First Fast-Ion D-alpha (FIDA) Measurements and Simulations on C-2U

In Tri Alpha Energy's C-2U experiment, advanced beam-driven field-reversed configuration (FRC) plasmas were sustained via tangential neutral beam injection$^{\mathrm{1}}$. The dominant fast ion population made a dramatic impact on the overall plasma performance. A fast-ion D-alpha (FIDA)$^{\mathrm{2}}$ diagnostic, which is based on the Doppler-shifted Balmer-alpha light from neutralized fast ions, was recently added to the C-2U fast-ion diagnostics suite. The first ever FIDA measurements on an FRC topology have been carried out. Bandpass-filtered FIDA measurements (\textgreater 6 keV ions) were made with a photomultiplier tube and are forward modeled by FIDASIM. Line-integrated signals were taken at eight radial locations and eight times during the FRC lifetime. While the measurements share some salient features with the simulation, they are 4.5x larger, suggesting a higher fast-ion content than the Monte Carlo distribution. Highly Doppler-shifted beam radiation is also measured with a high-speed camera and is spatially well-correlated with FIDASIM. Having shown the feasibility of FIDA on C-2U, we will further explore the use of FIDA on the upgraded C-2W machine to estimate fast-ion densities and to infer the local fast-ion distribution function. $^{\mathrm{1}}$M. Binderbauer \textit{et al}, Phys. of Plasmas \textbf{22}, 056110 (2015) $^{\mathrm{2}}$W.W. Heidbrink., Rev. Sci. Instr. \textbf{81,} 10D727 (2010)