from the OP1.2a campaign. The observed Alfvénic activity falls under two
major classes of behavior: modes that are excited during the early plasma within about
100 ms of breakdown, and those that are excited later in the plasma. In the early
transient phase, Alfvénic modes tend to be short-lived and with rapid frequency
variation, suggesting a sensitive response to evolution of the local equilibrium, perhaps
similar to reversed shear Alfvén eigenmodes observed in tokamak experiments. With
few exceptions, the Alfvén modes observed at later times are long-lived with steady
frequency signatures, and tend to closely track the density evolution. We compare
measurements from the phase contrast imaging diagnostic, Mirnov coils, and the soft x-
ray diagnostic system (XMCTS).
