Impact of relative phase shift on inward turbulent spreading

H., C.; Xu, X. Q.; Xi, Peng; Xia, Tianyang

doi:10.1063/1.4905644

Cited by 15 publications

(13 citation statements)

References 20 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…It has been observed that turbulence spreading persists in the region of reversed shear where local turbulence excitation is negligible, although no systematic parameter scan and analyses have been attempted. Finally, inward turbulence spreading of electron temperature fluctuation has been observed from a BOUT++ fluid simulation of H-mode pedestal collapse [81]. The spreading stops at the position where the radial turbulent correlation length is shorter than the distance between the rational surfaces.…”

Section: E Relation To Transport Barrier Dynamicsmentioning

confidence: 90%

Mesoscopic Transport Events and the Breakdown of Fick’s Law for Turbulent Fluxes

Hahm

Diamond

2018

J. Korean Phys. Soc.

View full text Add to dashboard Cite

This paper presents a pedagogical review of the physics of mesoscopic transport events and their role in the breakdown of Fick's Law for turbulent transport in magnetically confined plasma. It is now clear that the conventional picture of localized turbulence and quasi-linear calculation of fluxes fails to address and account for the phenomenology of tokamak transport. One key issue is the observed departure from the expected gyro-Bohm transport scaling. The causes of this breakdown of Fickian thinking include turbulent avalanching and pulse propagation (turbulence spreading). Both are mesoscopic transport events, and both tend to de-localize the flux-gradient relation. Turbulence spreading is the process of self-scattering and expansion of a slug or other local exciton of turbulence. Spreading is described by theoretically-motivated, phenomenological reaction-diffusion models for the turbulence activity (intensity) field, much in the spirit of Ginzburg-Landau theory. Such models imply that spreading will occur by propagation of intensity fronts. After discussing the basic theory, this paper presents several critical tests of turbulence spreading models using gyrokinetic simulation. Applications include rho-star scaling, penetration of transport barriers and core-edge coupling. Relevant experiment-theory comparisons are addressed, as well. Avalanching refers to a process whereby correlated topplings of nearby localized cells overturn sequentially and drive a burst of transport. Avalanching is a process intrinsic to systems that support a broad range of scales l between a cell size ∆ and system size L, i.e. ∆ < l < L. Avalanching is also a natural way to produce transport events on scales that exceed the cell size or correlation length. Therefore, the PDF (probability distribution function) of avalanches as a function of l is a crucial quantity, necessary for predicting confinement in a system like ITER, with a very large-scale separation between L and ∆. Avalanching emerged from the theory of selforganized criticality but is a more general phenomenon. The paper traces the intellectual prehistory of avalanching through the advent of self-organized criticality. Special focus is devoted to reduced continuum models of avalanching. The physics of avalanching in confined plasma is discussed in detail, via several multi-faceted comparisons to flux-driven fluid and gyrokinetic simulations. The dominance of bursty, large transport events in the flux is identified. Evidence for avalanching in basic and confinement experiments is summarized. The paper concludes with sections on selected special topics, a discussion of the relation between turbulence spreading and avalanching, and a list of possible future directions. Throughout the paper, an effort is made to set fusion theory and phenomenology in the context of ideas discussed in the broader scientific community.

show abstract

Section: E Relation To Transport Barrier Dynamicsmentioning

confidence: 90%

Mesoscopic Transport Events and the Breakdown of Fick’s Law for Turbulent Fluxes

Hahm

Diamond

2018

J. Korean Phys. Soc.

View full text Add to dashboard Cite

show abstract

“…The latest BOUT++ studies show an emerging understanding of dynamics of ELM crashes and the consistent collisionality scaling of ELM energy losses with the world multitokamak database [11]. The six-field two-fluid module has been developed to simulate the ELM crash in shifted-circular geometry [12,13]. The typical values for transport coefficients in the saturation phase after ELM crashes are…”

Section: Introductionmentioning

confidence: 99%

“…ir er m 2 s −1 . The theoretical and simulation results of a gyro-Landau-fluid (GLF) extension of the BOUT++ code are summarized in [13,14], which contributes to increasing the physics understanding of ELMs.…”

Section: Introductionmentioning

confidence: 99%

Nonlinear fluid simulation of particle and heat fluxes during burst of ELMs on DIII-D with BOUT++ code

Xia

2015

Nucl. Fusion

Self Cite

View full text Add to dashboard Cite

In order to study the distribution and evolution of the transient particle and heat fluxes during edge-localized mode (ELM) bursts, a BOUT++ six-field two-fluid model based on the Braginskii equations with non-ideal physics effects is used to simulate pedestal collapse in divertor geometry. The profiles from the DIII-D H-mode discharge #144382 with fast target heat flux measurements are used as the initial conditions for the simulations. A fluxlimited parallel thermal conduction is used with three values of the flux-limiting coefficient α j , free streaming model with α = 1 j , sheath-limit with α = 0.05 j , and one value in between.The studies show that a 20 times increase in α j leads to ∼6 times increase in the heat flux amplitude to both the inner and outer targets, and the widths of the fluxes are also expanded. The sheath-limit model of flux-limiting coefficient is found to be the most appropriate one, which shows ELM sizes close to the measurements. The evolution of the density profile during the burst of ELMs of DIII-D discharge #144382 is simulated, and the collapse in width and depth of n e are reproduced at different time steps. The growing process of the profiles for the heat flux at divertor targets during the burst of ELMs measured by IRTV (infrared television) is also reproduced by this model. The widths of heat fluxes towards targets are a little narrower, and the peak amplitudes are twice the measurements possibly due to the lack of a model of divertor radiation which can effectively reduce the heat fluxes. The magnetic flutter combined with parallel thermal conduction is found to be able to increase the total heat loss by around 33% since the magnetic flutter terms provide the additional conductive heat transport in the radial direction. The heat flux profile at both the inner and outer targets is obviously broadened by magnetic flutter. The lobe structures near the X-point at LFS are both broadened and elongated due to the magnetic flutter.

show abstract

“…Beyond the ELM energy losses, more sophisticated plasma models are obviously required to understand experimental observations and gain insight into possible new dynamics of particle losses and the ELM energy loss for different species. 7,13 To study the physics of nonlinear P-B mode dynamics, we choose circular cross-section toroidal equilibrium modified (cbm18_dens6) near the marginal P-B instability threshold, which has been simulated for H-mode plasmas with steep pressure gradient at the edge, but here with different density and temperature profiles and therefore large bootstrap current. 11 Major parameters are a minor radius a ¼ 1.2 m, major radius R 0 ¼ 3:4 m, toroidal field on axis B 0 ¼ 2T, the pedestal pressure 2/3 of the axis pressure, and a pedestal pressure half width 7% of the poloidal flux.…”

mentioning

confidence: 99%

Impact of the pedestal plasma density on dynamics of edge localized mode crashes and energy loss scaling

2014

Physics of Plasmas

Self Cite

View full text Add to dashboard Cite

The latest BOUTþþ studies show an emerging understanding of dynamics of edge localized mode (ELM) crashes and the consistent collisionality scaling of ELM energy losses with the world multitokamak database. A series of BOUTþþ simulations are conducted to investigate the scaling characteristics of the ELM energy losses vs collisionality via a density scan. Linear results demonstrate that as the pedestal collisionality decreases, the growth rate of the peeling-ballooning modes decreases for high n but increases for low n (1 < n < 5), therefore the width of the growth rate spectrum c(n) becomes narrower and the peak growth shifts to lower n. Nonlinear BOUTþþ simulations show a two-stage process of ELM crash evolution of (i) initial bursts of pressure blob and void creation and (ii) inward void propagation. The inward void propagation stirs the top of pedestal plasma and yields an increasing ELM size with decreasing collisionality after a series of microbursts. The pedestal plasma density plays a major role in determining the ELM energy loss through its effect on the edge bootstrap current and ion diamagnetic stabilization. The critical trend emerges as a transition (1) linearly from ballooning-dominated states at high collisionality to peelingdominated states at low collisionality with decreasing density and (2) nonlinearly from turbulence spreading dynamics at high collisionality into avalanche-like dynamics at low collisionality. V C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4905070]The phenomena of nonlocal transport, such as avalanches or turbulence spreading, are well-known in diverse systems. Examples include, but are not limited to: piles of granular matter, discrete energy dissipating events in earthquakes and solar flares, and turbulent overshoot and penetration in fluid turbulence. Nonlocal dynamics of turbulence, transport, and zonal flows are often observed in plasma turbulence simulations and experiments, such as Edge Localized Modes (ELMs) in tokamaks. ELMs are a common characteristic feature of the tokamak H-mode plasma regime, where the high frequency ELM instability repeats periodically throughout the high confinement mode (H-mode) phase of the discharge. 1 The instability causes quasi-periodic relaxations of the edge pedestal, resulting in a series of hot plasma eruptions on a fast MHD timescale and leading to large energy fluxes to the plasma facing components (PFCs), which will suffer from excessive ablation, fast erosion, or melting. The concern about the survival of PFCs in ITER has sparked intense interest in ELM dynamics and in the parameter scaling of ELM energy loss. Numerous experiments in divertor tokamaks have shown a decrease in the relative type I ELM energy loss with increasing pedestal density (collisionality) over a decade ago. 2 There are attempts to provide an explanation for the dependence of ELM energy loss on collisionality. 3 However, there is as yet no common accepted explanation of the observed scaling neither from analytical theory nor numerical simulations. 4He...

show abstract

Impact of relative phase shift on inward turbulent spreading

Cited by 15 publications

References 20 publications

Mesoscopic Transport Events and the Breakdown of Fick’s Law for Turbulent Fluxes

Mesoscopic Transport Events and the Breakdown of Fick’s Law for Turbulent Fluxes

Nonlinear fluid simulation of particle and heat fluxes during burst of ELMs on DIII-D with BOUT++ code

Impact of the pedestal plasma density on dynamics of edge localized mode crashes and energy loss scaling

Contact Info

Product

Resources

About