B-field diagnostics in transient plasmas

Abstract

Summary form only given. Measurements of magnetic fields are a key issue in many studies of equilibrium and transient laboratory plasmas. However, the traditional techniques are not useful in situations where the magnetic field has no definite direction during the time of observation, or for magnetic fields with amplitudes that vary in time or space, e.g. in laser-produced or pinch plasmas. To overcome these difficulties, a new spectroscopic approach was proposed, applicable to measurements of such isotropic magnetic fields. The technique is based on the spectroscopic analysis of line-shapes of different fine-structure components of the same atomic multiplet that undergo different splitting under the B-field. Another advantage of this method is that, when coupled with detailed line-shape calculations, it allows for an accurate B-field determination even when the line-shapes are strongly influenced by Stark and Doppler broadenings in the plasma. Here we report on an experimental implementation of the proposed method to measure the magnetic field in a laser-produced plasma injected into a coaxial vacuum transmission line carrying a 200-kA current. The pulsed magnetic and electric fields, generated in the transmission line, are of spatially varying orientations at the plasma boundary. This causes a complex motion of the plasma particles, that is further complicated due to the 3-D geometry of the experimental setup. This results in a twisting and a depolarization of the magnetic field lines in the plasma. The profiles of spectral lines emitted by the plasma demonstrate complex shapes due to the combined inhomogeneous Stark, Zeeman, and Doppler broadenings. Nevertheless, using the method presented, the magnetic field is determined unambiguously. Magnetic-field penetration into the plasma that is faster than the diffusing rate expected from the Spitzer resistivity is observed. The results here presented demonstrate the powerfulness of the method as a tool for single-shot measurements of magnetic fields with complex geometries