Triggers for plasma detachment bifurcation in the edge divertor region of tokamaks

TL;DR

This paper identifies the trigger mechanism for plasma detachment bifurcation in tokamak divertors, showing how a radiation front crossing and flow reversal lead to rapid temperature drops and deep detachment, crucial for plasma control.

Contribution

It reveals the detailed bifurcation mechanism in tokamak divertors through advanced simulations, highlighting the role of radiation front crossing and flow reversal in detachment transitions.

Findings

Electron temperature cliff at the outer target is linked to bifurcation.

Flow reversal occurs within 0.5 ms below the X-point.

Deep detachment develops within 3 ms after bifurcation.

Abstract

We report the discovery of the trigger for detachment bifurcation phenomenon in tokamak divertors, revealed through steady-state and time-dependent UEDGE simulations: The observed electron temperature cliff at the outer target in DIII-D H-mode plasmas with ion $B\times \nabla B$ drift driven into the active divertor results from a bifurcation-induced $T_e$ drop above the X-point accompanied by reversal of the $E\times B$ flow pattern in the private flux region. Time-dependent simulations reveal a two-phase transition mechanism: the high-field-side radiation front first extends across the last closed flux surface and stabilizes above the X-point, causing local $T_e$ to drop from $\sim 70\,\mathrm{eV}$ to $\sim 10\,\mathrm{eV}$ and inducing $E\times B$ flow reversal in a thin layer below the X-point, which lasts $< 0.5\,\mathrm{ms}$; Flow reversal below the X-point subsequently triggers the sharp drop in outer target temperature on a timescale of $1-2\,\mathrm{ms}$, establishing deep detachment a few ms thereafter. A bifurcation transition occurs when the high-field-side radiation front crosses the separatrix while the outer divertor remains attached, with the $T_e$ cliff manifesting distinctly when the outer target $T_e \gtrsim 10\,\mathrm{eV}$ prior to the bifurcation. These results demonstrate that the bifurcation is linked to in-out divertor asymmetry and asymmetric radiation front evolution.