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In recent years tokamak experiments and modelling have increasingly indicated that the interaction between suprathermal (fast) ions and thermal plasma can lead to a reduction of turbulence and an improvement of confinement. The regimes in which this stabilization occurs are relevant to burning plasmas, and their understanding will inform reactor scenario optimization. This review summarizes observations, simulations, theoretical understanding, and open questions on this emerging topic.

A novel nonlinear functional relation of turbulence potential intensity, zonal flow potential intensity, and ion thermal diffusivity that accurately reproduces nonlinear gyrokinetic simulations of toroidal ion temperature gradient (ITG) driven turbulence is proposed. Applying mathematical optimization techniques to find extremal solutions in high-dimensional parameter space, the optimal regression parameters in the functional form are determined to be valid for both near- and far-marginal regime of the ITG stability including the Dimits-shift. Then, the regression error of ∼5% is accomplished. In addition, it is clarified that the intensity ratio of the zonal flow and turbulence potential intensity is a crucial factor to determine the reproduction accuracy., source:Nakayama T et al 2022 Plasma Phys. Control. Fusion 64 075007, source:https://doi.org/10.1088/1361-6587/ac6ffc, identifier:0000-0002-6719-9013

This conference report summarizes the contributions to, and discussions at the joint meeting of the 9th Asia Pacific-Transport Working Group (APTWG) & EU-US Transport Task Force (TTF) workshop held online, hosted by Kyushu University, Japan, during 6–9 July 2021. The topics of the meeting were organized under five main topics: (1) isotope effect on transport and physics on isotope mixture plasma, (2) turbulence spreading and coupling in core-edge-SOL, (3) interplay between magnetohydrodynamic topology/instability and turbulent transport, (4) interaction between energetic particle driven instability and transport, (5) model reduction and experiments for validation., source:K. Ida et al 2022 Nucl. Fusion 62 037001, source:https://doi.org/10.1088/1741-4326/ac3f19, identifier:0000-0002-0585-4561

The impact of magnetic geometry on zonal-flow generation in ion temperature gradient driven turbulence is investigated by means of linear and nonlinear gyrokinetic simulations. The modulation of geodesic curvature on various configurations has revealed amplification of the zonal-flow intensity in relatively smaller geodesic curvature. Based on these findings, a nonlinear proxy model for explorations of novel magnetic geometry to activate the zonal-flow dynamics is proposed., source:https://doi.org/10.1585/pfr.17.1203077, identifier:0000-0003-2693-4859

A new method is presented for identifying waveforms of fluctuations in turbulent plasmas. The method is based on heartbeat analysis in which the convolution of a waveform is obtained by employing the phase tracking method. Phase tracking is performed by correlating raw time-series data with a template waveform; the template is evaluated through iteration procedure. The method is applied to fluctuations in a PANTA plasma, and the nonlinear waveform and its distribution of periods are obtained., source:https://doi.org/10.1585/pfr.9.1201016

We present a synthetic phase contrast imaging (PCI) diagnostic for studying turbulence at the Wendelstein 7-X (W7-X) stellarator. We first describe the implemented instrument response model, which captures diffraction effects, detector noise, and the long-wavelength cutoff due to the phase plate of the PCI system. To verify the instrument response model, we show that it is capable of reproducing the PCI signal generated by the sound wave speaker used for calibration at W7-X. Next, we discuss the calculation of synthetic PCI signals based on the global, nonlinear gyrokinetic codes GENE-3D and EUTERPE, including results from some of the first stellarator simulations of this type with kinetic electrons (KEs) in GENE-3D. While the simulations used in this work lack a neoclassical radial electric field, which is crucial for reproducing experimental PCI signals, they do indicate that the dominant rotation direction and velocities of the turbulent fluctuations can be inferred from the wave number-frequency spectra of the PCI signals, as expected. The synthetic PCI wave number spectra are further shown to be similar to those of the line-integrated fluctuating electron density, with distinct differences between adiabatic and KE simulations, explainable by previously published turbulence models. For example, the wave number spectra of all adiabatic electron simulations analyzed here follow a power law with an exponent close to −5 for sufficiently large wave numbers. This indicates that universal features of electron density turbulence at W7-X may be studied using the PCI system.

In this letter, we investigate the spatial structure of a low frequency global potential oscillation, which is likely ascribed as the low frequency zonal flows, in TJ-II laboratory plasmas. In two plasmas produced by different heating schemes (electron cyclotron resonance heating and neutral beam injection heating) and characterized by different mean radial electric field structures, frequency-space-decomposed spectra of the global potential oscillation are obtained. In both cases, the oscillatory field has a single-peaked potential structure and a dipole radial electric field structure. The oscillating structure depends on its frequency as well as on the heating scheme., source:Citation T. Kobayashi et al 2019 Nucl. Fusion 59 044006, source:https://doi.org/10.1088/1741-4326/ab0122, identifier:0000-0001-5669-1937

The scattering of millimeter-wave beams from electron density fluctuations and the associated beam broadening are experimentally demonstrated. Using a dedicated setup, instantaneous deflection and (de-)focusing of the beam due to density blobs on the beam path are shown to agree with full-wave simulations. The detected time-averaged wave power transmitted through the turbulent plasma is reproduced by the radiative-transfer model implemented in the WKBeam code, which predicts a ∼50% turbulence-induced broadening of the beam cross-section. The role of core turbulence for the considered geometry is highlighted., QC 20230724

The scattering of millimeter-wave beams from electron density fluctuations and the associated beam broadening are experimentally demonstrated. Using a dedicated setup, instantaneous deflection and (de-)focusing of the beam due to density blobs on the beam path are shown to agree with full-wave simulations. The detected time-averaged wave power transmitted through the turbulent plasma is reproduced by the radiative-transfer model implemented in the WKBeam code, which predicts a ∼50% turbulence-induced broadening of the beam cross-section. The role of core turbulence for the considered geometry is highlighted.

Poloidal asymmetries of the propagation velocity of density fluctuations perpendicular to the magnetic field measured with Doppler reflectometry have been reported in several magnetic confinement plasma devices. Careful analysis of a large variety of different low confinement mode plasma scenarios performed at the ASDEX Upgrade tokamak does not reveal such an asymmetry outside the uncertainties of the evaluation process of the measurement data. The perpendicular velocity is investigated between mid-radius and the plasma edge and follows the poloidal dependence of the E × B drift velocity regardless of the probed turbulence structure size. Compared to measurements of a charge exchange recombination spectroscopy diagnostic this points towards a significantly smaller phase velocity than the E × B drift velocity. The analysis technique is presented in a representative discharge together with a sensitivity study of the impact of density, magneto hydrodynamic equilibrium and diagnostic alignment on the interpretation of the measured Doppler shift using ray tracing and thus on poloidal asymmetries. Three more highly different plasma scenarios with poloidally symmetric velocity profiles are shown.

The magnetosheath (MSH) plasma turbulence depends on the structure and properties of the bow shock (BS). Under quasi-parallel (Q(||)) and quasi-perpendicular (Q(perpendicular to)) BS configurations the electromagnetic field and plasma quantities possess quite distinct behavior, e.g., being highly variable and structured in the Q(||) case. Previous studies have reported abundance of thin current sheets (with typical scales of the order of the plasma kinetic scales) in the Q(||) MSH, associated with magnetic reconnection, plasma heating, and acceleration. Here we use multipoint observations from Magnetospheric MultiScale (MMS) mission, where for the first time a comparative study of discontinuities and current sheets in both MSH geometries at very small spacecraft separation (of the order of the ion inertial length) is performed. In Q(||) MSH the current density distribution is characterized by a heavy tail, populated by strong currents. There is high correlation between these currents and the discontinuities associated with large magnetic shears. Whilst, this seems not to be the case in Q(perpendicular to) MSH, where current sheets are virtually absent. We also investigate the effect of the discontinuities on the scaling of electromagnetic fluctuations in the MHD range and in the beginning of the kinetic range. There are two (one) orders of magnitude higher power in the magnetic (electric) field fluctuations in the Q(||) MSH, as well as different spectral scaling, in comparison to the Q(perpendicular to) MSH configuration. This is an indication that the incoming solar wind turbulence is completely locally reorganized behind Q(perpendicular to) BS while even though modified by Q(||) BS geometry, the downstream turbulence properties are still reminiscent to the ones upstream, the latter confirming previous observations. We show also that the two geometries are associated with different temperature anisotropies, plasma beta, and compressibility, where the Q(perpendicula

The nonlinear evolution of collisionless plasma flows is a multi-scale process where the energy is injected at large, fluid scales and dissipated at small, kinetic scales. Accurately modelling the global evolution of collisionless plasmas requires to take into account the main micro-scale physical processes of interest, in particular the kinetic effects. We concentrate here on the nonlinear evolution of the magnetized Kelvin-Helmholtz instability in collisionless plasmas. First, we will summarize recent works concerning fluid and kinetic modelling of the magnetized Kelvin-Helmholtz instability. Comparisons of different plasma models is today an imperative task aiming at understanding cross-scale processes in collisionless plasma turbulence. We then report new results of two dimensional fully kinetic (both electrons and ions are treated kinetically) simulations of the magnetized Kelvin-Helmholtz instability. We make use of the implicit Particle-In-Cell (PIC) code iPIC3D in 2D-3V configuration. In this work, we focus on the analysis of the effects of kinetic physics and compare results obtained with the full kinetic model to those obtained in the context of MHD and hybrid simulations. We discuss the space physics implications of the nonlinear saturation of the Kelvin-Helmholtz instability, in the context of the interactions between the solar wind and the Earth magnetosphere., QC 20200630

To meet the future energy demands of the World, fusion energy has been proposed as a possible energy solution as it provides clean, sustainable and reliable means of energy production. However, a working fusion power plant has yet to be realised. Currently, the most promising reactor design is a magnetic confinement device to called a Tokamak. One of the problems this concept is facing is turbulent transport of particles and energy from the region of closed magnetic field lines to that of open magnetic field lines and onto plasma-facing materials of the reactor. Future reactors inherently contain multiple ion species due to fusing of deuterium and tritium. Consequently, understanding the influence of the multiple ion species on turbulent transport is of utmost importance. In this work, a drift fluid model is derived that consistently incorporates multiple ion species and collisional interactions between species. The model is derived from the Boltzmann equation and uses the Zhdanov closure to obtain a closed set of equations from which the drift fluid expansion can be performed. The model equations are solved numerically using the discontinuous Galerkin method and the numerical implementation of the equations is discussed in detail. With the numerical implementation of the equations, two topics are studied, seeded plasma blobs and turbulent transport. For the seeded blobs, the influence of different ion mixes on the propagation of the blobs is examined through scaling of the blob velocity. The mixes that are studied are deuterium-tritium mixtures as well as deuterium and doubly ionized helium mixes. Finally, fully developed turbulence is investigated. In particular, the impact of multiple ion species on the particle and energy fluxes across the last closed flux surface is examined. For the blobs, it was found that when the mixture is of deuterium and tritium, the velocity of the radial motion decreases as the amount of tritium increases. From the deuteriu

The process of conversion or dissipation of energy in nearly collisionless turbulent space plasma, is yet to be fully understood. The existing models offer different energy dissipation mechanisms which are based on wave particle interactions or non-resonant stochastic heating. There are other mechanisms of irreversible processes in space. For example, turbulence generated coherent structures, e.g., current sheets are ubiquitous in the solar wind and quasi-parallel magnetosheath. Reconnecting current sheets in plasma turbulence are converting magnetic energy to kinetic and thermal energy. It is important to understand how the multiple (reconnecting) current sheets contribute to spatial distribution of turbulent dissipation. However, detailed studies of such complex structures have been possible mainly via event studies in proper coordinate systems, in which the local inflow/outflow, electric and magnetic field directions, and gradients could be studied. Here we statistically investigate different energy exchange/dissipation (EED) measures defined in local magnetic field-aligned coordinates, as well as frame-independent scalars. The presented statistical comparisons based on the unique high-resolution MMS data contribute to better understanding of the plasma heating problem in turbulent space plasmas.

Turbulence at kinetic scales is an unresolved and ubiquitous phenomenon that characterizes both space and laboratory plasmas. Recently, new theories, in situ spacecraft observations and numerical simulations suggest a novel scenario for turbulence, characterized by a so-called phase-space cascade - the formation of fine structures, both in physical and velocity-space. This new concept is here extended by directly taking into account the role of inter-particle collisions, modeled through the nonlinear Landau operator or the simplified Dougherty operator. The characteristic times, associated with inter-particle correlations, are derived in the above cases. The implications of introducing collisions on the phase-space cascade are finally discussed.

Time resolving investigations of charged particles flow from the cathode region of the vacuum arc reveal that the basis of charged particles flow is short-term burst. These bursts can form sequences, groups with a relatively smooth waveform and superbursts. The Fourier analysis showed that ion flow has a brown-like frequency law. This fact can serve as evidence of the fractal nature of the cathode spot. However, the detailed analysis of ionsand accelerated electron signals revealed that the fractal nature of charged particle signal can reflect the fractal nature of plasma turbulences in discharge plasma. Also, the strong correlation between intensive charged particle flow appearance and arc discharge parameter instabilities was revealed. Thus, the plasma instabilities can be considered as the cause of ion and electron flow acceleration in the vacuum arc. © 2019 IOP Publishing Ltd. All rights reserved.

In recent years, a strong reduction of plasma turbulence in the presence of energetic particles has been reported in a number of magnetic confinement experiments and corresponding gyrokinetic simulations. While highly relevant to performance predictions for burning plasmas, an explanation for this primarily nonlinear effect has so far remained elusive. A thorough analysis finds that linearly marginally stable energetic particle driven modes are excited nonlinearly, depleting the energy content of the turbulence and acting as an additional catalyst for energy transfer to zonal modes (the dominant turbulence saturation channel). Respective signatures are found in a number of simulations for different JET and ASDEX Upgrade discharges with reduced transport levels attributed to energetic ion effects., The simulations presented in this work were performed at the Cobra HPC system at the Max Planck Computing and Data Facility (MPCDF), Germany. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No. 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. Furthermore, we acknowledge the CINECA award under the ISCRA initiative, for the availability of high performance computing resources and support. The authors would like to thank N. Bonanomi, J. Citrin, H. Doerk, Ph. Lauber, P. Manas, P. Mantica, M.J. Pueschel, K. Stimmel, P.W. Terry, D. Zarzoso, and A. Zocco for all the stimulating discussions, useful suggestions and comments., Peer Reviewed, Postprint (published version)

Classical stellarators suffer from strong neoclassical transport. The advanced stellarator Wendelstein 7-X is the first fully neo-classically optimized device posing the question about the role of turbulence in optimized stellarators. While there has been a growing number of theoretical investigations about turbulence in advanced stellarator geometry over the last two decades, the experimental investigation is a completely new field. The present thesis deals with the phase contrast imaging (PCI) diagnostic implementation on Wendelstein 7-X as a tool to address the lack of experimental research on the evolution of turbulent fluctuations in optimized stellarator geometry. Numerical simulations suggest the occurrence of microturbulence in the spatiotemporal range of ion scales such as the ITG and TEM instabilities. The PCI diagnostic allows for non-invasive measurements of line-of-sight integrated density fluctuations. It incorporates a flexible design to cover spatiotemporal scales from the ion down to the electron scale. The PCI uses a CO2 laser emitting light at 10.6µm. At this wavelength the plasma acts as an optical isotropic medium in which density fluctuations present phase objects causing a scattering of the laser. Key element of the PCI diagnostic is the phase plate which allows for a phase shift of the most energetic scattered light components in order to convert the phase modulation into a measurable intensity variation. An excellent alignment onto the phase plate is crucial for the linear diagnostic response on the density fluctuations. An optical beam path length of more than 20m imposes a challenge on this alignment. As a consequence, remote control capabilities are implemented in order to control and correct any misalignment during operation. Turbulent density fluctuations in the ion scale are analysed on the example of a typical electron cyclotron resonance heated discharge in Wendelstein 7-X for a moderate density and different heating steps. The spectral analys

Fast ions have recently been found to significantly impact and partially suppress plasma turbulence both in experimental and numerical studies in a number of scenarios. Understanding the underlying physics and identifying the range of their beneficial effect is an essential task for future fusion reactors, where highly energetic ions are generated through fusion reactions and external heating schemes. However, in many of the gyrokinetic codes fast ions are, for simplicity, treated as equivalent-Maxwellian-distributed particle species, although it is well known that to rigorously model highly non-thermalised particles, a non-Maxwellian background distribution function is needed. To study the impact of this assumption, the gyrokinetic code GENE has recently been extended to support arbitrary background distribution functions which might be either analytical, e.g., slowing down and bi-Maxwellian, or obtained from numerical fast ion models. A particular JET plasma with strong fast-ion related turbulence suppression is revised with these new code capabilities both with linear and nonlinear gyrokinetic simulations. It appears that the fast ion stabilization tends to be less strong but still substantial with more realistic distributions, and this improves the quantitative power balance agreement with experiments., Export Date: 9 May 2018; Article; CODEN: PHPAE. QC 20180604

Thermonuclear fusion of light atomic nuclei in future reactors promises a source of electric energy which is clean, safe, and sustainable. A propitious design for such reactor is the tokamak, in which hot plasma is suspended in a twisted toroidal magnetic field. The geometry of toroidal connement results in turbulent transport of particles and heat from the conned bulk region radially outwards towards the first wall of the vacuum chamber. The plasma passes through the edge layer of the conned region, for which the conditions have been found to be defining for the overall plasma connement. This makes the plasma edge a region of uttermost importance to understand, if energy from reactor based fusion is to be realized. In the scrape-off layer (SOL) region, between the edge region and the first wall, plasma is transported outwards in field-aligned laments. The SOL is characterized by open magnetic field lines which terminate on material surfaces, and it is signicantly colder and less dense than the conned plasma. This allows for maintaining a considerable population of neutral particles. It has been experimentally observed that the presence of neutrals in influences the conditions in the edge region. This makes the understanding of transport of neutrals and their interactions with the edge and SOL plasma equally important. In this work a novel numerical model for describing the transport of neutral particles, in particular the interactions with the electrons and ions, is formulated. Both molecular and atomic neutral species are included in the model to account for various neutral-plasma interactions. The model for the neutrals is coupled to an existing drift-fluid model for edge and SOL plasma, and the combined model is applied in a series of studies on the effect of neutral interactions on the plasma and vice versa. Particularly the relative roles of molecules and atoms in the plasma fuelling process, as well as the local effect that the field-aligned plasma l

Understanding the stabilising mechanism of fast particles on plasma turbulence is an essential task for a fusion reactor, where the energetic particles can constitute a significant fraction of the main ions. While the consideration of equivalent Maxwellian distributed fast ions in the simulations has greatly improved the agreement with experiments, fast ion electromagnetic stabilization seems to be somewhat over-estimated. Power balance is usually reached only with increased plasma gradients. However, it is well known that to rigorously model highly non thermalised particles, a non-Maxwellian background distribution function is needed. To this aim, a previous study on a particular JET plasma has been revised and analysed with the gyrokinetic code GENE. Fast particles have been modelled with a number of different analytic and numerical distributions. The latter have been imported from the modelling tools NEMO/SPOT and SELFO., QC 20190619

JET ITER-like wall (ILW) experiments show that the edge density evolution is strongly linked with the poloidal distribution of the ionization source. The fueling profile in the JET-ILW is more delocalized as compared to JET-C (JET with carbon-based plasma-facing components PFCs). Compared to JET-C the H-mode pedestal fueling cycle is dynamically influenced by a combination of plasma-wall interaction features, in particular: (1) edge-localized modes (ELMs) induced energetic particles are kinetically reflected on W divertor PFCs leading to distributed refueling away from the divertor depending on the divertor plasma configuration, (2) delayed molecular re-emission and outgassing of particles being trapped in W PFCs (bulk-W at the high field side and W-coated CFCs at the low field side) with different fuel content and (3) outgassing from Be co-deposits located on top of the high-field side baffle region shortly after the ELM. In view of the results of a set of well diagnosed series of JET-ILW type-I ELMy H-mode discharges with good statistics, the aforementioned effects are discussed in view of H-mode pedestal fueling capacity. The ongoing modelling activities with the focus on coupled core-edge plasma simulations and plasma-wall interaction are described and discussed also in view of possible code improvements required., Funding details: EC, European Commission; Funding text: This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.QC 20171201

Continuous plasma coherent emission is maintained by repetitive Langmuir collapse driven by the nonlinear evolution of a strong electron two-stream instability. The Langmuir waves are modulated by solitary waves in the linear stage and electrostatic whistler waves in the nonlinear stage. Modulational instability leads to Langmuir collapse and electron heating that fills in cavitons. The high pressure is released via excitation of a short-wavelength ion acoustic mode that is damped by electrons and reexcites small-scale Langmuir waves; this process closes a feedback loop that maintains the continuous coherent emission.

The Controlled Shear Decorrelation Experiment (CSDX) linear plasma device provides a unique platform for investigating the underlying physics of self-regulating drift-wave turbulence/zonal flow dynamics. A 3D drift-reduced nonlocal cold ion fluid model which evolves density, vorticity, and electron temperature fluctuations, with proper sheath boundary conditions, is used to simulate dynamics of the turbulence in CSDX. The simulations show density gradient-driven drift-waves are the dominant instability in CSDX. However, the choice of insulating or conducting end plate boundary conditions affects the linear growth rates and energy balance of the system due to the absence or addition of Kelvin-Helmholtz modes generated by the sheath-driven equilibrium E×B shear and sheath-driven temperature gradient instability.The physics study is then followed by an extensive quantitative validation study. For the direct comparison of nonlinear simulation results to experiment, a synthetic Langmuir probe diagnostic is used to generate a set of synthetic observables which are in turn used to construct the validation metrics. A significant improvement of model-experiment agreement relative to the previous 2D simulations is observed. An essential component of this improved agreement is found to be the effect of electron temperature fluctuations on floating potential measurements, which introduces clear amplitude and phase shifts relative to the plasma potential fluctuations in synthetically measured quantities. Moreover, systematic simulation scans show that the self-generated E×B zonal flows profile is very sensitive to the steepening of density equilibrium profile. To minimize the number of simulations required for uncertainty quantification, a more advanced seed sampling methods for efficient sampling of the input parameter space, and a rapidly converging non-intrusive Probabilistic Collocation method to map out a probabilistic model response is implemented. This approach is shown t

Turbulence is ubiquitous in the flows of fluids and plasmas. This thesis is devoted to studies of the statistical properties of turbulence in the three-dimensional (3D) Hall magnetohydrodynamic (Hall-MHD) equations, the two-dimensional (2D) MHD equations, the one-dimensional (1D) hyperviscous Burgers equation, and the 3D Navier-Stokes equations. Chapter 1 contains a brief introduction to statistically homogeneous and isotropic turbulence. This is followed by an over-view of the equations we study in the subsequent chapters, the motivation for the studies and a summary of problems we investigate in chapters 2-6. In Chapter 2 we present our study of Hall-MHD turbulence [1]. We show that a shell-model version of the 3D Hall-MHD equations provides a natural theoretical model for investigating the multiscaling behaviors of velocity and magnetic structure functions. We carry out extensive numerical studies of this shell model, obtain the scaling exponents for its structure functions, in both the low-k and high-k power-law ranges of 3D Hall-MHD, and find that the extended-self-similarity procedure is helpful in extracting the multiscaling nature of structure functions in the high-k regime, which otherwise appears to display simple scaling. Our results shed light on intriguing solar-wind measurements. In Chapter 3 we present our study of the inverse-cascade regime in two-dimensional magnetohydrodynamic turbulence [2]. We present a detailed direct numerical simulation (DNS) of statistically steady, homogeneous, isotropic, two-dimensional magnetohydrodynamic (2D MHD) turbulence. Our study concentrates on the inverse cascade of the magnetic vector potential. We examine the dependence of the statistical properties of such turbulence on dissipation and friction coefficients. We extend earlier work significantly by calculating fluid and magnetic spectra, probability distribution functions (PDFs) of the velocity, magnetic, vorticity, current, stream-function, and magnetic-vector-p

Understanding the turbulent transport in the plasma-edge in fusion devices is of utmost importance in order to make precise predictions for future fusion devices. The plasma turbulence observed in linear devices shares many important features with the turbulence observed in the edge of fusion devices, and are easier to diagnose due to lower temperatures and a better access to the plasma. In order to gain greater insight into this complex turbulent behavior, numerical simulations of plasma in a linear device are performed in this thesis. Here, a three-dimensional drift-fluid model is derived from first principles for a magnetized plasma in a linear device. To account for the fluctuations at the same level as the background plasma, the traditional split between background and fluctuations has not been made. The model is implemented using the BOUT++ framework and is solved numerically. Special attention is given to the treatment of the singularity at the cylinder axis, and at the inversion of the non-linear elliptic equation, which is done to obtain the electrical potential. The evolution of the plasma through the steady-state, linear phase, and turbulent phase is investigated and compared for different B-field strengths. It is found that drift-waves are responsible for the onset of turbulence, and that the turbulent radial flux is causing a flattening of the density profiles. Coherent structures from the intermittent radial flux in the turbulent state are investigated. Results of simulations using the Boussinesq approximation is compared to full simulations. It is found that the Boussinesq approximation leads to an unphysical increase of the electrical potential as ions and electrons are lost at a different rate. Finally, the results from the full simulations are compared with simulations performed at different ionization levels, using a simple model for plasma interaction with neutrals. It is found that the steady state and the saturated state of the sys

This article presents a model to describe the effects of non-equilibrium plasma discharges on gas temperature and species concentration, in the set of equations governing the combustion phenomena. Based on the results reported in the literature, the model is constructed by analysing the channels through which the electric energy is deposited. The two main channels by which the electrons produced during the discharge impact the flow are considered: (1) the excitation and the subsequent relaxation of electronic states of nitrogen molecules which leads to an ultrafast increase of gas temperature and species dissociation within the discharge characteristic time; and (2) the excitation and relaxation of vibrational states of nitrogen molecules which causes a much slower gas heating. The model is fully coupled with multi-dimensional flow balance equations with detailed transport coefficients and detailed combustion chemical kinetic mechanisms. This high level of NRP discharge modelling allows computing high Reynolds flows by means of Direct Numerical Simulations and, therefore, a better understanding of plasma-assisted ignition phenomena in practical configurations. A sequence of discharge pulses in air and methane-air mixture in quiescent and turbulent flow configurations are studied with this model. The results show the minor impact of the vibrational energy on mixture ignition and how the increase of the turbulence spreads this vibrational energy and intermediate combustion species around the discharge zone, minimizing the cumulative effect of multiple pulses. In contrast, the production of O atoms during the discharge has a strong impact on the ignition delays and ignition energies (number of discharge pulses). The results also underscore the impact of the initial turbulent flow Reynolds number and the spatial distribution of turbulent eddies, relative to the discharge channel, on the number of pulses needed to ignite the mixture., SCOPUS: ar.j, info:eu-repo/semantics/published

The interaction between fast particles and core turbulence has been established as a central issue for a tokamak reactor. Recent results predict significant enhancement of electromagnetic stabilisation of ITG turbulence in the presence of fast ions. However, most of these simulations were performed with the assumption of equivalent Maxwellian distributed particles, whereas to rigorously model fast ions, a non-Maxwellian background distribution function is needed. To this aim, the underlying equations in the gyrokinetic code GENE have been re-derived and implemented for a completely general background distribution function. After verification studies, a previous investigation on a particular JET plasma has been revised with linear simulations. The plasma is composed by Deuterium, electron, Carbon impurities, NBI fast Deuterium and ICRH 3He. Fast particle distributions have been modelled with a number of different analytic choices in order to study the impact of non-Maxwellian distributions on the plasma turbulence: slowing down and anisotropic Maxwellian. Linear growth rates are studied as a function of the wave number and compared with those obtained using an equivalent Maxwellian. Generally, the choice of the 3He distribution seems to have a stronger impact on the microinstabilities than that of the fast Deuterium., QC 20170308

On 8 September 2015, the four Magnetospheric Multiscale spacecraft encountered a Kelvin-Helmholtz unstable magnetopause near the dusk flank. The spacecraft observed periodic compressed current sheets, between which the plasma was turbulent. We present observations of large-amplitude (up to 100 mV/m) oscillations in the electric field. Because these oscillations are purely parallel to the background magnetic field, electrostatic, and below the ion plasma frequency, they are likely to be ion acoustic-like waves. These waves are observed in a turbulent plasma where multiple particle populations are intermittently mixed, including cold electrons with energies less than 10 eV. Stability analysis suggests a cold electron component is necessary for wave growth., QC 20161109

Validerat; 20160627 (global_studentproject_submitter)

The overall goal of this research is to analyze influence of plasma turbulence on hypersonic sensor systems and NGOTHR applications and to meet the Air Force's ever-increasing demand for reduced cost, size, weight, and power (CSWAP) while maintaining or increasing system functionality. This basic research effort consists of several sub-tasks that use experiments, analytical and numerical methods to examine linear and nonlinear stages of instabilities of plasma flow with velocity shear. Refraction and scattering of high frequency electromagnetic waves on nonlinear vortex structures produced in the process of instability development will be analyzed as well.

Extensive linear and non-linear gyrokinetic simulations and linear magnetohydrodynamic (MHD) analyses performed for JET hybrid discharges with improved confinement have shown that the large population of fast ions found in the plasma core under particular heating conditions has a strong impact on core microturbulence and edge MHD by reducing core ion heat fluxes and increasing pedestal pressure in a feedback mechanism. In the case of the ITER like wall, it is shown how this mechanism plays a decisive role for the transition to plasma regimes with improved confinement and it can explain the weak power degradation obtained in dedicated power scans. The mechanism is found to be highly dependent on plasma triangularity as it changes the balance between the improvement in the plasma core and the edge. The feedback mechanism can play a similar role in the ITER hybrid scenario as in the JET discharges analysed due to its high triangularity plasmas and the large amount of fast ions generated in the core by the heating systems and the alpha power., For complete list of authors see http://dx.doi.org/10.1088/0029-5515/55/5/053007

Mechanisms of current generation in edge localized mode (ELM) filaments are discussed. It is shown that dipole type currents should flow in the filaments and that these currents are short-circuited through the ambient plasma of the scrape-off layer. The steady-state monopole currents in the filaments cannot be closed through the ambient plasma and oscillatory currents with Alfven frequency should emerge instead. The dipole type currents in the filaments are sufficient to produce magnetic field perturbations which create a stochastic layer inside the edge transport barrier (ETB) region. The dynamic of penetration of the magnetic field perturbations is analyzed and it is shown that the corresponding time is comparable with the life time of the filaments. At the stage when the ETB region is stochastized by the currents of the filaments the radial electric field becomes less negative or even positive inside the ETB and the onset of outward convective flux takes place. It is shown that this outward flux could lead to the significant pump-out effect - the density decrease in the pedestal region., QC 20160212

Incoherent scatter radars are powerful ground based instruments for ionospheric measurements. By analysis of the Doppler shifted backscatter spectrum, containing the signature of electrostatic plasma waves, plasma bulk properties are estimated. At high latitudes the backscattered radar power is occasionally enhanced several orders of magnitude above the thermal backscatter level. These enhancements occur during geomagnetic disturbed conditions and are referred to as naturally enhanced ion acoustic echoes (NEIALs). NEIALs are linked to auroral activity with optical auroral emission observed in the vicinity of the radar measurement volume simultaneously to NEIALs. The backscatter enhancements are thought to be caused by wave activity above thermal level due to instability. A number of theories have been put forward including streaming instabilities and Langmuir turbulence to explain NEIAL observations. NEIALs occur in two classes distinct by their Doppler features. Observations of the first type, which has been studied more extensively, are generally modelled well by the Langmuir turbulence model. The difficulty in trying to understand the driving mechanism of the instability is the limited spatial resolution of the radar measurements. Observations of the second type, reported on more recently, have been interpreted as evidence for naturally occurring strong Langmuir turbulence by means of their Doppler features. Aperture synthesis is a technique to increase the spatial resolution of the radar measurements to below beam width of the single receiver antennas. The technique is employed to investigate the structure of NEIALs in the plane perpendicular to the magnetic field at sub-degree scale corresponding to hundreds of meters to a few kilometres at ionospheric altitudes. Calibration of the radar interferometer is necessary and a calibration technique is presented in paper I. Interferometry observations of a NEIAL event with receivers deployed at the EISCAT incoherent s, QC 20150303

Soliton solutions of nonlinear Schroedinger equation solitons represent an admixture of clockwise and anticlockwise polarized waves coupled due to the nonlinearity. They are excited in the ionosphere due to formation of strong field-aligned currents induced by the motion of neutrals against the charged particles in the dynamo region. This paper demonstrates that nonlinear dynamics of such solitary waves may be quite different depending on the type of a propagating medium. In unstable media with the linear driver, the stationary state is attained in the form of a traveling wave solution which results in the appearance of stable Alfven solitons traveling back and forth between the conjugate ionosphere. On the contrary, in case of a circular driver, the stationary state is not formed. The propagation of the nonlinear structures in the ionospheric plasma is then accompanied plasma turbulence excited by strong transverse electric currents in the front of the shock creating the turbulent track of a soliton. Lastly, generalization to the two dimensional structures is also presented., 非線形シュレディンガー方程式のソリトン解は非線形性により結合した時計回りと反時計回りの偏波を混合した波動を示す。これらのソリトンはダイナモ領域において荷電粒子に対する中性粒子の運動により誘起された強電磁場整合電流の形成により電離層中で励起される。ここでは、このような孤立波の非線形ダイナミクスは、おそらく伝搬媒質のタイプに全く異なる形で依存することを示した。線形ドライバを伴う不安定媒質において、共役電離層間を前後に進行する安定アルベーンソリトンを生成する進行波形式で安定状態を得た。これに反して、円形ドライバの場合は定常状態は形成されない。電離層プラズマにおける非線形構造の伝搬は、ソリトンの乱流トラックを生成する衝撃波前面での強い横方向電流が励起したプラズマ乱流をともなっている。最後に、2次元構造への一般化について言及した。

Electromagnetic features of turbulent filaments, emerging from a turbulent plasma background, have been studied in four different magnetic configurations: the stellarator TJ-II, the Reversed Field Pinch RFX-mod, a device that can be operated also as a ohmic tokamak, and the Simple Magnetized Torus, TORPEX. By applying an analogous diagnostic concept in all cases, direct measurements of both field-aligned current density and vorticity were performed inside the filament. The inter-machine comparison reveals a clear dependence of the filament vorticity upon the local time-averaged E x B flow shear. Furthermore, a wide range of local beta was explored allowing concluding that this parameter plays a fundamental role in the appearance of filament electromagnetic features.

A simple dynamical cascade model for the evolution of free energy is considered in the context of gyrokinetic formalism. It is noted that the dynamics of free energy, that characterize plasma micro-turbulence in magnetic fusion devices, exhibit a predator-prey character. Various key features of predator-prey dynamics such as the time delay between turbulence and large-scale flow structures, or the intermittency of the dynamics are identified in the quasi-steady-state phase of the nonlinear gyrokinetic simulations. A novel prediction on the ratio of turbulence amplitudes in different parts of the wavenumber domain that follows from this simple predator-prey model is compared to a set of nonlinear simulation results and is observed to hold quite well in a large range of physical parameters. Detailed validation of the predator-prey hypothesis using nonlinear gyrokinetics provides a very important input for the effort to apprehend plasma micro-turbulence, since the predator-prey idea can be used as a very effective intuitive tool for understanding and designing efficient transport models. © 2014 IOP Publishing Ltd., SCOPUS: ar.j, info:eu-repo/semantics/published