Blob Dynamics Research Articles

Radial drift of plasma blobs in a toroidal magnetic field with fully kinetic and reduced fluid models

In curved magnetic geometries, field-aligned regions of enhanced plasma pressure and density, termed ‘blobs,’ move as coherent filaments across the magnetic field lines. Coherent blobs account for a significant fraction of transport at the edges of magnetic fusion experiments and arise in naturally-occurring space plasmas. This work examines the dynamics of blobs with a fully kinetic electromagnetic particle-in-cell code and with a drift-reduced fluid code. In low-beta regimes with moderate blob speeds, good agreement is found in the maximum blob velocity between the two simulation schemes and simple analytical estimates. The fully kinetic code demonstrates that blob speeds saturate near the initial sound speed, which is a regime outside the validity of the reduced fluid model.

An improved TDE technique for derivation of 2D turbulence structures based on GPI data in toroidal plasma

This paper reports an improved time-delay estimation (TDE) technique for the derivation of turbulence structures based on gas-puff imaging data. The improved TDE technique, integrating an inverse timing search and hierarchical strategy, offers superior accuracy in calculating turbulent velocity field maps and analyzing blob dynamics, which has the power to obtain the radial profiles of equilibrium poloidal velocity, blob size and its radial velocity, even the fluctuation analysis, such as geodesic acoustic modes and quasi-coherent mode, etc. This improved technique could provide important 2D information for the study of edge turbulence and blob dynamics, advancing the understanding of edge turbulence physics in fusion plasmas.

Blob and hole dynamics in the rotating plasma column of a simple magnetised torus in open field line configuration

Blob and hole dynamics in the rotating plasma column of a simple magnetised torus in open field line configuration

Effect of neutral interactions on parallel transport and blob dynamics in gyrokinetic scrape-off layer simulations

The effect of neutral interactions on scrape-off layer (SOL) turbulence is investigated in a continuum gyrokinetic code that has been coupled to a continuum kinetic model of neutral transport. This extends the work of a previous paper [T. N. Bernard et al., Phys. Plasmas 9, 052501 (2022)], which compared two NSTX SOL simulations in simple helical geometry, one with neutrals and one without. The former included electron-impact ionization, charge exchange, and wall recycling. Here, the case with neutrals is compared to a gyrokinetic-only simulation that includes an effective ionization source to separate the effect of sourcing from charge exchange collisions. It is observed that sourcing accounts for many features of the simulated SOL with neutrals, including density and temperature magnitudes and reduced normalized density fluctuations, but differences persist. In particular, a flatter density profile results due to changes in parallel transport when neutral collisions are included, illustrating the importance of neutral drag on global plasma properties. An analysis of coherent turbulent structures, or blobs, in these simulations demonstrates the case with neutrals has slower and larger blobs. A series of seeded blob simulations corroborates the blob velocity observation. In general, the blob motion does not contribute significantly to radial transport in these simulations.

Fragmentation and coalescence dynamics of non-wetting blobs during immiscible two-phase flows in porous media

Fragmentation and coalescence dynamics of non-wetting blobs during immiscible two-phase flows in porous media

Transport barrier and spinning blob dynamics in the tokamak edge

In this work, we investigate the dynamics of plasma blobs in the edge of magnetic confinement devices using a full-f gyrokinetic particle-in-cell code with X-point geometry. In simulations, the evolution of a seeded blob is followed as it approaches a naturally-forming zonal shear layer near the separatrix, where the blob is stabilized by a large spin induced by the self-consistent adiabatic electron response, and blob bifurcation and trapping are observed during the cross-field propagation of blobs. A new theoretical explanation in both the zonal free and zonal shear layer is constructed, where the dominant spin motion is included. A theoretical condition for a transport barrier induced by the interaction between spinning blobs and the zonal shear layer is obtained, and its scaling is verified with simulations. The new theoretical framework, especially the transport barrier, is applicable to explain and predict various experimental phenomena. In particular, the transport barrier condition calculated with experimental parameters demonstrates that the blob radial transport for H mode is smaller than L mode in experiments.

Estimating cross-field particle transport at the outer midplane of TCV by tracking filaments with machine learning

Cross-field transport of particles in the boundary region of magnetically confined fusion plasmas is dominated by turbulence. Blobs, intermittent turbulent structures with large amplitude and a filamentary shape appearing in the scrape-off layer (SOL), are known from theoretical and experimental studies to be the main contributor to the cross-field particle transport. The dynamics of blobs differs depending on various plasma conditions, including triangularity (δ). In this work, we analyze triangularity dependence of the cross-field particle transport at the outer midplane of plasmas with , +0.15, −0.14, and −0.26 on the Tokamak à Configuration Variable, using our novel machine learning (ML) blob-tracking approach applied to gas puff imaging data. The cross-field particle flux determined in this way is of the same order as the overall transport inferred from KN1D, GBS, and SOLPS-ITER simulations, suggesting that the blobs identified by the ML blob-tracking account for most of the cross-field particle transport in the SOL. Also, the ML blob-tracking and KN1D show a decrease in the cross-field particle transport as δ becomes more negative. The blob-by-blob analysis of the result from the tracking reveals that the decrease of cross-field particle transport with decreasing δ is accompanied by a decrease in the number of blobs in a fixed time, which tend to have larger area and lower radial speed. Also, the blobs in these plasmas are in the connected sheath regime, and show a velocity scaling consistent with the two-region model.

Effect of electromagnetic wave reflection from conducting surfaces on blob dynamics in the tokamak scrape-off layer

Electromagnetic dynamics of blobs in hot scrape-off-layer plasmas of the tokamak are affected by excitation of the Alfvén waves and their subsequent propagation to the machine first wall along open magnetic field lines. In this study, the interaction of electromagnetic perturbations with the conducting tokamak wall and the resulting impact of these perturbations on the motion of filaments at the tokamak edge are analyzed. The model describing blob dynamics is presented. To describe the reflection of the Alfvén waves from the tokamak wall, the new form of sheath boundary conditions for the parallel current and electrostatic potential at the plasma–sheath interface is proposed. It is demonstrated that depending on the wall resistivity, the waves can be either absorbed or reflected by the wall, influencing the excitation of electromagnetic fluctuations inside the filament plasma. The theoretical conclusions of the study are supported with the BOUT++ numerical modeling of blob dynamics at the edge of the DIII-D and NSTX tokamaks. It is shown that taking the reflective boundary conditions into account leads to the excitation of the standing Alfvén waves in the filament, periodically canceling the electrostatic currents inside the blob.

Airglow Imaging Observations of Plasma Blobs: Merging and Bifurcation during Solar Minimum over Tropical Region

Plasma blobs are night-time ionospheric irregularities whose generation mechanism is still under investigation. A large number of observations highlighted several aspects of their morphology and dynamics. However, the plasma blobs have not been attributed convincingly to a known mechanism. We analyzed the OI 630.0 nm emission images during March and October of 2019 and 2020 (minimum solar activity) using the ground-based all-sky imager at ZF-2 (2.58° S, 60.22° W) in the Amazon region of Brazil. The novelties of the present study are the rarely reported observation of both plasma blob merging and bifurcation. We studied the evolutional dynamics of plasma blobs and observed that blobs are distinct phenomena with unique properties. We attribute the merging of plasma blobs to the “wind reversion effect” (WRE) mechanism caused by a change in the direction of the zonal thermospheric wind from east to west. In some cases, the slower-drifting plasma blobs may merge with the faster ones. Moreover, blobs were observed initially bifurcating at the topside and later divided into two. The activity of the polarized electric field inside the plasma bubble mapping along the magnetic field lines is possibly responsible for the blob’s bifurcation. Subjecting the two features of ionospheric plasma blobs to simulation may reveal further the physics of blobs’ merging and bifurcation.

Beyond the Oberbeck–Boussinesq and long wavelength approximation

We present the first simulations of a reduced magnetized plasma model that incorporates both arbitrary wavelength polarization and non-Oberbeck–Boussinesq effects. Significant influence of these two effects on the density, electric potential and vorticity and non-linear dynamics of interchange blobs are reported. Arbitrary wavelength polarization implicates so-called gyro-amplification that compared to a long wavelength approximation leads to highly amplified small-scale vorticity fluctuations. These strongly increase the coherence and lifetime of blobs and alter the motion of the blobs through a slower blob-disintegration. Non-Oberbeck–Boussinesq effects incorporate plasma inertia, which substantially decreases the growth rate and linear acceleration of high amplitude blobs, while the maximum blob velocity is not affected. Finally, we generalize and numerically verify unified scaling laws for blob velocity, acceleration and growth rate that include both ion temperature and arbitrary blob amplitude dependence.

Effects of edge biasing on blob dynamics and associated transport in the edge of the J-TEXT tokamak

Effects of edge radial electric field and × B flow shear on edge turbulence and turbulent transport, in particular, on large-scale blobs and blobby transport have been investigated in the positive and negative biasing discharges in the J-TEXT tokamak. The results show that under certain conditions, the positive electrode biasing induces better plasma confinement than the negative biasing. Further studies reveal that in addition to flow shear effects on blob dynamics, the local radial electric field at the edge region plays a significant role in repulsion of the blobs and associated transport, leading to improvement of particle confinement when the outward motion of the blobs is blocked. The results are consistent with the existing theory on blob dynamics.

Generalized pressure-correction for viscous potential flow simulations of multiphase flows using boundary element method

A new formulation for correcting the pressure field in Viscous Potential Theory (VPT) is developed, generalizing previous Viscous Pressure Correction (VPC) methods to simulate multi-phase flows based on VPT. The latter theory is now applicable to a wider range of flow problems. Without limiting the general applicability of the formulation, the present work focuses on the dynamics of two-dimensional liquid blobs and liquid films in a void. The governing equations of the method are solved using the traditional Boundary Element Method together with fourth-order Runge-Kutta time-integration, thereby reducing the spatial dimension of the problem by one and increasing the computational efficiency dramatically as compared to general purpose solvers for interfacial flows. Verification and Validation of the new method and algorithm are carried out through comparison of the simulation results with those obtained from (semi-)analytical models, a general-purpose OpenFOAM solver with Volume-of-Fluid interface treatment, and a Smoothed-Particle Hydrodynamics solver. Several benchmark cases and a wide range of viscosities are considered. Significantly improved accuracy of the newly proposed method as compared to the original Viscous Pressure Correction of Viscous Potential Flow (VCVPF) approach is demonstrated for the case of a retracting free liquid film or ligament. In order to demonstrate the capacity of the present method and algorithm in handling complex multiphase flow problems and in providing physical insight into such phenomena, the disintegration mechanism of a strongly modulated discharging liquid sheet under the influence of gravity is analyzed.

Impact of electromagnetic effects on macroscopic dynamics of blobs in hot low- β edge plasma of fusion devices

The impact of electromagnetic effects on macroscopic dynamics of blobs in hot low-β (β<me/mi) edge plasma of fusion devices is analyzed. The physical model governing filament dynamics is presented. The qualitative analysis of the blob motion in electrostatic and electromagnetic regimes is performed. The scalings for the plasma potential and filament advection velocity are obtained along with the estimates of plasma parameters, for which the physics of Alfvén waves and skin effect can have an impact on the filament motion. It is shown that required edge/scrape-off layer conditions can be found in modern tokamaks. Analytical expressions for the distributions of the parallel current and electrostatic potential in a blob, containing electrostatic and electromagnetic contributions, are derived. It is demonstrated that in sufficiently hot, weakly resistive plasma the electromagnetic effects can lead to noticeable variations of the potential along the magnetic field lines distorting the blob propagation pattern in contrast to the electrostatic case. Results of the theoretical analysis are compared with results of 3D BOUT++ modeling, which show that in the electromagnetic limit, the macroscopic motion of blobs can exhibit ballooning features due to the emission of Alfvén waves, absent in electrostatic simulations. It is concluded that, at least in the near scrape-off layer region of fusion devices, electromagnetic models of plasma dynamics may be appropriate for the analysis of blob motion instead of electrostatic ones.

Reduction of blob-filament radial propagation by parallel variation of flows: Analysis of a gyrokinetic simulation

Data from the XGC1 gyrokinetic simulation are analyzed to understand the three-dimensional spatial structure and the radial propagation of blob-filaments generated by quasi-steady turbulence in the tokamak edge pedestal and scrape-off layer plasma. Spontaneous toroidal flows vary in the poloidal direction and shear the filaments within a flux surface, resulting in a structure that varies in the parallel direction. This parallel structure allows the curvature and grad-B induced polarization charge density to be shorted out via parallel electron motion. As a result, it is found that the blob-filament radial velocity is significantly reduced from estimates that neglect parallel electron kinetics, broadly consistent with experimental observations. Conditions for when this charge shorting effect tends to dominate blob dynamics are derived and compared with the simulation.

Dynamics of seeded blobs under the influence of inelastic neutral interactions

Field-aligned filaments, the so-called blobs, born at the edge of the magnetically confined region of tokamaks propagate radially outward into the scrape-off layer (SOL) region that allows for a substantial population of neutral particles compared to the region of confinement. The electrons and ions constituting the blob undergo both elastic and inelastic collisions with the neutral particles, and the latter leads to sources and sinks of the blob density, momentum, and heat. The influence of the inelastic collisions with neutrals on the evolution of seeded blobs is investigated numerically by the nHESEL drift-fluid model through a series of discrete scans in interactions, active source terms, and blob plasma parameters. In light of the results, the potential influence of local inelastic collisions on the SOL density shoulder formation is discussed. It is found that density sources increase the blob compactness, which delays the blob dispersion and decreases the dispersion rate. Density sources or momentum sinks also influence the blob dynamics by increasing the vorticity layer around the perturbation, whereas the pressure sources/sinks only affect the blob dynamics marginally. The change to the vorticity structure leads, in most cases, to a decrease in the radial velocity of the blob center of mass, although, at high source rates, a radial acceleration of the blob center of mass is also observed. Density sources may, thus, contribute to shoulder formation not only by increasing the density locally but also by changing the filament dynamics.

Dynamics as a cause for the nanoscale organization of the genome

ABSTRACT Chromatin ‘blobs’ were recently identified by live super-resolution imaging of labeled nucleosomes as pervasive but fleeting structural entities. However, the mechanisms leading to the formation of these blobs and their functional implications are unknown. We explore here whether causal relationships exist between parameters that characterize the chromatin blob dynamics and structure, by adapting a framework for spatio-temporal Granger-causality inference. Our analysis reveals that chromatin dynamics is a key determinant for both blob area and local density. Such causality, however, could be demonstrated only in 10–20% of the nucleus, suggesting that chromatin dynamics and structure at the nanometer scale are dominated by stochasticity. We show that the theory of active semiflexible polymers can be invoked to provide potential mechanisms leading to the organization of chromatin into blobs. Our results represent a first step toward elucidating the mechanisms that govern the dynamic and stochastic organization of chromatin in the cell nucleus.

Up-down asymmetric exact solutions to the Navier-Stokes equations

Exact up-down asymmetric solutions to the Navier-Stokes equations for a viscous and incompressible fluid with time-dependent viscosity ν(t) are derived. Transformations of the exact solutions are defined that produce an infinite sequence of new solutions from each known one. The solutions are presented in terms of elementary functions and have no singularities. Three infinite-dimensional families of new exact axisymmetric unsteady solutions to the viscous magnetohydrodynamics equations are derived. Dynamics of vortex rings and vortex blobs is studied for some exact up-down asymmetric incompressible viscous fluid flows and viscous plasma flows.

Evolution of solute blobs in heterogeneous porous media

We study the mixing dynamics of solute blobs in the flow through saturated heterogeneous porous media. As the solute plume is advected through a heterogeneous porous medium it suffers a series of deformations that determine its mixing with the ambient fluid through diffusion. Key questions are the relation between the spatial disorder and the mixing dynamics and the effect of the initial solute distribution. To address these questions, we formulate the advection–diffusion problem in a coordinate system that moves and rotates along streamlines of the steady flow field. The impact of the medium heterogeneity is quantified systematically within a stochastic modelling approach. For a simple shear flow, the maximum concentration of a blob decays asymptotically as$t^{-2}$. For heterogeneous porous media, the mixing of the solute blob is determined by the random sampling of flow and deformation heterogeneity along trajectories, a mechanism different from persistent shear. We derive explicit perturbation theory expressions for stretching-enhanced solute mixing that relate the medium structure and mixing behaviour. The solution is valid for moderate heterogeneity. The random sampling of shear along trajectories leads to a$t^{-3/2}$decay of the maximum concentration as opposed to an equivalent homogeneous medium, for which it decays as$t^{-1}$.

On the dynamics of blobs in scrape‐off layer plasma: Model validation from two‐dimensional simulations and experiments in Tore Supra

An investigation is proposed on the dynamics of blobs in scrape‐off layer (SOL) plasmas. Starting from a common reduced model predicting the dynamics of isolated blobs, we emphasize the necessity of verifying the meaningfulness of the notion of generic or average blob. A definition and methodology is proposed and tested on turbulent data built from two‐dimensional (2D) flux‐driven simulations. Average blobs are shown to possess physical properties in relatively good agreement with predictions made using the isolated blob model. An application to experimental data from Tore Supra is then demonstrated. A striking agreement is found between average blob velocities measured in experiments and isolated blob model predictions. It suggests that the turbulent transport in the SOL of limited plasmas is correctly described by 2D interchange models. A discussion is finally proposed that blob models have to focus on intermittency in order to find a field of application.

Coronal rain in magnetic bipolar weak fields

We intend to investigate the underlying physics for the coronal rain phenomenon in a representative bipolar magnetic field, including the formation and the dynamics of coronal rain blobs. With the MPI-AMRVAC code, we performed three dimensional radiative magnetohydrodynamic (MHD) simulation with strong heating localized on footpoints of magnetic loops after a relaxation to quiet solar atmosphere. Progressive cooling and in-situ condensation starts at the loop top due to radiative thermal instability. The first large-scale condensation on the loop top suffers Rayleigh-Taylor instability and becomes fragmented into smaller blobs. The blobs fall vertically dragging magnetic loops until they reach low beta regions and start to fall along the loops from loop top to loop footpoints. A statistic study of the coronal rain blobs finds that small blobs with masses of less than 10^10 g dominate the population. When blobs fall to lower regions along the magnetic loops, they are stretched and develop a non-uniform velocity pattern with an anti-parallel shearing pattern seen to develop along the central axis of the blobs. Synthetic images of simulated coronal rain with Solar Dynamics Observatory Atmospheric Imaging Assembly well resemble real observations presenting dark falling clumps in hot channels and bright rain blobs in a cool channel. We also find density inhomogeneities during a coronal rain "shower", which reflects the observed multi-stranded nature of coronal rain.