The ECE radiation signature of runaway electrons in optically thick and thin plasmas*

Dedicated forward modeling is applied to interpret Electron Cyclotron Emission (ECE) radiation from runaway electrons (>500 keV) on the DIII-D tokamak. ECE measures the radiation temperature profile T_e,rad (R) using the 2^nd harmonic cold resonance in a tokamak. It is found the runaway’s effect on  depends heavily on the plasma optical depth. For T_e,rad (R)  near the plasma axis where ECE is optically thick, only the runaways at the outboard side of the cold resonance contribute to the radiation. In this case, the increased T_e,rad (R)  by runaway is made through emission from the relativistic downshift of the 3^rd or 4^th harmonic ECE resonance, and a large pitch angle is needed for runaways to make noticeable change on T_e,rad (R). For T_e,rad (R)  near the plasma edge where the ECE is optically thin, the downshifted 2^nd harmonic radiation from runaways at the inboard side play the dominant role increasing the radiation. In this case, a large pitch angle is not required for runaways to make a noticeable change on T_e,rad (R) . The knowledge from the modeling successfully explains T_e,rad (R)  in a DIII-D runaway discharge, where  T_e,rad (R) close to the axis is unsymmetrical between the low and high field side, and the edge T_e,rad (R)  is significantly higher than the thermal value. Future application will involve interpreting T_e,rad (R) when whistler waves scatter the runaways.