[en] Experiments on spatiotemporal open-loop synchronization of drift wave turbulence in a magnetized cylindrical plasma are reported. The synchronization effect is modeled by a rotating current profile with prescribed mode structure. Numerical simulations of an extended Hasegawa-Wakatani model show good agreement with experimental results

[en] The paradigm for transport barriers is based on the simple argument that sheared flows suppress turbulence and transport via a decorrelation mechanism. Here we present results from 3D simulations of drift-Alfven turbulence in a sheared geometry. An intrinsic relationship between the shear flows and magnetic field perturbations leads to a more complex behavior, where for larger values of beta the transport might rise in the presence of shear flows

[en] Results from three-dimensional numerical simulations of drift-Alfven turbulence in a toroidal geometry with sheared magnetic field are presented. The simulations show a relation between self-generated poloidal shear flows and magnetic field perturbations. For large values of the plasma β we observe an increase of the transport if the viscous damping of the self-generated shear flows is absent. This behavior is in contrast to the standard argument that sheared flows suppress turbulence and transport via a decorrelation mechanism. An explanation of this behavior in terms of the transport related to magnetic flutter is proposed. The characteristics of the ExB flux are investigated using probability density distribution functions (PDFs). Although they are not Gaussian, no signs of algebraic tails in the PDFs are observed. The PDFs of the pointwise transport are found to agree well with a folded Gaussian, while the PDFs of the spatially averaged transport are in good agreement with an extreme value distribution

[en] A low-frequency instability is investigated in a helicon plasma, which is characterized by comparably high plasma-β and high collision frequencies. Single movable Langmuir probes and a poloidal probe array are used for studies of spatiotemporal dynamics and for characterization of the background plasma parameters. All experimentally observed features of the instability are found to be consistent with drift waves. A linear nonlocal numerical model for drift modes, based on the two-fluid description of a plasma, is used for comparison between the experimental observations and theory. Comparing numerical and experimental frequencies, it is found that the experimentally observed frequencies are consistent with drift waves. The numerical results show that the high electron collision frequencies provide the strongest destabilization mechanism in the helicon plasma

[en] The connection between the diffusion of passive tracer particles and the anomalous turbulent flux in electrostatic drift-wave turbulence is investigated by direct numerical solutions of the 2D Hasegawa-Wakatani equations. The probability density functions for the point-wise and flux surface averaged turbulent particle flux are measured and compare well to a folded Gaussian, respectively a log-normal distribution. By following a large number of passive tracer particles we evaluate the diffusion coefficient based on the particle dispersion. It is found that the particle diffusion coefficient is in good agreement with the one derived from the turbulent ExB-flux by using Fick's law. Employing the Lagrangian conservation of the 'Potential Vorticity' in the Hasegawa-Wakatani equations, the analytical support for this result is obtained. The transport estimated by passive tracer dispersion and turbulent plasma flux are found to coincide

[en] In a cylindrical helicon plasma, mode structures of coherent drift waves are studied in the poloidal plane, the plane perpendicular to the ambient magnetic field. The mode structures rotate with a constant angular velocity in the direction of the electron diamagnetic drift and show significant radial bending. The experimental observations are compared with numerical solutions of a linear nonlocal cylindrical model for drift waves [Ellis et al., Plasma Phys. 22, 113 (1980)]. In the numerical model, a transition to bended mode structures is found if the plasma collisionality is increased. This finding proves that the experimentally observed bended mode structures are the result of high electron collisionality

No abstract available

[en] In the cylindrical plasma of the linear magnetized plasma device MIRABELLE the drift wave and interchange instability can be separately studied depending on plasma parameters and ambient magnetic field. Due to both instabilities azimuthal space-time mode structures of plasma potential and density fluctuations are observed, that are either coherent or turbulent. With a space-time open-loop control system consisting of an azimuthal octupole arrangement of electrodes the dynamics of both instabilities can be influenced. Synchronization of coherent and weakly developed turbulent modes is investigated and compared with 3D fluid code simulations. Fast camera imaging of the complete azimuthal range yields conclusions on the globality of the control. Mode coupling and energy transfer is investigated by means of wavelet bicoherence.

[en] Interactions between plasma and neutrals are investigated with particular attention to the influence of large amplitude blob structures that mediate a significant particle and energy transport through the scrape-off layer (SOL). We perform a statistical analysis of the mean-field approximation for plasma parameters in the SOL, and this approximation is shown to be poor in a SOL with a high level of fluctuations, as the plasma fields are strongly correlated. A 1D neutral fluid model which account for both cold and hot neutrals is formulated and the effects of blobs on the ionization in the SOL and edge are investigated. Simulations suggest that neutrals originating from dissociation of hydrogen molecules only fuel in the outermost edge region of the plasma, whereas hot neutrals from charge exchange collisions penetrate deep into the bulk plasma. The results are recovered in a simplified 2D model. (paper)

[en] The propagation of a cold pulse initiated by edge cooling in JET is compared to propagation of the heat wave originating from a modulation of the heating source roughly at mid radius. It is found that the propagation of the cold pulse is by far faster than what could be predicted on the basis of the heat wave propagation, and within local transport models no sufficient explanation for this behaviour can be found. Recently, modelling of the cold pulse propagation using non-local effects and a transport equation that uses fractional derivatives has been successfully applied to model the effect [1]. Here we discuss a model in which the non-locality is introduced by the process of turbulence spreading. Transport models with turbulence spreading have been proposed in [2] and conditions under which the perturbation in the turbulence profile could travel at the required high speed from edge to the core have been established [3]. Here we report on recent results in the modelling of cold pulse propagation using turbulence spreading transport models. (author)