Speaker
Description
Electron Cyclotron Emission (ECE) diagnostics have been widely adopted to measure local electron temperature in tokamaks assuming a Maxwellian electron distribution [1]. The presumption of local temperature measurement often fails for ECE emitted by nonthermal electrons, notably runaway electrons (REs). It is necessary to identify the nonthermal electron distribution function and correct the emissivity and absorption coefficients, which remains a challenging inverse problem in interpreting ECE signals in RE experiments. To address the challenge, we develop an analytic model that adopts a constrained characterization of nonthermal electrons, namely, the marginally stable runaway electron distribution function [2]: The RE distribution relaxes to a stable equilibrium where the kinetic drive from the anomalous Doppler resonance (l=-1) balances the collisional damping of whistler waves. We investigate spectral signatures of nonthermal ECE using the nonthermal emissivity and absorption coefficients computed from the marginally stable distribution. The modeling results confirm that the RE population generates distinct down-shifted nonthermal emission features at frequencies corresponding to the thermal bulk. As a model validation activity, we show a comparison between the radiation intensity reconstructed from our nonthermal ECE model and the measurements in KSTAR Quiescent Runaway Electron (QRE) discharges. Our analytic wave coefficient models will be efficiently implemented into synthetic ECE reconstruction frameworks, offering a resolution in analyzing nonthermal ECE signals in QRE plasmas.
References
[1] J. Leem, J. Jo, and G. S. Yun, Phys. Plasmas 26 (2019) 043303
[2] B. N. Breizman and D. I. Kiramov, Phys. Plasmas 30 (2023) 022301