Speaker
Ben Fynney McMillan
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.1100.pdf
Absolute versus convective instabilities in subcritical tokamak plasmas.
Ben F. McMillan1 , Chris C. T. Pringle2 , Bogdan Teaca2
1 Centre for Fusion, Space and Astrophysics, Department of Physics, University of Warwick,
Coventry, United Kingdom 2 Applied Mathematics Research Centre, Coventry University,
Coventry, CV1 5FB, United Kingdom
In tokamak plasmas, sheared flows perpendicular to the driving temperature gradients can
strongly stabilize linear modes. While the system is linearly stable, regimes with persistent
nonlinear turbulence may develop, i.e. the system is subcritical. A perturbation with small but
finite amplitude may be sufficient to push the plasma into a regime where nonlinear effects are
dominant and thus allow sustained turbulence. The resulting excitation of the system spreads
through the system and can progressively destabilise larger and larger regions of the device.
Interestingly, for sufficiently large values of shear flow, the excition propagates only in one
direction, and the turbulence is transient when viewed at a fixed spatial location. The system is
thus only convectively unstable, and in a bounded physical tokamak, the plasma will eventually
return to a quiscent state. This suggests a strong role for nonlocality in the system, and provides
a mechanism for trigerring of the plasma edge by core turbulence, even if the edge region is
locally quiescent. We numerically explore these issues using a standard tokamak testcase, the
CYCLONE benchmark, by scanning the size of the background flow shear. The relationship
between this phenomena is examined in light of propagating phenomena found in the edge of
chaos, and the the avalanche-like bursts found in earlier work.
