Turbulence Phase Space in Simple Magnetized Toroidal Plasmas (original) (raw)
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Turbulence and Transport in Simple Magnetized Toroidal Plasmas
Progress in understanding turbulence and related cross-field transport of fusion relevance is achieved in the simple magnetized plasmas of the TORPEX toroidal device, which provides an ideal test bed for code benchmarking and theory validation. In TORPEX (R = 1 m, a = 0.2 m), a small vertical magnetic field B z < 4 mT is superposed on a toroidal magnetic field B t < 100 mT, resulting in open helical magnetic field lines. Similarly to the Scrape-Off Layer of fusion devices, ∇B and magnetic field curvature are at play. Here, Hydrogen plasmas are produced by microwaves (2.45 GHz, P<10 kW) with electron temperatures ~5 eV and densities ~1x10 16 m -3 . Drift and interchange instabilities are observed and characterized in terms of dispersion relation, driving mechanisms and non-linear development into turbulence and blobs. Density fluctuations associated with driftinterchange waves exhibit universal statistical properties. Their probability density functions (PDFs) are best fitted by a Beta distribution, with a unique quadratic polynomial relation linking their skewness and flatness. At large vertical fields, PDFs with positive skewness are associated with blobs, which form from radially elongated structures that are sheared off by the ExB flow. These structures are in turn generated by interchange waves that increase in amplitude and extend radially following an increase of the radial pressure gradient. The transport of heat and particles associated with the interchange wave and the blobs is investigated, revealing that fundamentally different mechanisms are involved. Initial fluid simulations of the interchange-dominated regime show intermittent transport in the presence of plasma blobs.
High- and Low-Confinement Modes in Simple Magnetized Toroidal Plasmas
Physical Review Letters, 2008
Three-field simulations of interchange turbulence are presented for a simple magnetized toroidal plasma with a vertical magnetic field. The simulations show the presence of two turbulent regimes characterized by low (L) and high (H) confinement properties. We evaluate analytically the properties of the L regime, obtaining expressions for the plasma gradients and for the density and heat fluxes that agree well with the simulations. By increasing the plasma source strength or reducing the vertical magnetic field, a transition to a H regime occurs, in which a strong velocity shear limits the perpendicular transport with respect to the L scaling and the plasma profiles steepen. The analytic estimate of the transition condition is in accord with the simulations.
Turbulence and bias-induced flows in simple magnetized toroidal plasmas
Physical Review E, 2011
Turbulence and bias-induced flows in simple magnetized toroidal plasmas are explored with global threedimensional fluid simulations, focusing on the parameters of the Helimak experiment. The simulations show that plasma turbulence and transport in the regime of interest are dominated by the ideal interchange instability. The application of a bias voltage alters the structure of the plasma potential, resulting in the equilibrium sheared flows. These bias-induced vertical flows located in the gradient region appear to reduce the radial extent of turbulent structures, and thereby lower the radial plasma transport on the low field side.
Edge turbulence measurements in toroidal fusion devices
Plasma Physics and Controlled Fusion, 2007
This paper reviews measurements of edge plasma turbulence in toroidal magnetic fusion devices with an emphasis on recent results in tokamaks. The dominant feature of edge turbulence is a high level of broadband density fluctuations with a relative amplitude δn/n ∼ 5-100%, accompanied by large potential and electron temperature fluctuations. The frequency range of this turbulence is ∼10 kHz-1 MHz, and the size scale is typically ∼0.1-10 cm perpendicular to the magnetic field but many metres along the magnetic field, i.e. the structure is nearly that of 2D 'filaments'. Large intermittent bursts or 'blobs' are usually observed in the scrape-off layer. Diagnostic and data analysis techniques are reviewed and the main experimental results are summarized. Recent comparisons of experimental results with edge turbulence theory are discussed, and some directions for future experiments are suggested.
Turbulence and transport in the edge region of toroidally magnetized plasmas
Intermittent transport of particles and heat in the boundary region of toroidally magnetized plasmas is investigated by means of two-dimensional fluid turbulence simulations. The model describes the non-linear dynamics of interchange modes, coupling a plasma edge region with localized sources of particles and heat to an effective scrape-off layer with linear damping of the dependent variables due to transport along open magnetic field lines. An intermittent turbulent state is maintained by a steep pressure profile in the edge region. The turbulence spreads out through the scrape-off layer in a bursty manner in the form of blob structures. The rescaled probability distribution functions of the local particle density fluctuations as well as the turbulent radial particle flux at different radial positions in the scrape-off layer fall into coincidence, thus indicating universality of the fluctuation and transport statistics. Coarse graining further reveals that the particle density fluctuations possess a range of self-similarity and long range correlations on time scales from the correlation time to that of the bursting in the turbulence intensity.
Statistical properties of electrostatic turbulence in toroidal magnetized plasmas
Plasma Physics and Controlled Fusion, 2007
A unique parabolic relation is observed to link skewness and kurtosis of density fluctuation signals, measured over the whole cross-section of the simple toroidal device TORPEX for a broad range of experimental conditions. This relationship is also valid for density fluctuation signals measured in the scrape-off layer of the TCV tokamak. All the probability density functions (PDFs) of the measured signals, including those characterized by a negative skewness, are universally described by a special case of the beta distribution. In TORPEX, fluctuations in the drift-interchange frequency range are necessary and sufficient to assure that PDFs can be described by this specific beta distribution. For a more detailed plasma scenario, it is shown that electron temperature and plasma potential fluctuations have different statistical properties compared with the density.
The poloidal distribution of turbulent fluctuations in the Mega-Ampère Spherical Tokamak
Physics of Plasmas, 2005
Recently, it was shown that intermittency observed in magnetic fusion devices is caused by large-scales events with high radial velocity reaching about 1 / 10th of the sound speed ͑called avaloids or blobs͒ ͓G. Antar et al., Phys. Rev. Lett. 87 065001 ͑2001͔͒. In the present paper, the poloidal distribution of turbulence is investigated on the Mega-Ampère Spherical Tokamak ͓A. Sykes et al., Phys. Plasmas 8 2101 ͑2001͔͒. To achieve our goal, target probes that span the divertor strike points are used and one reciprocating probe at the midplane. Moreover, a fast imaging camera that can reach 10 s exposure time looks tangentially at the plasma allowing us to view a poloidal cut of the plasma. The two diagnostics allow us to have a rather accurate description of the particle transport in the poloidal plane for L-mode discharges. Turbulence properties at the low-field midplane scrape-off layer are discussed and compared to other poloidal positions. On the low-field target divertor plates, avaloids bursty signature is not detected but still intermittency is observed far from the strike point. This is a consequence of the field line expansion which transforms a structure localized in the poloidal plane into a structure which expands over several tens of centimeters at the divertor target plates. Around the X point and in the high-field side, however, different phenomena enter into play suppressing the onset of convective transport generation. No signs of intermittency are observed in these regions. Accordingly, like "normal" turbulence, the onset of convective transport is affected by the local magnetic curvature and shear.
Transport and Turbulence with Innovative Plasma Shapes in the TCV Tokamak
2010
We present recent results on turbulence measurements in TCV L-mode plasmas. It has been shown that the heat transport is reduced by a factor of two for a plasma at negative triangularity compared with a plasma at positive triangularity. This transport reduction is reflected in the reduction of the temperature fluctuation level, in the low frequency part of the spectrum (20-150 kHz), measured by correlation ECE in the outer equatorial plane. Moreover, the radial correlation length of the turbulence is typically reduced by a factor of two at negative triangularity compared with positive triangularity. Nonlinear gyrokinetic simulations predict that the TEM turbulence might be dominant for these TCV plasmas. The TEM induced transport is shown to decrease with decreasing triangularity and increasing collisionality. Both dependences are in fairly good agreement with experimental observations. We also report on an innovative divertor magnetic configuration: the snowflake (SF) divertor whose properties are expected to affect the local heat load to the divertor plates in particular during ELMs when compared with the classical single-null (SN) divertor. In L-mode plasmas, the intermittent particle and heat transport in the SOL is associated with the presence of "blobs" propagating in the radial direction. Intermittency is compared between SN and SF configurations by looking at the statistical properties of the ion saturation current J sat measured with Langmuir probes (LPs) in the LFS scrape-off layer. For ELMy H-mode SF plasmas, the time evolution of J sat during ELMs is estimated with LPs covering the strikepoints target zones.
Comparisons of nonlinear toroidal turbulence simulations with experiment
1994
The anomalously large thermal transport observed in tokamak experiments is the outstanding physics based obstacle in the path to a commercially viable fusion reactor. Although decades of experimental and theoretical work indicate that anomalous transport and collective instabilities in the gyrokinetic regime are linked,. no widely accepted description of this transport exists so far. Here, detailed comparisons of first-principles gyrofluid and gyrokinetic simulations of tokamak microinstabilities with experimental data are presented. With no adjustable parameters, more than 50 TFTR L mode discharges have been simulated with encouraging success. Given the local plasma parameters and the temperatures at rIa = 0.8, the simulations typically predict T; (r) and T e (r) within ±25 % throughout the core and confinement zones. In these zones, the predicted thermal diffusivity increases robustly with minor radius. For parameters typical of rIa > 0.8, toroidal stability studies confirm the importance of impurity density gradients as a source of free energy potentially strong enough to explain the large edge thermal diffusivity, as first emphasized by Coppi and his coworkers. Advanced confinement discharges have also been simulated. The dramatic increase of T j (0) observed in supershots (from 5 to 30 keY) is recovered by our model for dozens of simulated experiments. Finally, simulations of YH and PEP mode-like plasmas show that velocity-shear stabilization of toroidal microinstabilities is quantitatively significant for realistic experimental parameters.
Recent progress made with our global gyrokinetic simulations in understanding the origin of intrinsic rotation and non-diffusive transport characteristics in tokamaks is reported. Key results include the finding of an important nonlinear flow generation process due to the residual stress produced by the fluctuation intensity and the intensity gradient, acting with the zonal flow shear induced k symmetry breaking, which offers a universal mechanism to drive intrinsic rotation via wave-particle momentum exchange. This turbulence nonlinearly-driven intrinsic rotation scales close to linearly with plasma gradients and the inverse of the plasma current in various turbulence regimes, reproducing and extending empirical scalings obtained in multiple fusion devices. The underlying physics governing these characteristic dependences is elucidated. Particularly, the current scaling is found to result from the magnetic shear effect on k symmetry breaking. Highlighted results also include robust radial pinches in toroidal flow, heat and particles driven by CTEM turbulence, which emerge "in phase", and are shown to play remarkable roles in determining plasma transport. Particularly, the "flow pinch" phenomenon amazingly reproduces the experimental result of radially inward penetration of perturbed flows created by modulated beams in peripheral regions, and thus is highly illuminating. Finally, the ∇T e -driven CTEM turbulence in specific parameter regimes is found to generate remarkably large fluctuation structures via inverse energy cascades, which may have a natural connection to the generation of blobs in the edge.