Modeling electron cyclotron emission signatures from runaway electrons in tokamak plasmas

The presence of suprathermal electrons and runaway electrons (REs) often causes a significant increase in measured electron cyclotron emission (ECE) radiation. As a result, while ECE diagnostics are commonly used to conduct electron temperature measurements, they can also be used to detect REs. A more comprehensive understanding of the contribution to ECE radiation from a nonthermal electron population would potentially allow the use of ECE to diagnose the RE energy distribution. This work develops a model to investigate ECE signatures from an arbitrary electron momentum distribution by calculating the emission and absorption coefficients in the radiation transport equation using a relativistic framework [R. W. Harvey et al., Phys. Fluids B 5, 446 (1993)]. The result is benchmarked against an analytical formula for a Maxwellian electron distribution [M. Austin et al., Phys. Plasmas 3, 3725 (1996)]. The simulations reveal a stronger effect on ECE from REs at oblique viewing angles. They also show a link between RE distribution and ECE emissivity at different frequencies. With future development, this work would provide a substantial advantage for RE detection and measurement in ITER and future reactor relevant tokamaks.