Experimental Verification of the MHD Dynamo in the Axisymmetric Linear Machine

We report the first experimental verification of the magnetohydrodynamic (MHD) dynamo in the axisymmetric linear machine. The dynamo phenomenon, in which the magnetic-field-aligned electric current is self-generated by plasma dynamics, has been a puzzle not only in astrophysical plasmas, but also in magnetically confined laboratory plasmas for many decades. The mirror trap axisymmetric plasma, in which the unstable differential rotation of plasma column in crossed E×B fields excites the helical MHD turbulence, is a new and particularly vivid example of the dynamo effect.

By manipulating the trap's magnetic and plasma conditions, we have obtained both the parallel and the antiparallel to the magnetic field electric current with density to the order of 100 A/cm² (total current up to 6 kA) in the plasma. The measured mean electromotive force F_em has linear dependence from the turbulent diffusion coefficient D_T (r,t) and reachs up to 50 V/m. By measuring each term of F_em, the parallel MHD mean-field Ohm's law has been observed to hold within experimental error bars during plasma flow pulse. A comprehensive physical picture of the dynamo phenomenon has been obtained.