Measurement of the dynamo effect in a plasma

A series of detailed experiments has been conducted in three laboratory plasma devices to measure the dynamo electric field along the equilibrium field line (the alpha effect) arising from the correlation between the fluctuating flow velocity and magnetic field. The fluctuating flow velocity is obtained from probe measurement of the fluctuating ExB drift and electron diamagnetic drift. The three major findings are the following: (1) The alpha effect accounts for the dynamo current generation, even in the time dependence through a ''sawtooth'' cycle; (2) at low collisionality the dynamo is explained primarily by the widely studied pressureless magnetohydrodynamic (MHD) model, i.e., the fluctuating velocity is dominated by the ExB drift; (3) at high collisionality, a new ''diamagnetic dynamo'' is observed, in which the fluctuating velocity is dominated by the electron diamagnetic drift. In addition, direct measurements of the helicity flux indicate that the dynamo activity transports magnetic helicity from one part of the plasma to another, but the total helicity is roughly conserved, verifying Taylor's [Phys, Rev. Lett. 33, 1139 (1974); Rev, Mod. Phys, 58, 741 (1986)] conjecture. (C) 1996 American Institute of Physics.