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Miki, Kazuhiro; Diamond, P. H.*; Hahn, S.-H.*; Xiao, W. W.*; Grcan, . D.*; Tynan, G. R.*
Physics of Plasmas, 20(8), p.082304_1 - 082304_11, 2013/08
Times Cited Count:14 Percentile:52.63(Physics, Fluids & Plasmas)We report on model studies of stimulated LH transitions. These studies use a reduced mesoscale model. Model studies reveal that LH transition can be triggered by particle injection into a subcritical state. Particle injection changes edge mean flow shear via changes of density and temperature gradients. The change of edge mean flow shear is critical to turbulence collapse and the subsequent stimulated transition. For low ambient heating, strong injection is predicted to trigger a transient turbulence collapse. Repetitive injection at a period less than the lifetime of the collapsed state can thus maintain the turbulence collapse. The total number of injected particles required is much smaller than that required for a transition by gas puffing. We thus show that internal injection is more efficient than gas puffing of comparable strength. We also observe that zonal flows do not play a critical role in stimulated transitions.
Miki, Kazuhiro; Diamond, P. H.*; Fedorczak, N.*; Grcan, . 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
Times Cited Count:25 Percentile:72.11(Physics, Fluids & Plasmas)Understanding the LH and HL 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 micromacro 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 LIH transition, (b) the physics origin of the well-known B-drift asymmetry in power threshold, (c) the role of heat avalanches in the intrinsic variability of the LH transition, (d) the dynamics of the HL back transition and the physics of hysteresis.
Miki, Kazuhiro; Diamond, P. H.*; Schmitz, L.*; McDonald, D. C.*; Estrada, T.*; Grcan, . D.*; Tynan, G. R.*
Physics of Plasmas, 20(6), p.062304_1 - 062304_9, 2013/06
Times Cited Count:20 Percentile:64.05(Physics, Fluids & Plasmas)Since ITER will operate close to threshold and with limited control, the HL back transition is a topic important for machine operations as well as physics. Using a reduced mesoscale model, we investigate ELM-free HL 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.
Miki, Kazuhiro; Diamond, P. H.*; Hahn, S.-H.*; Xiao, W. W.*; Grcan, . D.*; Tynan, G. R.*
no journal, ,
A central issue in H-mode physics is to achieve control, not only understanding. Work in control has focused mainly on fueling by pellet injection at the near edge. We here introduce a developed one-dimensional mesoscale model, to report on the L-H transition stimulated by pellet injection. For highly heated plasmas, transition occurs spontaneously; when heat flux increases, a strong zonal flow is sufficiently excited, leading to I-phase oscillations or a single burst. When the zonal flow is excited, turbulence is reduced, allowing an ion pressure gradient to steepen. Then, mean flow shear increases to lock in the H-mode. On the other hand, studies reveal that L-H transition can be triggered by particle injection into a subcritical state. We also observe that zonal flows do not play a critical role in stimulated transitions.