A Brief History and Evolution of Electron Cyclotron Emission Imaging (ECEI) Systems

This paper presents the history and evolution of Electron Cyclotron Emission Imaging (ECEI) systems, from its first applications in the mid 1990-ies until the present. ECEI has emerged as a transformative diagnostic tool for magnetically confined fusion plasmas, providing 2D measurements of electron temperature fluctuations with high resolution (centimeter-scale, and microsecond). Deployed globally on major fusion devices (e.g., ASDEX-Upgrade, DIII-D, KSTAR, EAST), ECEI has enabled critical studies of plasma instabilities, including sawtooth crashes, edge-localized modes (ELMs), and energetic particle driven instabilities. Recent breakthroughs with millimeter-wave system-on-chip technology applications have significantly enhanced ECEI performance, achieving 400× higher signal gain, 85% lower noise, and a 2000x footprint reduction. The 2024 development of radiation-hardened GaN-based receiver chips further ensures compatibility with reactor harsh environments. Additionally, AI-driven analysis has expanded ECEI’s diagnostic capabilities, enabling early disruption prediction, plasma shape detection, and locked mode identification with greater accuracy than the conventional diagnostics. These advancements position millimeter-wave diagnostics as the key for next-generation fusion reactors, meeting demands for compact integration, neutron tolerance, and real-time stability control. Future developments will focus on further optimizing ECEI for Fusion Pilot Plant (FPP) applications, solidifying its role in enabling stable, high-performance plasmas.