A new, frequency modulation mechanism for zonal flow pattern formation is presented. The model predicts the probability distribution function of the flow strength as well as the evolution of the characteristic spatial scale. Magnetic toroidicity-induced global phase dynamics is shown to determine the spatial structure of the flow. A key result is the observation that global phase patterning can lead to zonal flow formation in the absence of turbulence inhomogeneity.

We present a new model of turbulence spreading in magnetically confined plasma. A basic question in turbulence spreading is how to sustain finite amplitude fluctuations in a stable subcritical region, where linear dissipation of the turbulence is strong? The answer to this question relies on a consistent treatment of mesoscale temperature profile corrugation and microscale turbulence. We argue that inhomogeneous mixing of the turbulence corrugates the mean temperature profile and that the temperature corrugation then induces subcritical bifurcation of the turbulence. Thus, the system will transition from a metastable “laminar” state to an absolutely stable, excited state. Incorporating spatial coupling of the locally excited turbulent regions, a front forms. This front connects the excited and laminar states and penetrates the linear stable region efficiently. We argue that such bistable turbulence spreading can explain observations of hysteresis in the intensity of L-mode core turbulence.

We demonstrate that E×B shear, V_{E×B}^{'}, governs the dynamics of the cross phase of the peeling-ballooning-(PB-)mode-driven heat flux, and so determines the evolution from the edge-localized (ELMy) H mode to the quiescent (Q) H mode. A physics-based scaling of the critical E×B shearing rate (V_{E×B,cr}^{'}) for accessing the QH mode is predicted. The ELMy H mode to the QH-mode evolution is shown to follow from the conversion from a phase locked state to a phase slip state. In the phase locked state, PB modes are pumped continuously, so bursts occur. In the slip state, the PB activity is a coherent oscillation. Stronger E×B shearing implies a higher phase slip frequency. This finding predicts a new state of cross phase dynamics and shows a new way to understand the physics mechanism for ELMy to the QH-mode evolution.

We examine the influence of noise and Alfvén wave turbulence on magnetic reconnection in a reduced magnetohydrodynamics model. We focus on the dynamics of magnetic helicity density. Helicity conservation is then used to calculate the global reconnection rate in terms of the helicity density flux. Two specific scenarios are explored - noisy reconnection and Alfvén wave turbulent reconnection. For noisy reconnection, the current sheet is assumed to sit in a noisy state, marginal to plasmoid formation instability. The scaling of the reconnection rate in the presence of noise is proportional to (S 20/VAL2)1/11, where S20/VAL2 is the relative amplitude of the noise. We obtain this prediction using a symmetry analysis of the helicity density flux. For Alfvén wave turbulent reconnection, a mean field closure scheme is applied. A reconnection rate proportional to is obtained, where and 〈B〉 are the relative energy of Alfvén wave turbulence and the reconnecting field. The constraint on reconnection rate enforced mean-square magnetic potential conservation is reexamined. A critical magnetic Reynolds number R m, c is identified. For Rm≫ Rm, c, the reconnection rate becomes independent of Spitzer resistivity and thus can be higher than the Sweet-Parker model prediction. Both cases exhibit a weak dependence of the reconnection rate on the amplitude of the turbulence. Therefore, even noise or weak turbulence can trigger fast reconnection if the system is marginally stable. The important distinction between turbulent reconnection and turbulent dissipation of magnetic energy is also discussed. © 2012. The American Astronomical Society. All rights reserved.

We show that the radial electric field (E_{r}) plays a dual role in edge magnetohydrodynamics (MHD) activity. While E_{r} shear (first spatial derivative of E_{r}) dephases radial velocity and displacement, and so is stabilizing, a new finding here is that E_{r} curvature (second spatial derivative of E_{r}) tends to synchronize the radial velocity and displacement, and so destabilizes MHD. As a highlighted result, we analytically demonstrate that E_{r} curvature can destabilize an otherwise stable kink mode, and so form a joint vortex-kink mode. The synergetic effects of E_{r} shear and E_{r} curvature in edge MHD extend the familiar E×B shearing paradigm. This theory thus explains the experimental findings that a deeper E×B well may aggravate edge MHD, and so trigger the formation of the edge harmonic oscillation. A simple criterion linking E_{r} structure and the edge MHD activity is derived.

We present a paradigm for the generation of small scale coherent vortex (SSCV) in drift wave-zonal flow (DW-ZF) turbulence. We demonstrate that phases of DWs can couple coherently, mediated by the ZF shearing. A SSCV is formed when the phases of the DWs are "attracted" to form a stable "phase cluster." We show that the ZF shearing induces asymmetry between "attractive" and "repulsive" phase couplings, so that a net attractive phase coupling results. The turbulent DWs will (partially)synchronize into a stable SSCV at locations, where the attractive phase coupling induced by the ZF shearing exceeds the "detuning" effects by the DW dispersion and random phase scattering. We also discuss the "self-binding" effect of the newly formed SSCV.

We present a new, unified model of transport barrier formation in " elastic" drift wave-zonal flow (DW-ZF) turbulence. A new physical quantity - the delay time (i.e., the mixing time for the DW turbulence) - is demonstrated to parameterize each stage of the transport barrier formation. Quantitative predictions for the onset of limit-cycle-oscillation (LCO) among DW and ZF intensities (also denoted as I-mode) and I-mode to high-confinement mode (H-mode) transition are also given. The LCO occurs when the ZF shearing (|〈v〉ZF′|) enters the regime Δωk<|〈V〉 ZF′|<τcr-1, where Δωk is the local turbulence decorrelation rate and τcr is the threshold delay time. In the basic predator-prey feedback system, τcr is also derived. The I-H transition occurs when |〈V〉E×B′|>τ cr-1, where the mean E×B shear flow driven by ion pressure "locks" the DW-ZF system to the H-mode by reducing the delay time below the threshold value. © 2014 AIP Publishing LLC.

We present a theory of turbulent elasticity, a property of drift-wave-zonal-flow (DW-ZF) turbulence, which follows from the time delay in the response of DWs to ZF shears. An emergent dimensionless parameter |〈v〉'|/Δωk is found to be a measure of the degree of Fickian flux-gradient relation breaking, where |〈v〉'| is the ZF shearing rate and Δωk is the turbulence decorrelation rate. For |〈v〉'|/Δωk>1, we show that the ZF evolution equation is converted from a diffusion equation, usually assumed, to a telegraph equation, i.e., the turbulent momentum transport changes from a diffusive process to wavelike propagation. This scenario corresponds to a state very close to the marginal instability of the DW-ZF system, e.g., the Dimits shift regime. The frequency of the ZF wave is ΩZF=±γd1/2γmodu1/2, where γd is the ZF friction coefficient and γmodu is the net ZF growth rate for the case of the Fickian flux-gradient relation. This insight provides a natural framework for understanding temporally periodic ZF structures in the Dimits shift regime and in the transition from low confined mode to high confined mode in confined plasmas.

We carry out several numerical simulations to illustrate how the radial electric field (Er) impacts the edge magnetohydrodynamic (MHD) instabilities. The analyses reveal that Er-shear (Er′, here the prime denotes the derivative with respect to the radial direction) tends to stabilize the kink/Peeling–Ballooning modes by dephasing the perturbed radial velocity (ṽr) and displacement (ξ̃r). However, Er-curvature (Er″) tends to destabilize the kink/peeling modes by inducing a phase lock between ṽr and ξ̃r. More specifically, the ratio between them could be measured to quantify their relative competition strength. Consequently, the shape of Er is crucial to the shape of linear growth rate spectrum γ(n) (here n is the toroidal mode number), which further determines the nonlinear dynamics. On the one hand, relatively larger Er-curvature causes narrower γ(n), leading to larger nonlinear energy loss fraction. On the other hand, relatively larger Er-shear has the opposite effect.