A statistical analysis of avalanching heat transport in stationary enhanced core confinement regimes

Abstract

We present a statistical analysis of heat transport in stationary enhanced confinement regimes obtained from flux-driven gyrofluid simulations. The probability density functions of heat flux in improved confinement regimes, characterized by the Nusselt number, show significant deviation from Gaussian, with a markedly fat tail, implying the existence of heat avalanches. Two types of avalanching transport are found to be relevant to stationary states, depending on the degree of turbulence suppression. In the weakly suppressed regime, heat avalanches occur in the form of quasiperiodic (QP) heat pulses. Collisional relaxation of zonal flow is likely to be the origin of these QP heat pulses. This phenomenon is similar to transient limit cycle oscillations observed prior to edge pedestal formation in recent experiments. On the other hand, a spectral analysis of heat flux in the strongly suppressed regime shows the emergence of a 1/f (f is the frequency) band, suggesting the presence of self-organized criticality (SOC)-like episodic heat avalanches. This episodic 1/f heat avalanches have a long temporal correlation and constitute the dominant transport process in this regime. V

Phase space structures in gyrokinetic simulations of fusion plasma turbulence

“…In Section 4, the zonal flow pattern of the simulation 1/ρ * = 300 is investigated in detail. The radial profile of the zonal flows is characterised by a series of minima and maxima and includes flow reversal [9]. This flow pattern determines the regions where the shear |ρ 0 ∂ r v z | is strong and can modify the radial transport properties [10].…”

“…A remarkable feature of these flux-driven simulation is the self-organisation of the turbulence that couples all available scales from the size of the device, typically the minor radius of the torus a, to the size of the turbulent cells, of the order of the ion Larmor radius ρ i . An outstanding issue is that of turbulence self regulation with on the one hand long range radial transport [6][7][8][9] and on the other hand zonal flows [10]. These combine in a selfconsistent way to determine the level of turbulent transport.…”

Experimental observation of the non-diffusive avalanche-like electron heat transport events and their dynamical interaction with the shear flow structure

“…These non-diffusive transport processes have been observed in many flux-driven fluid [13][14][15][16][17] and gyrokinetic [18][19][20][21][22][23][24][25][26] simulations. In particular, avalanches are near-ballistic radial transport events of heat accompanied by front propagation of fluctuation intensity, arising from nonlinear critical gradient dynamics.…”

A mixed SOC-turbulence model for nonlocal transport and Lévy-fractional Fokker–Planck equation

“…but finite driving rates (i.e., δ 1) such as magnetically confined plasma systems, keeping in mind the natural limitations of associate power-law behavior. Indeed spectra resembling a 1/ f type noise have been observed in edge plasma fluctuations and their connection with SOC has been also discussed [31,33,63,64,65,66,67].…”

“…In magnetically confined fusion plasma, SOC has been proposed as an alternative to the turbulence theoretical framework to explain and control the anomalous particle and heat transport across the magnetic field lines and the extreme transport events that may have destructive effect on very expensive plasma-facing components [31,32,33]. Here by contrast with this way of thinking we suggest that the phenomena of turbulence and SOC are not really separable in tokamaks in the regime of strong nonlinearity and that there is a theoretical possibility that the turbulence fuels the avalanching dynamics due to SOC through inverse cascade of the energy, giving rise to transport events of anomalously large size beyond the range of predictability of the "conventional" SOC.…”