Abstract Details
Abstracts
Author: Megan B Evans
Requested Type: Poster
Submitted: 2025-03-04 12:03:15
Co-authors: P. G. Ivanov, M. A. Barnes
Contact Info:
University of Oxford
Beecroft Building, Parks Road
Oxford, Oxfordshire OX1 3PU
Oxfordshire
Abstract Text:
It is widely known that cross-field E×B flow shear stabilises ion temperature gradient (ITG) turbulence in magnetised plasma via stretching eddies to dissipation scales. However, in the presence of a sheared magnetic field, this effect can be negated by turbulence moving at an angular frequency of ΩE = γE/s, where γE is our shearing rate and s is our magnetic shear. Due to the variations in curvature along a given magnetic field line in a tokamak, our instantaneous growth rates for modes moving at ΩE oscillate periodically in time. These modes are known as Floquet modes and are seen in a range of physically different systems. Although they appear to not be relevant for example in the determination of the 'Waltz quench rule' seen in Waltz et al. 1998, their importance in the bistable turbulence seen in Christen et al. 2022 inspired us to derive a minimal physics model to explore them.
Starting with a large aspect ratio tokamak with circular flux surfaces, we transform the gyrokinetic equation into the frame moving at the frequency ΩE. We then neglect all derivatives parallel to the field line and integrate to produce fluid equations, which we then study numerically. We find that even without parallel derivatives we still get suppression of linear and nonlinear fluxes at sufficiently high ΩE . However, at lower values of ΩE, we find that increasing flow shear can in fact drive instability. We use qualitative models to explain the instability and predict the point at which suppression happens.
Characterization: 1.0
Comments: