Intermittent Convective Transport Research Articles

Intermittent convective transport suppressed by supersonic molecular beam injection on the HL-2A tokamak

Edge localized mode (ELM)-filament statistical characteristics and intermittent convective transport are studied during ELM mitigation (10–30 ms) induced by supersonic molecular beam injection (SMBI) on the HL-2A tokamak. Langmuir probe data show that the burst rate of large amplitude filaments (δne > 4σ) decreases by 30–50% and their radial velocity decreases by 40–60%, which is an indication of an amplitude reduction. A decrease of long-range correlation along the magnetic flux surface is observed after SMBI, indicating a role of electron–ion collisionality on the filament parallel current in the ELM mitigation time. The transient particle and heat fluxes decrease about 40–50% and 50–60%, respectively. These observations indicate that SMBI might be an effective method to suppress the intermittent convective transport.

Evidence of determinism for intermittent convective transport in turbulence processes

In this letter we present evidence that intermittent transport observed in nature, e.g., the scrape off layer of magnetically confined plasma devices, has a strong signature of determinism. We show that the universal distribution of density fluctuation as well as the unique parabolic relation between skewness and kurtosis observed in experimental data can be obtained by a superposition of stochastic and deterministic events. A well known deterministic effect, namely, the loss of transversal stability of periodic orbits embedded in an invariant (inertial) manifold is used to model the spiky nature of the emissions. The intermittent emissions are proposed to be due to local unstable transversal directions of the invariant manifold resulting in an ejection of particles and a consequent burst in the signal. We show that characteristics observed by the emissions namely an impulsive ejection followed by a slow recovery phase can be directly related to the deterministic mechanism proposed.

Observation of Blobs and Holes in the Boundary Plasma of EAST Tokamak

Intermittent convective transport at the edge and in the scrape-off layer (SOL) of EAST was investigated by using fast reciprocating Langmuir probe. Holes, as part of plasma structures, were detected for the first time inside the shear layer. The amplitude probability distribution function of the turbulence is strongly skewed, with positive skewed events (“blobs") prevailing in the SOL region and negative skewed events (“holes") dominant inside the shear layer. The statistical properties coincide with previous observations from JET. The generation mechanism of blobs and holes is also discussed. In addition burst structure and dynamics character of them are also presented.

Intermittent convective transport carried by propagating electromagnetic filamentary structures in nonuniformly magnetized plasma

Drift-Alfvén vortex filaments associated with electromagnetic turbulence were recently identified in reversed field pinch devices. Similar propagating filamentary structures were observed in the Earth magnetosheath, magnetospheric cusp and Saturn’s magnetosheath by spacecrafts. The characteristics of these structures closely resemble those of the so-called mesoscale coherent structures, prevailing in fusion plasmas, known as “blobs” and “edge localized mode filaments” in the boundary region, and propagating avalanchelike events in the core region. In this paper the fundamental dynamics of drift-Alfvén vortex filaments in a nonuniformly and strongly magnetized plasma are revisited. We systemize the Lagrangian-invariant-based method. Six Lagrangian invariants are employed to describe structure motion and the resultant convective transport, namely, magnetic flux, background magnetic energy, specific entropy, total energy, magnetic momentum, and angular momentum. The perpendicular vortex motions and the kinetic shear Alfvén waves are coupled through the parallel current and Ampere’s law, leading to field line bending. On the timescale of interchange motion τ⊥, a thermal expansion force in the direction of curvature radius of the magnetic field overcomes the resultant force of magnetic tension and push plasma filament to accelerate in the direction of curvature radius resulting from plasma inertial response, reacted to satisfy quasineutrality. During this process the internal energy stored in the background pressure gradient is converted into the kinetic energy of convective motion and the magnetic energy of field line bending through reversible pressure-volume work as a result of the plasma compressibility in an inhomogeneous magnetic field. On the timescale of parallel acoustic response τ∥⪢τ⊥, part of the filament’s energy is transferred into the kinetic energy of parallel flow. On the dissipation timescale τd⪢τ⊥, the kinetic energy and magnetic energy are eventually dissipated, which is accompanied by entropy production, and in this process the structure loses its coherence, but it has already traveled a distance in the radial direction. In this way the propagating filamentary structures induce intermittent convective transports of particles, heat, and momentum across the magnetic field. It is suggested that the phenomena of profile consistency, or resilience, and the underlying anomalous pinch effects of particles, heat, and momentum in the fusion plasmas can be interpreted in terms of the ballistic motion of these solitary electromagnetic filamentary structures.

Disparate Scale Nonlinear Interactions in Edge Turbulence

AbstractIn this topical review, we explain the recent achievement in the study of nonlinear interactions, putting an emphasis on the relevance to edge turbulence. First, we start from the survey of the essence in the nonlinear theory of drift wave ‐zonal flows systems, and visit the experimental observations of the nonlinear interactions of tokamak edge turbulence. Secondly, the universality of intermittent convective transport in the SOL of different magnetic devices are shown. Then, we discuss evolution of collisional drift wave instability in the linear plasma configuration, which is bounded by end plates having analogy to SOL plasmas. By introducing the Numerical Linear Device, the intermittent evolution of large‐amplitude instabilities, generation mechanism of the poloidal flow and other nonlinear process are examined. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Transport phenomena in the edge of Alcator C-Mod plasmas

Two aspects of edge turbulence and transport in Alcator C-Mod are explored. The quasi-coherent mode (QCM), an edge fluctuation present in Enhanced Dα H-mode plasmas, is examined with regard to its role in the enhanced particle transport found in these plasmas, its in/out asymmetry, its poloidal wave number and its radial width and location. It is shown to play a dominant role in the perpendicular particle transport. The QCM is not observed at the inboard midplane, indicating that its amplitude there is significantly smaller than on the outboard side. The peak amplitude of the QCM is found just inside the separatrix, with a radial width ≳5 mm, leading to a non-zero amplitude outside the separatrix and qualitatively consistent with its transport enhancement. Also examined are the characteristics of the intermittent convective transport, associated with the larger scale turbulent structures, also called blobs, and typically occurring in the scrape-off-layer (SOL). These turbulent structures are qualitatively similar in L- and H-mode. When their perpendicular extent, occurrence frequencies and magnitudes are compared, it is found that their size is somewhat smaller in ELMfree H-Mode, while their frequency is similar. A clear difference is seen in the magnitude of these turbulent fluctuations in the far SOL, with ELMfree H-mode showing a smaller perturbation there than L-mode. As the Greenwald density limit is approached (n/nGW ⩾ 0.7), blobs are seen inside the separatrix consistent with the observation that the high cross-field transport region, normally found in the far SOL, penetrates the closed flux surfaces at high n/nGW.

On the properties of turbulence intermittency in the boundary of the TEXTOR tokamak

Intermittent convective transport has been investigated in the edge and the scrape-off layer (SOL) of TEXTOR using Langmuir probe signals. The probability distribution function (PDF) of the density fluctuations and the turbulence-induced flux are all positively skewed, while a Gaussian shape is recorded for the negative fluctuations. The deviation of the signals from Gaussian statistics clearly increases from the plasma edge to the SOL. Conditional averaging reveals that in the SOL region the waveform of intermittent structures is asymmetric in time and the burst events move radially outwards with Eθ × BT/B2 velocities of ∼ 450 m s−1. It is found that the large burst fluctuations (⩾2.5 × rms) account for nearly 40% of the total transport in the SOL. Statistics of the waiting-time between successive bursts indicate that the PDF of the time interval follows a Poisson-distribution for small-duration events (selected by size ⩽2.5 × rms) and changes into a power-law form for larger ones. Moreover, the intermittency density fluctuation data clearly show self-similar characters and long-range time correlations through the presence of (1) sandpile-like frequency spectra and existence of the f−1 region; (2) a long tail in the autocorrelation function and (3) Hurst exponents H > 0.5 from R/S analysis, suggesting a possible role of avalanche-like transport in the turbulence intermittency.

Universality of Intermittent Convective Transport in the Scrape‐off Layer of Magnetically Confined Devices

AbstractThis article describes briefly recent results on the universality of intermittent convective transport reported in the scrape‐off layer. We show that the statistical properties of turbulence are identical for four different magnetic fusion devices. The four devices are the PISCES linear plasma device, the Tore Supra tokamak with limiter configuration, the Alcator C‐MOD with divertor configuration and high magnetic field, and theMAST spherical tokamak where the walls are far from the last closed flux surface (LCFS). These four devices do not only have different magnetic configurations but also have four different probes designs. Nine different types of statistical analyses are used, here we present just a few. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Universality of intermittent convective transport in the scrape-off layer of magnetically confined devices

The nature of intermittency, long observed in magnetic fusion devices, was revisited lately [G. Antar et al., Phys. Rev. Lett. 87, 065001 (2001)]. It was shown that intermittency is caused by large-scale events with high radial velocity reaching about 1/10th of the sound speed. These type of structures were named “avaloids.” In the present article, the universality of convective turbulence in magnetically confined plasmas is investigated. Turbulence properties in the scrape-off layer of four different magnetic fusion devices are compared. Namely, the Tore Supra tokamak [Tore Supra Team, Nuclear Fusion, 40, 1047 (2000)] with circular cross-section limiter-bounded plasma, the Alcator C-Mod tokamak [B. LaBombard et al., Phys. Plasmas 8, 2107 (2001)] which is a divertor device, the Mega-Ampere Spherical Tokamak (MAST) [A. Sykes et al., Phys. Plasmas 8, 2101 (2001)] with vacuum chamber walls far from the plasma last closed flux surface and the PISCES linear plasma device [D. Geobel et al., Rev. Sci. Istrum. 56, 1717 (1985)]. The statistical properties of the turbulent signals in the four devices are found to be identical allowing us to conclude that intermittent convective transport by avaloids is universal in the sense that it occurs and has the same properties in many confinement devices with different configurations.