README.md

2026ZRZS

Codex/ChatGPT (July 2026)

Summary

Steady-state and time-dependent UEDGE simulations identify the trigger of a detachment bifurcation associated with the abrupt outer-target electron-temperature cliff observed in DIII-D. In the forward toroidal-field configuration, a high-field-side radiation front crosses the last closed flux surface while the outer divertor remains attached and settles above the X-point. The local electron temperature then collapses from about $70,\mathrm{eV}$ to $10,\mathrm{eV}$, changing the electrostatic potential and reversing the $E\times B$ flow in a thin private-flux-region layer below the X-point in less than $0.5,\mathrm{ms}$. This reversal transports plasma toward the outer leg and initiates feedback that drops the outer-target temperature within $1$--$2,\mathrm{ms}$, followed by deep detachment. The bifurcation can occur without a visually sharp cliff when the target is already cold. The simulations further connect the transition to in--out divertor asymmetry: the corresponding radiation-front behavior and deep detachment are absent for reverse field. Direct experimental confirmation of the predicted flow reversal remains needed.

Contributions

1. Identified private-flux-region $E\times B$ flow reversal as the immediate trigger of the outer-target temperature cliff.
2. Resolved the detachment transition into two successive sub-millisecond and millisecond phases.
3. Connected radiation-front penetration across the separatrix to the X-point temperature and potential collapse.
4. Distinguished the general bifurcation from the special case in which it appears as a sharp temperature cliff.
5. Related bifurcation and deep-detachment access to toroidal-field direction and in--out divertor asymmetry.