Physics of the trigger for a detachment bifurcation

This work investigates the physics underlying the onset of detachment bifurcation based on UEDGE simulation results. Steady-state UEDGE simulations in a DIII-D configuration reveal that a sharp drop in electron temperature at the outer target (an electron temperature cliff with ion BxGradB drift directed into the divertor) with increasing upstream density is correlated with a reversal of the E×B flow beneath the X-point in the private flux region (PFR). This flow reversal coincides with the high-field side (HFS) radiation front moving into the last closed flux surface (LCFS). Time-dependent UEDGE simulations across the bifurcation further indicate that, prior to the T_e cliff, increasing density drives the HFS radiation front into the LCFS, cooling electrons above the X-point and causing a bifurcation-like drop in T_e to < 10 eV, and lowering the local potential. This leads to a reversed electric field around the X-point, thus causing the reversal of E×B flow beneath the X-point. The reversed E×B flow then rapidly transports particles from the inner to the outer divertor, leading to a sharp drop in Te at the outer target. This transition occurs on a timescale of ~ 1 ms.

As a comparison, in steady-state UEDGE simulations of a KSTAR configuration with the carbon divertor from attached to detached outer divertor, the HFS radiation front does not enter the LCFS when the outer divertor is detached, therefore, the potential drop above the X-point and the following bifurcation of the E×B flow reversal are not present in KSTAR.