Plasma flow in a magnetic field is similar to the flow of a fluid or gas in that it can be either laminar, i.e., in smooth layers, or turbulent, i.e., vortex flow. In most cases a magnetic field directed along the flow inhibits the formation of turbulence, that is to say, it stabilizes the laminar motion. The stabilizing action of the magnetic field is strongest in a highly conducting fluid, in which the field-freezing effect applies. In this case turbulent motion leads to tangling of the field lines as a result of which the kinetic energy of the motion is transformed into magnetic energy. In a weakly conducting plasma the magnetic field also inhibits the growth of turbulence, but the stabilization mechanism is altogether different. It is due to the fact that the motion of a conducting medium in a magnetic field gives rise to an electric current which, for finite conductivity, leads to an energy dissipation, i.e., the kinetic energy of the motion is converted into thermal energy (Joule heat). A similar dissipation process occurs in nonconducting media because of viscosity. For this reason the quantity 1/σμ0 (the magnetic diffusion coefficient) is sometimes called the magnetic viscosity.
KeywordsHartmann Number Magnetic Reynolds Number Magnetohydrodynamic Flow Magnetic Viscosity Laminar Motion
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