Dynamical Pathway to Radiative Divertor Driven by Transient X-Point Vortex in Tokamaks

Area of Science:

• Plasma Physics
• Fusion Energy Research
• Tokamak Operations

Background:

• X-point radiator (XPR) experiments in the WEST tokamak demonstrate stable, long-pulse operation (approx. 70s) of the XPR regime.
• The XPR regime effectively mitigates divertor heat loads and tungsten contamination while preserving core plasma performance.
• Understanding the rapid formation dynamics of the XPR regime is essential for its implementation in future fusion reactors.

Purpose of the Study:

• Investigate the macroscopic dynamics of the rapid transition from an attached divertor to a stable XPR regime.
• Identify the key mechanisms driving the millisecond-scale transition to the XPR state.
• Explore how plasma conditions and particle transport influence XPR formation and stability.

Main Methods:

• Utilized time-dependent fluid simulations incorporating magnetic drifts.
• Investigated the transition dynamics from an attached divertor to a stable XPR regime.
• Qualitatively reproduced the experimentally observed millisecond-scale transition sequence.

Main Results:

• Identified a transient X-point-centered vortex as the primary mechanism driving the transition to the XPR regime.
• Simulations qualitatively reproduced the millisecond-scale transition sequence observed in WEST experiments.
• The vortex regulates cold particle and impurity entry, increasing neutral density and impurity concentration without edge overcooling.

Conclusions:

• The X-point-centered vortex is crucial for enabling stable XPR access by managing particle and impurity transport.
• This vortex prevents the onset of unstable XPR conditions, such as The Multifaceted Asymmetric Radiation From the Edge (MARFE).
• The vortex sustains a self-amplifying cycle through particle mixing and gradient steepening, ultimately enabling stable, long-pulse XPR operation.