Spatio-temporal evolution of the H → L back transition
- WCI Center for Fusion Theory, National Fusion Research Institute, Daejeon 305-333 (Korea, Republic of)
- University of California, Los Angeles, California 90095 (United States)
- JET-EFDA, Culham Centre for Fusion Energy, Arbingdon (United Kingdom)
- Laboratorio Nacional de Fusión, Asociación Euratom-CIEMAT, Madrid (Spain)
- LPP, Ecole Polytechnique, CNRS (France)
- Center for Momentum Transport and Flow Organization, University of California, San Diego, California 92093 (United States)
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=χ{sub i,turb}/χ{sub i,neo}. Relative hysteresis of temperature gradient vs density gradient is sensitive to the pedestal Prandtl number, where Pr{sub ped}=D{sub ped}/χ{sub i,neo}. We expect the H-mode to be somewhat more resilient in density than in temperature.
- OSTI ID:
- 22228026
- Journal Information:
- Physics of Plasmas, Vol. 20, Issue 6; Other Information: (c) 2013 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); ISSN 1070-664X
- Country of Publication:
- United States
- Language:
- English
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