Theory-based coherent structure transport model for the Tokamak SOL region

Managing plasma exhaust heat on the divertor surface is a major challenge for future tokamaks. If the heat-flux width λ_q is inversely proportional to the poloidal magnetic field Β_p in ITER, it will fall below 1 mm during ITER's H-mode, requiring radiative divertor mitigation strategies. Notably, coherent structure turbulence (blob) can possibly widen λ_q.  While first-principles kinetic edge codes offer a reliable way to study this, their application is limited due to complex geometry and high computational requirements.

 

To overcome these challenges, we construct a theory-based model for coherent structure transport (CST) in a turbulence-limited scrape-off layer (SOL). This model utilizes the SOLPS-ITER steady-state solution for the 2D electrostatic potential and background plasma density equilibrium, and a theoretically parameterized turbulent field with minimal input parameters. The transport is measured through the evolution of gyrokinetic particles in these combined fields.

 

Our model, first validated against the Goldston model and its λ_q scaling law, establishes the foundation for further analysis. We then introduce the 2D electrostatic potential and a turbulent field of non-interacting blobs, characterized by a comprehensive theoretical description. The fluid vorticity equation under different conditions (such as sheath-connected, spinning, collisional, inertial, etc.) is used to determine the blob potential.  This enables a study of blob transport and its effect on λ_q.