Comprehensive Imaging of C-2W Plasmas: Instruments and Applications

The C-2W device (“Norman”), has produced and sustained beam-driven field-reversed configuration (FRC) plasmas embedded in a magnetic mirror geometry using neutral beams and end-bias electrodes located in expander divertors. Many discrete vessels comprise this device, and a suite of spatially and radiometrically calibrated, high-speed camera systems have been deployed to visualize the plasma throughout. Besides global visualization of the plasma evolution, this imaging suite has been used in a variety of applications. Reconstruction of the magnetic field in the equilibrium vessel is complicated by eddy currents in conducting structures and thus far, non-perturbative measurements of internal field have not been available. Tomographic reconstruction of O4+ impurity emission provides an independent check of magnetic modeling and indirect evidence for field reversal within the FRC. Voltages up to 3.5 kV are applied to electrodes in the expander divertors to control the radial electric field in the plasma located on open field-lines. This has been shown to improve the macroscopic stability of the FRC; however, a full model for how electrode potentials propagate to the center of the plasma is the subject of ongoing work. Imaging in the expander divertors is used to study gas ionization and to identify metal arcing from electrode surfaces.

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