Speaker
Guanqi Dong
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.1037.pdf
Non-linear interplay between edge localized infernal mode and plasma flow
G. Q. Dong,1 Y. Q. Liu,2,1 Y. Liu,1 S. Wang,1 N. Zhang,1 G. Z. Hao,1 and G. L. Xia1
1
Southwestern Institute of Physics, Chengdu, People’s Republic of China
2
General Atomics, PO Box 85608, San Diego, California 92186-5608, USA
Quiescent H-mode (QH-mode) was first discovered in DIII-D as an ELM-free H-mode regime,
which is usually achieved at relatively low plasma density and found to be accompanied by the
presence of edge harmonic oscillations (EHOs). EHOs are believed to provide necessary
transport to eliminate ELMs by dynamics of the plasma itself. The saturated kink-peeling mode
has been suggested as a possible candidate for EHO. In this work, we consider another
instability – the edge localized infernal mode (ELIM) – as a possible candidate, for plasmas
where the large edge bootstrap current causes local flattening of the plasma edge safety factor,
or even the magnetic shear reversal in the pedestal region. An ELIM is a low-n (n is the toroidal
mode number) instability similar to the conventional infernal mode, but being localized at the
plasma edge where safety factor is locally flattened. Finite plasma pressure in the pedestal
region drives this mode. A saturated ELIM, due to non-linear interaction with toroidal plasma
edge flow, can be responsible for EHO. A systematic numerical investigation, utilizing the free
boundary MARS-F/K codes, shows that both plasma resistivity and toroidal flow shear
destabilize the ELIM. The drift kinetic effects, due to mode resonance with precessional and
bounce motions of trapped thermal particles, are found to be stabilizing for the mode, albeit not
dramatic. We also find that the low-n ELIM instability is strongly affected by a close-fitting
resistive wall. The presence of a resistive wall can fully stabilize an otherwise flow-shear
destabilized ELIM. The ELIM instability, like other MHD instabilities, generates toroidal
torques which in turn can affect the plasma flow. The non-linear interplay between the ELIM
and the plasma flow, by running initial value simulations with the quasi-linear code MARS-Q,
is investigated. We start simulations at a prescribed rotation speed. Compared to the linear runs,
the quasi-linear run shows partial saturation of the mode. Meanwhile, the toroidal rotation
profile especially that near the plasma edge where the mode is located, is significantly reduced,
together with the local flow shear near the q=4 surface. It is this reduction of the local flow
shear that leads to the eventual mode saturation in this simulation.
