Measurements of the time-dependent spatial magnetic field distribution and structure of a Z-pinch plasma throughout the stagnation process

Summary form only given. Knowledge of the time dependent magnetic field spatial distribution in imploding plasmas (such as a Z-pinch plasma) is of high importance, due to the key role of the magnetic field in determining the characteristics of the imploding and stagnating plasma. Theoretical models of Z pinch plasmas strongly rely on the magnetic field distribution for the predictions of the hydrodynamic and atomic processes, and of the energy coupling. Polarization spectroscopy previously employed in our laboratory, only yielded the field distribution prior to stagnation and away from the pinch axis¹. Indeed, at stagnation and generally in high-energy-density experiments, this technique is impractical, since the Stark and Doppler broadenings dominate the spectral line shape, disallowing the determination of the Zeeman effect².Recently, we have employed a new technique that overcomes these limitations by recording the individual shapes of the left and right circularly polarized components of Zeeman-split emission lines3. Furthermore, while the system integrates over the chordal views, measuring selected lines from the various charge states allowed for determining unambiguously the spatial dependence of the magnetic field and current, at stagnation and near the pinch axis. It was found that at stagnation the current spreads over the entire plasma column (20 mm in diameter), and the current at the stagnating plasma is a rather small fraction of the total current. The data show a rise of the current near the axis before, during and after stagnation, from a tiny fraction of the total current to larger values. These results are combined with the determination of the radial distribution of the electron and ion densities and temperatures. In addition, the variation in the z dimension is studied too. Understanding the magnetic field distribution and the detailed plasma structure here found, should be pursued with magnetohydrodynamics modelling.