An investigation of the plasma focus by Faraday rotation polarimetry

Abstract

The azimuthal magnetic field and current density structure of a Plasma Focus device, have been investigated by the observation of the Faraday rotation of a ruby laser probe beam. The magnetic field and current distributions play important roles in plasma compression, heating, confinement, stability, and particle acceleration. However, in over two decades of research, no satisfactory experimental data on the field or the current have been produced. The results reported are of the first measurements of magnetic field and current distribution in the Plasma Focus, using a non perturbing diagnostic technique. A full description of the experimental apparatus and method, the physics of the differential polarimetry (with refraction effects included), and theoretical reviews of Faraday rotation, birefringence, dichroism, and refraction, are given. It was found that during the collapse phase of the discharge, times t = -10 ns to t = 0 ns (peak compression), the current and field are confined to the plasma skin. The penetration depth is 0.56 mm, and the resistivity is classical. During the dense pinch phase, between t = 0 ns and t = +10 ns, the plasma develops a turbulent core, of radius 2 mm, in which the resistivity is highly anomalous (by a factor 6000). This results in a rapid diffusion (lasting approximately 10 ns) of field and current (typically 20 of the total) into the core. Particle acceleration is suppressed at this stage because the ion Hall term is less than unity. Outside this core, the resistivity is classical, and the current is carried in the plasma skin. At times t = +10 ns to t = +15 ns, axial current filamentation was observed. These filaments last less than 2.5 ns, and carry in excess of 12% of the current. Future studies of the filamentation should lead to a better understanding of the intense neutron production observed in Plasma Focus devices.