Magnetohydrodynamics 38, No. 1/2, 191-210, 2002

The a dynamo effects in laboratory plasmas

H. Ji1 - S. C. Prager2

1Princeton Plasma Physics Laboratory Princeton, New Jersey 08543, USA
2Department of Physics, University of Wisconsin Madison, Wisconsin 53706, USA

A concise review of observations of the a dynamo effect in laboratory plasmas is given. Unlike many astrophysical systems, the laboratory pinch plasmas are driven magnetically. When the system is overdriven, the resultant instabilities cause magnetic and flow fields to fluctuate, and their correlation induces electromotive forces along the mean magnetic field. This a-effect drives mean parallel electric current, which, in turn, modifies the initial background mean magnetic structure towards the stable regime. This drive-and-relax cycle, or the so-called self-organization process, happens in magnetized plasmas in a time scale much shorter than resistive diffusion time, thus it is a fast dynamo process active in a strong magnetic field. The observed a-effect redistributes magnetic helicity (a measure of twistedness and knottedness of magnetic field lines) but conserves its total value. It can be shown that fast dynamos are natural consequences of a driven system, where fluctuations are statistically either not stationary in time or not homogeneous in space, or both. Implications to astrophysical phenomena will be discussed. Figs 15, Refs 57.

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