Spectroscopic study of axial porpagation of bias potentials in C-2W

In TAE Technologies’ current experimental device, C-2W (also called ``Norman'') [1], record breaking, advanced beam-driven field reversed configuration (FRC) plasmas are routinely produced and sustained in steady state. Variations in axial and radial electric fields are determining factors in electron-ion confinement as well as plasma rotation and heating. Axial drops in electric potential along magnetic field lines due to mirror field expansion and in the near-electrode sheath region in the divertors define the effective potentials in the central confinement region relative to the ends. Understanding how the applied biasing voltage is proportioned between axial and radial potential drops is thus a critical area of FRC research at TAE.  To this end, the existing Doppler spectroscopy instruments have been extended with sightlines at multiple axial locations. Simultaneous measurements of impurity ion rotation and temperature for different bias and magnetic field configurations will be used to compare with physics models of how bias potentials connect between the divertors and confinement vessel.  Here, details of the experimental setup and initial results are presented and discussed.

[1] H. Gota et al., Nucl. Fusion 59, 112009 (2019).