Spatio-temporal evolution of the H → L back transition

Since ITER will operate close to threshold and with limited control, the H -> L back transition is a topic important for machine operations as well as physics. Using a reduced mesoscale model [Miki et al., Phys. Plasmas 19, 092306 (2012)], we investigate ELM-free H -> L back transition dynamics in order to isolate transport physics effects. Model studies indicate that turbulence spreading is the key process which triggers the back transition. The transition involves a feedback loop linking turbulence and profiles. The I-phase appears during the back transition following a slow power ramp down, while fast ramp-downs reveal a single burst of zonal flow during the back transition. The I-phase nucleates at the pedestal shoulder, as this is the site of the residual turbulence in H-mode. Hysteresis in the profile gradient scale length is characterized by the Nusselt number, where Nu = chi(i,turb)/chi(i,neo). Relative hysteresis of temperature gradient vs density gradient is sensitive to the pedestal Prandtl number, where Pr-ped = D-ped/chi(i,neo). We expect the H-mode to be somewhat more resilient in density than in temperature. (C) 2013 AIP Publishing LLC.