Investigations of detachment bifurcations in DIII-D with UEDGE*

UEDGE simulations with cross-field drifts indicate that changes in the divertor electric potential can be largely responsible for the experimentally observed bifurcated onset of detached conditions in DIII-D. Partially detached divertor conditions are presently favored for ITER for optimal plasma performance with tolerable divertor heat loads. Divertor Thomson scattering measurements in DIII-D show that in high confinement mode plasmas with the B$\times$$\bigtriangledown$B-drift towards the active X-point, the low field side (LFS) divertor plate undergoes a rapid transition from well attached to fully detached conditions within a few percent increase in plasma density. UEDGE simulations indicate that at the onset of LFS detachment, the poloidal E$\times$B drift in the private flux region is reduced from about 15$\%$ of the LFS recycling flux to less than 1$\%$, effectively eliminating a strong particle sink in the LFS divertor. This increases the LFS divertor particle content further leading to fully detached conditions.