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Measurements of the magnetic field distribution in a z-pinch plasma during and near stagnation, using polarization spectroscopy*


  Guy Rosenzweig [1]  ,  Eyal Kroupp [1]  ,  Alexander Starobinets [1]  ,  Amnon Fisher [2]  ,  Yitzhak Maron [1]  ,  Henry R. Strauss [3]  ,  John L. Giuliani [4]  ,  J. Ward Thornhill [4]  ,  Alexander L. Velikovich [4]  
 [1] Weizmann Institute of Science, Rehovot, Israel
[2] Technion - Israel Institute of Technology, Haifa, Israel
[3] HRS Fusion, West Orange, NJ, USA
[4] Plasma Physics Division, Naval Research Laboratory, Washington, DC, USA

Knowledge of the magnetic field distribution in a z‑pinch plasma is of high importance since the plasma-field interaction plays a key role in determining the characteristics of the stagnation process and the efficiency of the magnetic-energy coupling to the plasma. However, plasma conditions that are typical of high-energy-density systems often render the common Zeeman-splitting magnetic field diagnostic impossible. The high densities and high ion velocities result in broad spectral line-shapes that smear out the Zeeman-split patterns1. Polarization spectroscopy previously employed in our laboratory, only yielded the field distribution prior to stagnation and away from the pinch axis2.

The limitations on the radial and temporal proximities to the stagnation region and time have been overcome by recording the individual shapes of the left- and right-circularly polarized components of Zeeman-split emission lines3. The radial distribution of the magnetic field was determined unambiguously, at stagnation and near the pinch axis, by measuring selected lines from various charge states that reside only at certain radii due to the radial temperature gradient. It was found that the current flowing through the stagnating plasma is a rather small fraction of the total current. These results are combined with measurements of the electron and ion densities and temperatures. In addition, the variations of the plasma and field parameters in the z dimension are studied too.

Understanding the magnetic field distribution and the detailed plasma structure here found is presently being pursued with magnetohydrodynamics modelling and numerical simulations.

 

1. R. Doron et al. Determination of magnetic fields based on the Zeeman effect in regimes inaccessible by Zeeman-splitting spectroscopy. High Energy Density Phys. 10: 56–60, 2014.

2. G. Davara et al. Spectroscopic determination of the magnetic-field distribution in an imploding plasma. Phys. Plasmas, 5(4):1068–1075, April 1998.

3. R. P. Golingo, U. Shumlak, and D. J. Den Hartog. Note: Zeeman splitting measurements in a high-temperature plasma. Review of Scientific Instruments, 81(12):126104, 2010.

 

* This research is supported by the Israel Science Foundation.