Synchrotron spectra, images, and polarization measurements from runaway electrons in Alcator C-Mod*

In Alcator C-Mod, runaway electrons (REs) generated during flattop plasma discharges emit synchrotron radiation (SR) in the visible wavelength range. Spectrometers, cameras, and the Motional Stark Effect diagnostic measure absolutely-calibrated spectra, distortion-corrected images, and polarization information of SR, respectively. Due to the complex interplay of the RE phase-space distribution, magnetic topology, and detector geometry, the synthetic diagnostic SOFT [M. Hoppe, et al., Nucl. Fusion 58 (2018)] is used to simulate all three measurements. As inputs, RE distributions are calculated using a test-particle model [J.R. Martin-Solis, et al., Phys. Plasmas 5 (1998)] and kinetic solver CODE [M. Landreman, et al., Comput. Phys. Commun. 185 (2014)]. Synchrotron spectra measured at three magnetic fields (B₀ = 2.7, 5.4, and 7.8 T) indicate a decrease in RE energy as higher B enhances synchrotron power loss [R.A. Tinguely, et al., Nucl. Fusion 58 (2018)]. MHD activity appears to increase RE transport and decrease the RE beam size as observed in synchrotron images. Profiles of linearly-polarized SR and polarization angle are explored as a novel RE diagnostic.