Magnetohydrodynamic turbulence with net currents

Abstract

Recent work concerning magnetohydrodynamic (MHD) turbulence with net electric currents is reviewed. Most of the problems considered derive from those encountered in toroidal confinement devices. In the presence of net currents and externally imposed dc magnetic fields, the standard symmetries of “homogeneous turbulence” theory do not apply, and spatially periodic boundary conditions may not be invoked. The turbulence which results tends to be less than fully developed, and its properties are not controlled by Reynolds-like numbers alone. Its description requires occasional departures from the strict MHD framework. Recognizable structures are plentiful in unstable regimes, and include paired helical vortices and helical distortions of the current channel; these may be “large scale” or “small scale”, depending upon the value of the on-axis Hartmann number at which the pinch ratio is raised above its critical value. Plasma rotation, due to departures from strict charge neutrality, may be used to suppress this MHD activity. Unsolved problems in microscopic plasma kinetic theory add uncertainty to the macroscopic MHD predictions, particularly in connection with the viscous stress tensor.