Experimental and synthetic measurements of polarized synchrotron emission from runaway electrons in Alcator C-Mod

Journal article, 2019

© 2019 IAEA, Vienna. 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 spatial profiles of the polarization angle pol and the fraction fpol of detected light that was linearly-polarized. Spatial transitions in 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 pol flips by 90° and fpol is minimal. Using Soft, we determine the dominant RE pitch angle which reproduces measured pol and fpol values. The spatiotemporal evolutions of pol and fpol 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.