Turbulence-Driven Edge-Localized-Mode-Free High-Confinement Mode with Divertor Detachment in a Metal-Wall Tokamak

Area of Science:

• Fusion energy research
• Plasma physics
• Tokamak reactor technology

Background:

• Achieving stable, high-confinement plasma is crucial for fusion energy.
• Edge-localized modes (ELMs) are disruptive instabilities that limit plasma performance.
• Metal-wall tokamaks present unique challenges for plasma control.

Purpose of the Study:

• To demonstrate a novel edge-localized-mode-free (ELM-free) high-confinement plasma regime.
• To investigate the underlying physical mechanisms enabling this ELM-free state.
• To assess the applicability of this regime for future fusion reactors like ITER.

Main Methods:

• Experimental operation of the Experimental Advanced Superconducting Tokamak (EAST) with a metal wall.
• Inducing divertor partial detachment and analyzing its effect on pedestal properties.
• Utilizing gyrokinetic simulations to identify and characterize plasma turbulence.
• Investigating the role of trapped electron modes (TEMs) in maintaining the ELM-free state.

Main Results:

• Successfully demonstrated a minute-scale, ELM-free high-confinement plasma regime.
• Identified a new mechanism where divertor detachment leads to increased pedestal temperature gradients and high-frequency broadband turbulence.
• Gyrokinetic simulations confirmed this turbulence as a temperature-gradient-driven trapped electron mode (ηe-TEM).
• The ηe-TEM was shown to drive outward transport, stabilizing the pedestal and maintaining the ELM-free state.

Conclusions:

• The demonstrated ELM-free regime is compatible with divertor partial detachment and enhanced pedestal performance.
• This regime is particularly promising for ITER due to anticipated favorable conditions for ηe-TEM excitation.
• The integrated scenario offers a viable solution for managing heat loads and impurities in future fusion reactors.