Fluctuation entrainment and SOL width broadening in small/grassy ELM regime

Abstract

This study delves into the phenomena of fluctuation within the Scrape-Off Layer (SOL) of tokamak fusion reactors, with a specific focus on its impact on SOL width, particularly during grassy Edge Localized Modes (ELMs). Employing a comprehensive approach involving analysis and simulations, including BOUT++ and UEDGE simulations, we examine the role played by the fluctuation energy flux (${\Gamma }_{\epsilon }$) and various control parameters in shaping the dynamics of fluctuation entrainment. Our findings shed light on the intricate process of transporting fluctuation energy from the pedestal to the SOL, a process influenced by critical factors such as fluctuation correlation length and SOL electric field shear. These insights yield valuable strategies for effectively managing SOL width and achieving plasma detachment, both of which are essential for optimizing fusion reactor performance and ensuring operational stability. Notably, the adoption of H-mode with small/grassy ELMs holds significant promise in addressing three pivotal challenges for future tokamak fusion reactors: minimizing the size of ELMs, expanding the SOL width, and aiding in the detachment of divertor plasma at lower plasma densities, all while avoiding back transitions to low confinement and disruptions.