arXiv subjects

Alexander Kendl

Publications and source records attributed to Alexander Kendl.

At least 19 recordsLinked to original sources

arXiv2026

Merging of zonal flows in gyrofluid resistive drift-wave turbulence

Non-linear dynamics of zonal flows is investigated in the context of the gyrofluid modified Hasegawa-Wakatani model. Merging of zonal flows and the chaotic developement of the initial zonal flow pattern is explored. Conservation equations for zonal flow momentum and energy with consistent finite Larmor radius (FLR) effects are derived and used for a quantitative analysis of zonal flow mergers in numerical simulations. The nonlinear local Reynolds stress transfer as opposed to (hyper)viscous dissipation is found to be the main cause of merging. The applicability of the concept of a phase transition in the strict thermodynamical sense is discussed in context of zonal flow transition hysteresis.

physics.plasm-ph↗

arXiv2023

TIFF: Gyrofluid Turbulence in Full-f and Full-k

A model and code (TIFF) for isothermal gyrofluid computation of quasi-two-dimensional interchange and drift wave turbulence in magnetized plasmas with arbitrary fluctuation amplitudes (full-f) and arbitrary polarization wavelengths (full-k) is introduced. The model reduces to the paradigmatic Hasegawa-Wakatani model in the limits of small turbulence amplitudes (delta-f), cold ions (without finite Larmor radius effects), and homogeneous magnetic field. Several solvers are compared for the generalized Poisson problem, that is intrinsic to the full-f gyrofluid (and gyrokinetic) polarization equation, and a novel implementation based on a dynamically corrected Fourier method is proposed. The code serves as a reference case for further development of three-dimensional full-f full-k models and solvers, and for fundamental exploration of large amplitude turbulence in the edge of magnetized plasmas.

physics.plasm-ph↗

arXiv2020

Padé-based arbitrary wavelength polarization closures for full-F gyro-kinetic and -fluid models

We propose a solution to the long-standing short wavelength polarization closure shortfall of full-F gyro-fluid models. This is achieved by first finding an appropriate quadratic form of the gyro-fluid moment over the polarization part of the gyro-center Hamiltonian. Secondly, we deduce Padé-based approximations to the latter expression that produce a polarization charge density with the desired order of accuracy and retain linear polarization effects for arbitrary wavelengths. The proposed closures feature proper energy conservation and the anticipated Oberbeck-Boussinesq and long wavelength limits.

physics.plasm-ph↗

arXiv2018

The collisional drift wave instability in steep density gradient regimes

The collisional drift wave instability in a straight magnetic field configuration is studied within a full-F gyro-fluid model, which relaxes the Oberbeck-Boussinesq (OB) approximation. Accordingly, we focus our study on steep background density gradients. In this regime we report on corrections by factors of order one to the eigenvalue analysis of former OB approximated approaches as well as on spatially localised eigenfunctions, that contrast strongly with their OB approximated equivalent. Remarkably, non-modal phenomena arise for large density inhomogeneities and for all collisionalities. As a result, we find initial decay and non-modal growth of the free energy and radially localised and sheared growth patterns. The latter non-modal effect sustains even in the nonlinear regime in the form of radially localised turbulence or zonal flow amplitudes.

physics.plasm-ph↗

arXiv2018

Interchange transport in electron-positron plasmas with ion impurities

Interchange drive and cross-field transport of density filaments in quasi-neutral inhomogeneously magnetized electron-positron plasmas is shown to be strongly reduced by the presence of minority ions. Two mechanisms are identified for the reduction in radial propagation and plasma transport: effective mass related inertia, and collisionality dependent Boltzmann spin-up of the filaments. Numerical results are obtained with a three-dimensional full-F multi-species gyrofluid model.

physics.plasm-ph↗

arXiv2018

Non-Oberbeck-Boussinesq zonal flow generation

Novel mechanisms for zonal flow (ZF) generation for both large relative density fluctuations and background density gradients are presented. In this non-Oberbeck-Boussinesq (NOB) regime ZFs are driven by the Favre stress, the large fluctuation extension of the Reynolds stress, and by background density gradient and radial particle flux dominated terms. Simulations of a nonlinear full-F gyro-fluid model confirm the predicted mechanism for radial ZF propagation and show the significance of the NOB ZF terms for either large relative density fluctuation levels or steep background density gradients.

physics.plasm-ph↗

arXiv2017

Gyrofluid vortex interaction

Low-frequency turbulence in magnetised plasmas is intrisically influenced by gyroscale effects across ion Larmor orbits. Here we show that fundamental vortex interactions like merging and co-advection in gyrofluid plasmas are essentially modified under the influence of gyroinduced vortex spiraling. For identical initial vorticity, the fate of co-rotating eddies is decided between accelerated merging or explosion by the asymmetry of initial density distributions. Structures in warm gyrofluid turbulence are characterised by gyrospinning enhanced filamentation into thin vorticity sheets.

physics.plasm-ph↗

arXiv2017

Isotope effect on blob-statistics in gyrofluid simulations of scrape-off layer turbulence

We investigate time-series obtained from gyrofluid simulations in coupled edge/scrape-off layer turbulence characteristic for fusion edge-region plasmas. Blob birth near the separatrix produces intermittent signals whose statistics depend on the ion mass of the reactor fuel, pointing towards overall slower dynamics for heavier isotopes. We find that a recently established shot-noise stochastic model for scrape-off layer fluctuations coincides reasonably well with the numerical simulations performed in this contribution.

physics.plasm-ph↗

arXiv2017

Isotope effect on filament dynamics in fusion edge plasmas

The influence of the ion mass on filament propagation in the scrape-off layer of toroidal magnetised plasmas is analysed for various fusion relevant majority species, like hydrogen isotopes and helium, on the basis of a computational isothermal gyrofluid model for the plasma edge. Heavy hydrogen isotope plasmas show slower outward filament propagation and thus improved confinement properties compared to light isotope plasmas, regardless of collisionality regimes. Similarly, filaments in fully ionised helium move more slowly than in deutrium. Different mass effects on the filament inertia through polarisation, finite Larmor radius, and parallel dynamics are identified.

physics.plasm-ph↗

arXiv2016

Ergodicity of gyrofluid edge localised ideal ballooning modes

The magnetic field structure associated with edge localised ideal ballooning mode (ELM) bursts is analysed by nonlinear gyrofluid computation. The linear growth phase is characterised by the formation of small scale magnetic islands. Ergodic magnetic field regions develop near the end of the linear phase when the instability starts to perturb the equilibrium profiles. The nonlinear blow-out gives rise to an ergodisation of the entire edge region. The time-dependent level of ergodicity is determined in terms of the mean radial displacement of a magnetic field line. The ergodicity decreases again during the nonlinear turbulent phase of the blow-out in dependence on the degrading plasma beta in the collapsing plasma pedestal profile.

physics.plasm-ph↗

arXiv2016

Gyrofluid computation of magnetic perturbation effects on turbulence and edge localized bursts

The effects of non-axisymmetric resonant magnetic perturbation fields (RMPs) on saturated drift-wave turbulence and on ballooning mode bursts in the edge pedestal of tokamak plasmas are investigated by numerical simulations with a nonlinear six-moment electromagnetic gyrofluid model including zonal profile evolution. The vacuum RMP fields are screened by plasma response currents, so that magnetic transport by perturbed parallel motion is not significantly changed. Radial transport of both particles and heat is dominated by turbulent convection even for large RMP amplitudes, where formation of stationary convective structures leads to edge profile degradation. Modelling of ideal ballooning mode unstable edge profiles for single bursts including RMP fields causes resonant mode locking and destabilization.

physics.plasm-ph↗

arXiv2016

Interchange instability and transport in matter-antimatter plasmas

Symmetric electron-positron plasmas in inhomogeneous magnetic fields are intrinsically subject to interchange instability and transport. Scaling relations for the propagation velocity of density blob perturbations relevant to transport in isothermal magnetically confined electron-positron plasmas are deduced, including damping effects when Debye lengths are large compared to Larmor radii. The relations are verified by nonlinear full-F gyrofluid computations. Results are in favour of sufficient magnetic confinement for planned electron-positron plasma experiments. The model is generalised to other matter-antimatter plasmas. Magnetised electron-positron-proton-antiproton plasmas are susceptible to interchange driven local matter-antimatter separation, which can be expected to impede (so far unrealised) sustained laboratory magnetic confinement.

physics.plasm-ph↗

arXiv2016

Isotope effect on gyro-fluid edge turbulence and zonal flows

The role of ion polarisation and finite Larmor radius on the isotope effect on turbulent tokamak edge transport and flows is investigated by means of local electromagnetic multi-species gyro-fluid computations. Transport is found to be reduced with the effective plasma mass for protium, deuterium and tritium mixtures. This isotope effect is found for both cold and warm ion models, but significant influence of finite Larmor radius and polarisation effects are identified. Sheared flow reduction of transport through self generated turbulent zonal flows and geodesic acoustic modes in the present model (not including neoclassical flows) is found to play only a minor role on regulating isotopically improved confinement.

physics.plasm-ph↗

arXiv2015

Splitting methods for time integration of trajectories in combined electric and magnetic fields

The equations of motion of a single particle subject to an arbitrary electric and a static magnetic field form a Poisson system. We present a second-order time integration method which preserves well the Poisson structure and compare it to commonly used algorithms, such as the Boris scheme. All the methods are represented in a general framework of splitting methods. We use the so-called $\phi$ functions, which give efficient ways for both analyzing and implementing the algorithms. Numerical experiments show an excellent long term stability for the new method considered.

math.NA↗

arXiv2015

Inertial blob-hole symmetry breaking in magnetised plasma filaments

Symmetry breaking between the propagation velocities of magnetised plasma filaments with large positive (blob) and negative (hole) amplitudes, as implied by a dimensional analysis scaling, is studied with global ("full-n") non-Boussinesq gyrofluid computations, which include finite inertia effects through nonlinear polarisation. Interchange blobs on a flat density background have higher inertia and propagate more slowly than holes. In the presence of a large enough density gradient, the effect is reversed: blobs accelerate down the gradient and holes are slowed in their propagation up the gradient. Drift wave blobs spread their initial vorticity rapidly into a fully developed turbulent state, whereas primary holes can remain coherent for many eddy turnover times. The results bear implications for plasma edge zonal flow evolution and tokamak scrape-off-layer transport.

physics.plasm-ph↗

arXiv2014

Modelling of turbulent impurity transport in fusion edge plasmas using measured and calculated ionization cross sections

Turbulent transport of trace impurities impurities in the edge and scrape-off-layer of tokamak fusion plasmas is modelled by three dimensional electromagnetic gyrofluid computations including evolution of plasma profile gradients. The source function of impurity ions is dynamically computed from pre-determined measured and calculated electron impact ionization cross section data. The simulations describe the generation and further passive turbulent E-cross-B advection of the impurities by intermittent fluctuations and coherent filamentary structures (blobs) across the scrape-off-layer.

physics.plasm-ph↗

arXiv2012

Asymmetric chiral alignment in magnetized plasma turbulence

Multi species turbulence in inhomogeneous magnetised plasmas is found to exhibit symmetry breaking in the dynamical alignment of a third species with the fluctuating electron density and vorticity with respect to the magnetic field direction and the species' relative background gradients. The possibility of truly chiral aggregation of charged molecules in magnetized space plasma turbulence is discussed.

physics.plasm-ph↗

arXiv2012

Turbulence and structure formation in complex plasmas and fluids

The formation and evolution of nonlinear and turbulent dynamical structures in two-dimensional complex plasmas and fluids is explored by means of generalised (drift) fluid simulations. Recent numerical results on turbulence in dusty magnetised plasmas, strongly coupled fluids, semi-classical ("quantum") plasmas and in rotating quantum fluids are reviewed and discussed.

physics.plasm-ph↗