Understanding Bias Potential Variation in C-2W with Multi-axial Doppler Spectroscopy

In C-2W, TAE's fifth generation field-reversed configuration plasma device [1], biasing of end electrodes is used to manipulate radial electric fields and drive sheared rotation of the scrape-off-layer plasma for mode stabilization. Applied potentials drop axially and radially along magnetic field lines and define the effective potentials in the central confinement region of the device, which makes understanding the mechanisms behind this axial variability and its implications on electron-ion confinement and machine performance critically important. For this purpose, multi-axial Doppler spectroscopy targeting discrete radiation from doped helium impurities was used to monitor impurity rotation at various locations along the C-2W device. Optical fibers coupled to a high-resolution spectrometer (Acton AM-510-8, 1m focal length, f/8, 2400 grooves/mm) were used to collect light emission and resolve the Doppler shift of the 468.56 nm He-II transition. Here, we discuss the experimental setup, present obtained results, and deliberate on plans to utilize this Doppler spectroscopy diagnostic system in Copernicus, TAE's next FRC fusion device.

[1] H. Gota et al., Nucl. Fusion 61, 106039 (2021)