Helium line ratio imaging in the C-2W divertor

In TAE Technologies' current experimental device, C-2W (also called ``Norman'')[1], voltages applied to concentric annular electrodes located in divertors are used to stabilize beam-driven field reversed configuration (FRC) plasmas. In addition, magnetic field expansion is employed to thermally isolate electrons from the end electrodes.  Measurements of electron temperature and density in the divertor are important in order to compare the experiment with models for the axial electrostatic potential and electron confinement. To this end, a three-wavelength, 2D imaging instrument has been deployed to study the plasma in the divertor using the helium line ratio method.  The neutral helium target is provided by a super-sonic gas injector located inside the divertor vessel which injects helium toward the magnetic axis and perpendicular to the camera sight-cone.  Three wavelength channels are isolated and imaged simultaneously on an image intensifier via an optical system containing dichroic beam-splitters, narrow-band interference filters, and mirrors.

Images of helium emission and reconstructed electron density and temperature profiles under various operational conditions will be presented.  These first measurements in the C-2W expander divertor will be compared to Thomson scattering measurements at the machine midplane and jet regions.

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