Observation of a magnetic dynamo driven by heat flow in a laser-heated plasma*

We have observed the generation of an azimuthal magnetic from an axial magnetic field in a cylindrical system due only to a radial temperature gradient. A gas jet in a cusp magnetic field was heated by a 2-ns, 2-kJ laser propagating along the axis of the cusp and the magnetic field was measured by protons accelerated from a foil by a 0.7-ps, 30-J laser, crossing the axis at a slight angle. A clear signature of an azimuthal magnetic generated from the axial magnetic field was observed in proton radiographs. The measurements are in good agreement with MHD simulations including the full magnetized electron transport coefficients. The magnetized electrothermal tensor leads to advection-like terms in Faraday’s law that can be attributed to magnetic field being carried by electron heat flow. The cross-field or Righi-Leduc heat flow, which is perpendicular to both the magnetic field and the temperature gradient, rotates an axial magnetic field in the presence of a radial temperature gradient. Differential rotation leads to generation of an azimuthal magnetic field, which was achieved by using a cusp to give an axially varying magnetic field. Cowling’s anti-dynamo theorem is overcome by the anisotropy of the electrothermal tensor.