Nonlinear mechanism for the suppression of error field magnetic islands by plasma flow
Non-axisymmetric magnetic field perturbations generated, for example, by errors in the alignment of the field coils are known to lead to reduced confinement in a tokamak. By inducing the formation of small, stationary, magnetic islands on all rational surfaces they can enhance radial transport and, under certain circumstances, interact with MHD instabilities to trigger the onset of locked modes leading, in some cases, to a disruption of the plasma discharge. Given the stationary nature of the error field islands, it is natural to consider whether they can be reduced significantly by the viscous drag of a sheared flow resulting from a bulk rotation of the plasma. This paper examines this interaction by modelling the nonlinear growth and saturation of force-reconnected magnetic islands driven by a corrugated boundary in a slab plasma with an initially uniform flow. A systematic parameter study is made of the time asymptotic steady state revealing a transition to a "suppressed state" as a function of the flow velocity, which becomes discontinuous for sufficiently small viscosity. As such, a critical velocity, V(c), is defined and its dependence on corrugation amplitude, viscosity, resistivity, wall separation and the perturbation wave number is explored. Besides the flow suppression mechanism, a variety of nonlinear phenomena are observed in different regions of the parameter space, such as flow amplified islands, flow induced secondary reconnection and hysteresis.
Bibliographic Reference: Paper presented: 19th EPS Conference on Controlled Fusion and Plasma Physics, Innsbruck (AT), June 29 - July 3, 1992
Availability: Available from (1) as Paper EN 36862 ORA
Record Number: 199210698 / Last updated on: 1994-12-02
Original language: en
Available languages: en