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Report No.

Spatio-temporal evolutions of L-H and H-L transitions in the one-dimensional model

Miki, Kazuhiro; Diamond, P. H.*; Schmitz, L.*; McDonald, D. C.*; Gurcan, O.*; Tynan, G. R.*

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, we investigate ELM-free H-L back transition dynamics. 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-down 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 profile gradient is characterized by the Nusselt number. Relative hysteresis of temperature gradient vs density gradient is sensitive to the pedestal Prandtl number. We expect the H-mode to be somewhat more resilient in density than in temperature.



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