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Träfflista för sökning "WFRF:(Rypdal V.) "

Sökning: WFRF:(Rypdal V.)

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1.
  • Rypdal, K., et al. (författare)
  • Non-equilibrium quasi-stationary states in a magnetized plasma
  • 2003
  • Ingår i: Nonlinear processes in geophysics. - : Copernicus GmbH. - 1023-5809 .- 1607-7946. ; 10:02-jan, s. 139-149
  • Tidskriftsartikel (refereegranskat)abstract
    • Non-equilibrium quasi-stationary states resulting from curvature driven interchange instabilities and driftwave instabilities in a low beta, weakly ionized, magnetized plasma are investigated in the context of laboratory experiments in a toroidal configuration. Analytic modelling, numerical simulations and experimental results are discussed with emphasis on identifying the unstable modes and understanding the physics of anomalous particle and energy fluxes and their linkage to self-organized pressure profiles.
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2.
  • Castaldo, C., et al. (författare)
  • Diagnostics of fast dust particles in tokamak edge plasmas
  • 2007
  • Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 47:7, s. L5-L9
  • Tidskriftsartikel (refereegranskat)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.
  • Demidov, V. I., et al. (författare)
  • Electric probes for plasmas : The link between theory and instrument
  • 2002
  • Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 73:10, s. 3409-3439
  • Forskningsöversikt (refereegranskat)abstract
    • Electric probe methods for diagnostics of plasmas are reviewed with emphasis on the link between the appropriate probe theories and the instrumental design. The starting point is an elementary discussion of the working principles and a discussion of the physical quantities that can be measured by the probe method. This is followed by a systematic classification of the various regimes of probe operation and a summary of theories and methods for measurements of charged particle distributions. Application of a single probe and probe clusters for measurements of fluid observables is discussed. Probe clusters permit both instantaneous and time-averaged measurements without sweeping the probe voltage. Two classes of applications are presented as illustrations of the methods reviewed. These are measurements of cross sections and collision frequencies (plasma electron spectroscopy), and measurements of fluctuations and anomalous transport in magnetized plasma.
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4.
  • Demidov, V. I., et al. (författare)
  • Reducing influence of ion current on measurements of electron energy distributions in collisional plasmas
  • 2001
  • Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 72:11, s. 4106-4109
  • Tidskriftsartikel (refereegranskat)abstract
    • A method for reducing the influence of ion current on probe measurements of electron velocity distributions in plasmas has been developed and tested in noble gas afterglow plasmas. It is valid for diffusive transport of ions to the probe. Experiments have demonstrated high accuracy of the proposed method.
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5.
  • Demidov, V. I., et al. (författare)
  • The analysis of probe I-V characteristics in a magnetized low-temperature plasma
  • 2001
  • Ingår i: Contributions to Plasma Physics. - 0863-1042 .- 1521-3986. ; 41:5, s. 443-448
  • Tidskriftsartikel (refereegranskat)abstract
    • The validity of probe theories based on the classical electron transport to probes in low-temperature magnetized plasma of the toroidal device `Blaamann has been demonstrated. The analysis was carried out for the conditions when global transport of charged particles in the device is anomalous, namely for magnetic field up to 0.3 T and pressure range 0.1 - 1 Pa. It was shown also that for the magnetic field larger than 0.1 T probes longer than 15 mm provide electron saturation current practically independent of probe potential, hence more accurate measurements of the plasma parameters. The experiments have revealed that application of long probes oriented parallel to the magnetic field may cause an anomaly of the I-V characteristics in the sense that a local increase of the electron current appears near the plasma potential.
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6.
  • Ratynskaia, Svetlana V., et al. (författare)
  • A probe for measurements of electrostatic fluctuations in a low-temperature magnetized plasma
  • 2000
  • Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 71:3, s. 1367-1369
  • Tidskriftsartikel (refereegranskat)abstract
    • A cylindrical probe with insulating end plugs for investigations of fluctuations of electron temperature and plasma potential is proposed. The radii of the metallic rod and plugs are chosen to optimize the ratio of ion saturation current to electron saturation current (> 1) for the probe oriented parallel to the magnetic field. This probe is applicable when the electron temperature is much larger than the ion temperature.
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7.
  • Ratynskaia, Svetlana V., et al. (författare)
  • Hypervelocity dust impacts in FTU scrape-off layer
  • 2008
  • Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 48:1
  • Tidskriftsartikel (refereegranskat)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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8.
  • Ratynskaia, Svetlana V., et al. (författare)
  • Measurements of anomalous particle and energy fluxes in a magnetized plasma
  • 2002
  • Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1063-651X .- 1095-3787. ; 65:6
  • Tidskriftsartikel (refereegranskat)abstract
    • Electrostatic probe measurements of low frequency plasma fluctuations and anomalous particle and energy flux densities in a magnetized plasma are presented. A method allowing the simultaneous recording of instantaneous electric field, electron density, and temperature is invoked. The method is applied to flux density measurements in a weakly ionized, low beta plasma created in a toroidal device without magnetic rotational transform. It is also used to identify modes belonging to different dispersion branches and to obtain the dispersion relations for these modes. For the plasma states studied, the phase velocities and the cross phase between the electron density and electric field agree with those predicted from a local, linear stability analysis for electrostatic flute modes and drift waves. The instability threshold, however, is one order of magnitude higher than predicted by theory for unsheared flow. The fluxes measured are consistent with the estimated ionization rate.
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9.
  • Ratynskaia, Svetlana V., et al. (författare)
  • Measurements of plasma potential and electron temperature fluctuation in a low-temperature magnetized plasma
  • 2001
  • Ingår i: Contributions to Plasma Physics. - 0863-1042 .- 1521-3986. ; 41:5, s. 449-454
  • Tidskriftsartikel (refereegranskat)abstract
    • Measurements of fluctuations of plasma potential and electron temperature in a toroidal magnetized plasma is carried out by applying a cylindrical probe with insulating end plugs oriented parallel to the B-field in conjunction with another cylindrical probe oriented perpendicularly. Coherency and cross-phase between (V) over tilde (f) and (T) over tilde (e) are estimated, and typically have values close to 0.6 and pi respectively. Power-law spectra are found for frequencies well above the poloidal rotation frequency with spectral index typically around 4.0 for (V) over tilde (f) and around 2.5 for (T) over tilde (e). The density gradient is above the threshold for flute interchange instability, and the results are consistent with theory and global numerical simulations of this plasma.
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10.
  • Ratynskaia, Svetlana V., et al. (författare)
  • Probe measurements of electron temperature and density in strongly magnetized plasma
  • 2000
  • Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 71:9, s. 3382-3384
  • Tidskriftsartikel (refereegranskat)abstract
    • It was demonstrated that with increasing probe length the electron saturation current becomes more independent of the probe potential. This provides higher accuracy of measurements of the electron density and plasma potential in a strongly magnetized plasma (B > 0.1 T). Long probes oriented parallel to the magnetic field yield better spatial resolution in the perpendicular direction. Parallel probes, however, are very sensitive to alignment errors. To avoid this problem, a cylindrical probe with a side surface coated by a thin insulating layer is proposed.
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11.
  • Ratynskaia, Svetlana V., et al. (författare)
  • Probe measurements of ion energy distribution in magnetized plasmas
  • 2002
  • Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 73:12, s. 4232-4236
  • Tidskriftsartikel (refereegranskat)abstract
    • A probe method for measurement of the ion energy distribution in magnetized plasmas is proposed. A cylindrical probe with end plugs is oriented parallel to the magnetic field and used to reduce the electron contribution to the total probe current. The contribution of ions to the second derivative of the total current is identified by experiments with different probe orientation and strength of the magnetic field. The ion density obtained from the measured ion energy distribution is in good agreement with the electron density obtained from measured electron energy distribution.
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12.
  • Ratynskaia, Svetlana V., et al. (författare)
  • Probe measurements of low-frequency plasma potential and electric field fluctuations in a magnetized plasma
  • 2002
  • Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 9:10, s. 4135-4143
  • Tidskriftsartikel (refereegranskat)abstract
    • A system of two cylindrical probes aligned along the magnetic field, and equipped with insulating end plugs, is proposed for measurement of low-frequency fluctuations of the electrostatic field in a magnetized plasma. It is demonstrated by modeling and experiments that the plug probe floats close to the plasma potential. The electric field component in a given direction is obtained by subtracting the plasma potentials obtained on two spatially separated plug probes. The probe system is applied to low-frequency electrostatic fluctuations in a simple magnetized torus, and reveals the presence of global oscillations, large scale propagating structures (m=1 modes), and developed turbulence with power-law spectra. Two different mode branches for the fluctuations are identified by comparing results from plug probes with results from conventional probes. Sources of errors arising from applying floating potential of conventional probes for electric field measurements are pointed out and discussed.
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13.
  • Ratynskaia, Svetlana V., et al. (författare)
  • Superdiffusion and viscoelastic vortex flows in a two-dimensional complex plasma
  • 2006
  • Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 96:10
  • Tidskriftsartikel (refereegranskat)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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14.
  • Rypdal, K., et al. (författare)
  • Scale-free vortex cascade emerging from random forcing in a strongly coupled system
  • 2008
  • Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 10
  • Tidskriftsartikel (refereegranskat)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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16.
  • Mikola, K., et al. (författare)
  • Transitioning patients with juvenile idiopathic arthritis to adult care: the Nordic experience
  • 2022
  • Ingår i: Pediatric Rheumatology. - : Springer Science and Business Media LLC. - 1546-0096. ; 20:1
  • Tidskriftsartikel (refereegranskat)abstract
    • Background With juvenile idiopathic arthritis (JIA), there are several protocols and practices used worldwide for the transition from paediatric to adult care. In this study, we examined the transferral rates and disease activity after transition, as well as the disease- and health-related outcomes. We also introduce the transition practices employed in the Nordic countries. Methods The study population comprised 408 participants with a disease onset from 1997 to 2000 who attended an 18-year follow-up visit in this population-based Nordic JIA cohort study. The patients were retrospectively divided into three subgroups: Patients transferred directly from paediatric care to adult rheumatology care, patients referred there later, and patients never transferred during the 18-year follow-up period. Results One hundred and sixty-three (40%) JIA patients had been directly transferred to an adult clinic. The cumulative transition rate was 52%, but there were significant differences between the participating centres. Fifty patients had later been referred to an adult clinic. Among the 195 patients who had never been transferred, 39% were found to have disease activity at the study visit. Conclusion This study highlights the need to reconsider transition practices to avoid our undesirable finding of patients with disease activity in JIA, but no appropriate health care follow-up.
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18.
  • Nordal, E., et al. (författare)
  • Incidence and predictors of Uveitis in juvenile idiopathic arthritis in a Nordic long-term cohort study
  • 2017
  • Ingår i: Pediatric Rheumatology. - : Springer Science and Business Media LLC. - 1546-0096. ; 15
  • Tidskriftsartikel (refereegranskat)abstract
    • Background: The incidence of uveitis associated with juvenile idiopathic arthritis (JIA) varies around the world. Our aim was to investigate the incidence and predictors of uveitis in a Nordic population-based cohort. Methods: Consecutive JIA cases from defined geographical areas in Denmark, Finland, Sweden and Norway with disease onset between January 1997 to June 2000 were followed for median 98 months in this prospective longitudinal cohort study. Potential clinical and immunological predictors of uveitis were identified with logistic regression analysis. Results: Uveitis occurred in 89 (20.5%) of the 435 children with regular ophtalmologic follow-up among the 500 included. Chronic asymptomatic uveitis developed in 80 and acute symptomatic uveitis in 9 children. Uveitis developed at a median interval of 0.8 (range - 4.7 to 9.4) years after onset of arthritis. Predictors of uveitis were age < 7 years at JIA onset (Odds ratio (OR) 2.1, 95% confidence interval (CI) 1.3 to 3.5), presence of antihistone antibodies (AHA) > 15 U/ml (OR 4.8 (1.8 to 13.4)) and antinuclear antibodies (ANA) (OR 2.4 (1.5 to 4.0)). Mean combined IgM/IgG AHA was significantly higher in the uveitis group (19.2 U/ml) than in the non-uveitis group (10.2 U/ml) (p = 0.002). Young age at JIA onset predicted uveitis in girls (p < 0.001), but not in boys (p = 0.390). Conclusion: Early-onset arthritis and presence of AHA in girls, as well as presence of ANA in both genders, were significant predictors of chronic uveitis. The high incidence of uveitis in this long-term Nordic JIA cohort may have severe implications in a lifelong perspective.
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19.
  • Nordal, E., et al. (författare)
  • Participation in school and physical education in juvenile idiopathic arthritis in a Nordic long-term cohort study
  • 2019
  • Ingår i: Pediatric Rheumatology. - : Springer Science and Business Media LLC. - 1546-0096. ; 17
  • Tidskriftsartikel (refereegranskat)abstract
    • BackgroundThe aim of the study was to describe school attendance and participation in physical education in school among children with juvenile idiopathic arthritis (JIA).MethodsConsecutive cases of JIA from defined geographical areas of Finland, Sweden and Norway with disease onset in 1997 to 2000 were followed for 8 years in a multi-center cohort study, aimed to be as close to population-based as possible. Clinical characteristics and information on school attendance and participation in physical education (PE) were registered.ResultsParticipation in school and in PE was lowest initially and increased during the disease course. Eight years after disease onset 228/274 (83.2%) of the children reported no school absence due to JIA, while 16.8% reported absence during the last 2 months due to JIA. Full participation in PE was reported by 194/242 (80.2%), partly by 16.9%, and none by 2.9%. Lowest participation in PE was found among children with ERA and the undifferentiated categories. Absence in school and PE was associated with higher disease activity measures at the 8-year visit. School absence >1day at baseline predicted use of disease-modifying anti-rheumatic drugs, including biologics (DMARDs) (OR 1.2 (1.1-1.5)), and non-remission off medication (OR 1.4 (1.1-1.7) 8 years after disease onset.ConclusionSchool absence at baseline predicted adverse long-term outcome. In children and adolescents with JIA participation in school activities is mostly high after 8years of disease. For the minority with low participation, special attention is warranted to promote their full potential of social interaction and improve long-term outcome.
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20.
  • Ratynskaia, Svetlana V., et al. (författare)
  • Statistics of particle transport in a two-dimensional dusty plasma cluster
  • 2005
  • Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 12:2
  • Tidskriftsartikel (refereegranskat)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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21.
  • Rypdal, K., et al. (författare)
  • Fluctuation threshold and profile resilience in weakly ionized plasma in a curved, unsheared magnetic field
  • 2004
  • Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 11:10, s. 4623-4633
  • Tidskriftsartikel (refereegranskat)abstract
    • A study of electrostatic fluctuations in a weakly ionized plasma confined by a purely toroidal magnetic field B is presented. At low B field the plasma is quiescent and transport is provided by stationary ExB flows along open equipotential surfaces. For B above a certain threshold, strong electrostatic fluctuations are excited, and the electron pressure profile on the low-field side is resilient with scale length much shorter than the threshold scale length for flute instability. At threshold the fluctuations exhibit the signatures of a monochromatic drift mode, and above threshold they are dominated by flute modes growing on the low-field side. Threshold behavior and profile resilience are explained as a two-stage process, where drift waves act as a seed for the unstable flute modes, and where a negative feedback mechanism involving anomalous transport regulates the electron pressure profile to maintain the imposed particle flux.
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22.
  • Rypdal, K., et al. (författare)
  • Onset of turbulence and profile resilience in the Helimak configuration
  • 2005
  • Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 94:22
  • Tidskriftsartikel (refereegranskat)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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23.
  • Rypdal, K., et al. (författare)
  • Plasma profiles, waves and anomalous transport in a purely toroidal plasma modified by a biased internal anode
  • 2006
  • Ingår i: Physica Scripta. - 0031-8949 .- 1402-4896. ; T122, s. 52-65
  • Tidskriftsartikel (refereegranskat)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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24.
  • Rypdal, K., et al. (författare)
  • Statistics of low-frequency plasma fluctuations in a simple magnetized torus
  • 2003
  • Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 10:7, s. 2686-2695
  • Tidskriftsartikel (refereegranskat)abstract
    • Statistical analysis is performed on time series of plasma potential and electron pressure fluctuations for two particular plasma states in a simple magnetized torus. In spite of the occurrence of cyclic trends in the signals, the analysis demonstrates the existence of long-range dependence on much longer time-scales than the plasma confinement time, and both Gaussian and non-Gaussian self-affinity is demonstrated in the probability distribution functions (PDFs) of increasingly coarse grained time series. Non-Gaussian PDFs for electron pressure fluctuations p are believed to arise from a nonlinear relationship between p and the plasma potential V. Non-Gaussian PDFs for V are well approximated by distributions derived from extreme value statistics.
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