| 1. |
- Rypdal, K., et al.
(author)
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Scale-free vortex cascade emerging from random forcing in a strongly coupled system
- 2008
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In: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 10
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Journal article (peer-reviewed)abstract
- We elucidate the unifying aspects of self-organized criticality (SOC) and turbulence through analysis of data from a laboratory dusty plasma monolayer. We compare analysis of experimental data with simulations of a two-dimensional (2D) many-body system, of 2D chaotic fluid flow, and two different SOC-models, the Zhang and the Bak-Tang-Wiesenfeld (BTW) models, all subject to steady random forcing at small scales. The scale-free vortex cascade is apparent from structure functions as well as spatio-temporal avalanche analysis. We find similar scaling exponents for the experiment, the many-body simulation, and the fluid simulation, indicating some common dynamical features. However, the exponents of the Zhang model are different from those of the BTW model, and they are all different from those of the dust and fluid systems. Thus, we conclude that the dust monolayer dynamics can be viewed as turbulent as well as avalanching, but a fluid model is a better representation of the dust dynamics for this particular experiment than the sandpile models considered. The experiment exhibits global fluctuation statistics consistent with a recent hypothesis predicting universal non-Gaussian probability density functions, but the model systems yield this result only in a restricted range of forcing conditions.
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| 2. |
- Castaldo, C., et al.
(author)
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Diagnostics of fast dust particles in tokamak edge plasmas
- 2007
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In: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 47:7, s. L5-L9
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Journal article (peer-reviewed)abstract
- The use of electrostatic probes as a diagnostic tool of the dust particles in the tokamak edge plasmas is investigated. Probe measurements of electrostatic fluctuations in the scrape-off layer of the Frascati Tokamak Upgrade revealed that some features of the signals can be explained only by a local non-propagating phenomenon. These signal features are shown to be both in qualitative and quantitative agreement with ionization, and consequent extra charge collected by the probes, due to the impact of micrometre-sized dust at a velocity of the order of 10 km s(- 1). Electron microscope analysis of the probe surface yielded direct support for such an interpretation.
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| 3. |
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| 4. |
- Ratynskaia, Svetlana V., et al.
(author)
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Hypervelocity dust impacts in FTU scrape-off layer
- 2008
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In: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 48:1
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Journal article (peer-reviewed)abstract
- The first evidence of dust-impact ionization processes in the scrape-off layer of the Frascati tokamak upgrade (FTU) was reported in Nucl. Fusion 47 L5 (2007). In this work an extended data analysis (both of electrostatic probe signals and probe surface analysis) is presented, using results of empirical studies of hypervelocity impacts. Within the accuracy and limits of application of such results to tokamak plasma environment, the data are shown to be consistent with occurrence of impacts by micrometre size iron particles at velocities of the order of 10 km s(-1).
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| 5. |
- Ratynskaia, Svetlana V., et al.
(author)
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Statistics of particle transport in a two-dimensional dusty plasma cluster
- 2005
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In: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 12:2
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Journal article (peer-reviewed)abstract
- Statistical analysis is performed on long time series of dust particle trajectories in a two-dimensional dusty plasma cluster. Particle transport is found to be superdiffusive on all time scales until the range of particle displacements approaches the size of the cluster. Analysis of probability distribution functions and rescaled range analysis of the position increments show that the signal is non-Gaussian self-similar with Hurst exponent H=0.6, indicating that the superdiffusion is caused by long-range dependencies in the system. Investigation of temporal and spatial characteristics of persistent particle slips demonstrates that they are associated with collective events present on all time scales and responsible for the non-Gaussianity and long-memory effects.
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| 6. |
- Ratynskaia, Svetlana V., et al.
(author)
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Superdiffusion and viscoelastic vortex flows in a two-dimensional complex plasma
- 2006
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 96:10
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Journal article (peer-reviewed)abstract
- Viscoelastic vortical fluid motion in a strongly coupled particle system has been observed experimentally. Optical tracking of particle motion in a complex plasma monolayer reveals high grain mobility and large scale vortex flows coexistent with partial preservation of the global hexagonal lattice structure. The transport of particles is superdiffusive and ascribed to Levy statistics on short time scales and to memory effects on the longer scales influenced by cooperative motion. At these longer time scales, the transport is governed by vortex flows covering a wide spectrum of temporal and spatial scales.
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| 7. |
- Rypdal, K., et al.
(author)
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Onset of turbulence and profile resilience in the Helimak configuration
- 2005
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 94:22
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Journal article (peer-reviewed)abstract
- An experimental study of the onset of drift wave and flute interchange instabilities in the Helimak configuration is presented. It is shown that the Helimak offers the opportunity to separate the regions where these instabilities are active and to assess their relative role in cross-field anomalous transport and in the self-organization of exponential plasma density profiles with resilient scale length. Some results indicating a period doubling route to turbulence are also presented.
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| 8. |
- Rypdal, K., et al.
(author)
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Plasma profiles, waves and anomalous transport in a purely toroidal plasma modified by a biased internal anode
- 2006
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In: Physica Scripta. - 0031-8949 .- 1402-4896. ; T122, s. 52-65
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Journal article (peer-reviewed)abstract
- Two-dimensional (2D) plasma profiles, gradient-driven low frequency electrostatic waves and anomalous particle transport are studied experimentally in a purely toroidal plasma configuration. The plasma is produced by a negatively biased, emissive cathode in conjunction with a variably biased anode plate localized on the same field line. Nearly circularly symmetric potential profiles, positive or negative, can be obtained by varying the anode plate bias. Confinement loss in the form of plasma ejection in the major radius direction is only observed when the anode is biased close to ground potential. For the cases of circularly symmetric plasma flow electrostatic flute modes with poloidal mode number m = 1 and m = 2 are identified as unstable on the low- field side of the density maximum, but indications are also given that they coexist with spatially quasi-uniform oscillations and, for low frequencies, with an ion acoustic mode with toroidal mode number n = 1 propagating parallel to the magnetic field. The anomalous particle flux density is found to be non- uniformly distributed on a toroidal surface, and the main flux is passed through two lobes directed almost vertically upwards and downwards, while almost no flux takes place along the major radius direction.
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