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Journal Articles

Spatio-temporal evolution of the L$$rightarrow$$H and H$$rightarrow$$L transitions

Miki, Kazuhiro; Diamond, P. H.*; Fedorczak, N.*; G$"u$rcan, $"O$. D.*; Malkov, M.*; Lee, C.*; Kosuga, Yusuke*; Tynan, G. R.*; Xu, G. S.*; Estrada, T.*; et al.

Nuclear Fusion, 53(7), p.073044_1 - 073044_10, 2013/07

https://doi.org/10.1088/0029-5515/53/7/073044
 Times Cited Count：25 Percentile：72.36(Physics, Fluids & Plasmas)

Understanding the L$$rightarrow$$H and H$$rightarrow$$L transitions is crucial to successful ITER operation. In this paper we present novel theoretical and modelling study results on the spatio-temporal dynamics of the transition. We place a special emphasis on the role of zonal flows and the micro$$rightarrow$$macro connection between dynamics and the power threshold dependences. The model studied evolves five coupled fields in time and one space dimension, in simplified geometry. The content of this paper is (a) the model fundamentals and the space-time evolution during the L$$rightarrow$$I$$rightarrow$$H transition, (b) the physics origin of the well-known $$nabla$$ B-drift asymmetry in power threshold, (c) the role of heat avalanches in the intrinsic variability of the L$$rightarrow$$H transition, (d) the dynamics of the H$$rightarrow$$L back transition and the physics of hysteresis.

Journal Articles

Spatio-temporal evolution of the H$$rightarrow$$L back transition

Miki, Kazuhiro; Diamond, P. H.*; Schmitz, L.*; McDonald, D. C.*; Estrada, T.*; G$"u$rcan, $"O$. D.*; Tynan, G. R.*

Physics of Plasmas, 20(6), p.062304_1 - 062304_9, 2013/06

https://doi.org/10.1063/1.4812555
 Times Cited Count：20 Percentile：64.31(Physics, Fluids & Plasmas)

Since ITER will operate close to threshold and with limited control, the H$$rightarrow$$L back transition is a topic important for machine operations as well as physics. Using a reduced mesoscale model, we investigate ELM-free H$$rightarrow$$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. Relative hysteresis of temperature gradient vs density gradient is sensitive to the pedestal Prandtl number.

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