Experimental and synthetic measurements of polarized synchrotron emission from runaway electrons in Alcator C-Mod
This paper presents the first experimental analysis of polarized synchrotron emission from relativistic runaway electrons (REs) in a tokamak plasma. Importantly, we show that the polarization information of synchrotron radiation can be used to diagnose spatially-localized RE pitch angle distributions. Synchrotron-producing REs were generated during low density, Ohmic, diverted plasma discharges in the Alcator C-Mod tokamak. The ten-channel Motional Stark Effect diagnostic was used to measure
... tial profiles of the polarization angle θ_pol and the fraction f_pol of detected light that was linearly-polarized. Spatial transitions in θ_pol of 90—from horizontal to vertical polarization and vice versa—are observed in experimental data and are well-explained by the gyro-motion of REs and high directionality of synchrotron radiation. Polarized synchrotron emission is modeled with the synthetic diagnostic SOFT; its output Green's (or detector response) functions reveal a critical RE pitch angle at which θ_pol flips by 90 and f_pol is minimal. Using SOFT, we determine the dominant RE pitch angle which reproduces measured θ_pol and f_pol values. The spatiotemporal evolutions of θ_pol and f_pol are explored in detail for one C-Mod discharge. For channels viewing REs near the magnetic axis and flux surfaces q = 1 and 4/3, disagreements between synthetic and experimental signals suggest that the sawtooth instability may be influencing RE dynamics. Furthermore, other sources of pitch angle scattering, not considered in this analysis, could help explain discrepancies between simulation and experiment.