The effects of kinetic instabilities on the electron cyclotron emission from runaway electrons

TL;DR

This study demonstrates that kinetic instabilities significantly influence non-thermal electron cyclotron emission from runaway electrons, with simulations aligning well with experimental data and highlighting the role of wave-particle interactions.

Contribution

The paper introduces a kinetic simulation model including whistler wave instabilities to explain non-thermal ECE from runaway electrons, showing the importance of scattering effects.

Findings

Runaway electrons in low-energy, large pitch angle regime dominate non-thermal ECE.

Non-thermal ECE is nonlocal and has a higher emission-absorption ratio than thermal ECE.

ECE spectrum becomes flatter as runaway electron population increases.

Abstract

In this paper we show that the kinetic instabilities associated with runaway electron beams play an essential role for the production of high-level non-thermal electron-cyclotron-emission (ECE) radiation. Most of the non-thermal ECE comes from runaway electrons in the low-energy regime with large pitch angle, which are strongly scattered by the excited whistler waves. The power of ECE from runaway electrons is obtained using a synthetic diagnostic model based on the reciprocity method. The electron distribution function is calculated using a kinetic simulation model including the whistler wave instabilities and the quasilinear diffusion effects. Simulations based on DIII-D low-density discharge reproduces the rapid growth of the ECE signals observed in DIII-D experiments. Unlike the thermal ECE where radiation for a certain frequency is strongly localized inside the resonance region, the non-thermal ECE radiation from runaway electrons is nonlocal, and the emission-absorption ratio is higher than that of thermal electrons. The runaway electron tail is more significant for ECE with higher frequencies, and the ECE spectrum becomes flatter as RE population grows. The nonlinear behavior of the kinetic instabilities is illustrated in the osculations of the ECE waves. The good agreement with the DIII-D experimental observations after including the kinetic instabilities clearly illustrate the significance of the scattering effects from wave-particle interactions, which can also be important for runaway electrons produced in disruptions.