Empirical evidence indicates that the turbulent transport which limits core densities and temperatures in magnetic confinement fusion plasmas can be suppressed by strongly sheared mean flows. An apparent result of this turbulence suppression is the transition of the plasma to an equilibrium with significantly steeper density and temperature gradients. Our theoretical and numerical work has shown that strong subcritical turbulence is driven by large gradients in the parallel flow velocity, resulting in an optimal (finite) flow shear value for confinement. Furthermore, we discovered the existence of a possible bifurcation in which the temperature gradient has two well-separated solutions at the same input power and torque. The nature of this bifurcation and its relation to experimental observations of transport barriers were explored.

Publications:

M. Barnes, F. I. Parra, E. G. Highcock, A. A. Schekochihin, S. C. Cowley, and C. M. Roach, Turbulent transport in tokamak plasmas with rotational shear, Physical Review Letters, submitted, arxiv:1007.3390.

E. G. Highcock, M. Barnes, A. A. Schekochihin, F. I. Parra, C. M. Roach, and S. C. Cowley, Transport bifurcation in a rotating tokamak plasma, Physical Review Letters, submitted.

F. I. Parra, M. Barnes, A. A. Schekochihin, E. G. Highcock, and S. C. Cowley, A plausible scenario for reduced transport in rotating plasmas, manuscript in preparation.

I. G. Abel, M. Barnes, S. C. Cowley, G. G. Plunk, and A. A. Schekochihin, Multiscale gyrokinetics in the high flow limit, manuscript in preparation.

C. M. Roach, I. G. Abel, R. J. Akers, W. Arter, M. Barnes et al., Gyrokinetic simulations of spherical tokamaks, Plasma Physics and Controlled Fusion 51, 124020 (2009).

Presentations:

Rotational shear in tokamak plasmas. Invited talk at the Workshop on Kinetic-scale turbulence in laboratory and space plasmas: empirical constraints, fundamental concepts and unsolved problems. Isaac Newton Institute, Cambridge (UK). July 22, 2010.

The role of flow shear in the formation of tokamak transport barriers. Invited talk at the 2010 European Mechanics Society (EUROMECH) Colloquium on interfaces and inhomogeneous turbulence. University College, London (UK). 29 June, 2010.

Core momentum transport and flow shear suppression of turbulence. Invited talk at the 17th UK Fusion Theory Meeting. Culham Centre for Fusion Energy, Culham (UK). 6 May, 2010.