The role of nonlinear absorption of an ECR beam for fusion plasma pre-ionization

Abstract

This study investigates the nonlinear interactions between electrons and electron-cyclotron resonance (ECR) heating beams in magnetized, low-temperature fusion plasmas, focusing on enhancing pre-ionization efficiency. We analyse key parameters affecting ECR absorption, including phase angle, temperature inhomogeneity, magnetic field gradients, and beam characteristics like width and frequency. Using 2D simulations for these low temperature plasmas, we demonstrate electrons gain energy through nonlinear trapping in velocity space near the resonance layer, achieving energy levels up to 1 keV under optimal conditions. Notably, narrow beam widths allow electrons to reach higher energy levels more efficiently than broader beams, highlighting the spatial localization of the nonlinear interactions to regions where the field is strong and frequency mismatch is small. Our findings show that in devices with lower-frequency ECR systems, such as TCV (82.7 GHz), electrons can gain up to 800 eV, while higher-frequency devices, like ASDEX-Upgrade (140 GHz) and ITER (170 GHz), achieve lower energy gains (480 eV and 420 eV, respectively) under similar conditions. These insights are directly applicable to improving ECR heating strategies, informing design choices in future fusion devices to optimize energy transfer during plasma breakdown and startup.