Non-thermal electron cyclotron emission during runaway plateau in tokamak disruptions from a highly anisotropic dielectric tensor

Abstract

During the runaway plateau phase in a tokamak, a cold background electron temperature of O(1 eV) forbids the onset of kinetic instability due to strong collisional damping. Nevertheless, non-thermal ECE anomalies at the level of 100 eV to keV have been observed in this phase without externally injected waves. To explore this, we characterize a highly anisotropic hot plasma medium with a Gaussian pitch-angle distribution. We derive an analytic hot plasma dielectric tensor, yielding direct expressions for the non-thermal emission coefficients and the kinetic instability drive rate. These analytic forms are verified against the KIAT and SYNO codes at small pitch-angle spread and are numerically complemented at large pitch-angle spread. Using the method of images, we define a fictitious global temperature of the entire plasma medium as measured by a horizontal ECE system. Because this representative medium temperature can exceed the keV level, the radiative temperature measured under incomplete wall reflection can be highly non-thermal without invoking any kinetic instability. This interpretation provides a conceptual basis for quantitative validation against experimental ECE measurements under realistic conditions.