Radiation front dynamics at detachment onset in DIII-D H-mode discharges*

Fast imaging of the radiation front in DIII-D discharges reveals that detachment bifurcations are always triggered by an ELM. The rapid bifurcation of divertor conditions (detachment cliff) observed experimentally in DIII-D H-mode discharges is attributed to the effect of collisionality on E×B drifts. This work identifies the trigger for the cliff via radiation front imaging in discharges with different radiators: C, N, and Ne. An ELM precedes every cliff with a 2-4 ms lag, consistent with timescales of D outgassing from tiles. ELM-induced detachment is observed above a threshold in ELM size, and either upstream density or impurity seeding rate. Its duration increases with ELM size, density, and seeding in a phase typically labeled as dithering detachment. Above a certain density, no reattachment between ELMs is observed, leading to a solidly detached state. The histogram of the radiation front location is bimodal, with peak emission either at the strike point or below the X-point, consistent with the T_e distribution. Reattachment timescales following a cliff can increase from 1 ms to several ms when approaching the density needed for sustained detachment. Small ELMs and different fuel retention in metallic walls in future reactors can benefit the detachment onset controllability.