Edge Plasmas In Fusion Devices Research Articles

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.

On ablation of large Tungsten dust grains in edge plasma of fusion devices

The model describing vapor induced shielding effects for W dust grain evaporation in the edge plasmas is considered. It is shown that unlike the case of pellet ablation models, where it is assumed that the heat flux to pellet is due to free streaming electrons, the heat flux to W grain in edge plasma is due to electron heat conduction. By using some reasonable simplifications, tractable equations for the evaporation rate of the grain are obtained. Approximate analytical analysis of these equations, supported by numerical solutions, shows that the vapor induced screening effects can reduce the evaporation rate of the grain in edge plasmas by an order of magnitude. This model can also be applied for other high-Z grain materials (e.g. Mo).

Simple and Fast Poisson Solver with Arbitrary Boundary Shape and Condition for PIC Simulation

AbstractParticle‐in‐cell (PIC) simulations are the very powerful method to understand physics of edge plasmas in fusion devices. Poisson solver with a regular grid in a rectangular vessel is very fast, but it cannot simply be applicable to simulations in complicated divertor configuration. In this paper, a simple and fast Poisson solver with arbitrary boundary shape and condition for PIC simulation is proposed. Boundary points are essential for this Poisson solver, on which the charge density is appropriately given and the boundary condition is satisfied. For particle simulations in the open system, it is necessary to distinguish whether a traced particle moves freely or hits the wall boundary during a time step. A new method for PIC simulation to discriminate a particle position before the boundary or beyond the boundary is also presented. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Direct Measurement of Current Filament Structures in a Magnetic-Confinement Fusion Device

Turbulent structures detected in the edge plasma of fusion devices, often described as blobs, are generally believed to be responsible for confinement degradation. Recent experimental evidence and theories have suggested their filamentary electromagnetic nature. In this Letter the first direct experimental measurements of the parallel current density associated with turbulent structures in a fusion experiment are reported. The electromagnetic nature of structures is clearly shown by identifying the current filaments with a vortexlike velocity pattern and the associated pressure perturbation.

Novel Technique for Direct Measurement of the Plasma Diffusion Coefficient in Magnetized Plasma

AbstractThe diffusion coefficient in the edge plasma of fusion devices is measured by a new technique. The method is based on studying the decay of the plasma fluctuation spectrum inside the so‐called “Ball‐pen probe”: a small ceramic tube having its mouth flush with a magnetic surface and a movable electrode inside the tube. When the electrode protrudes from the tube, the measured signal shows the floating potential fluctuations of the plasma or the ion saturation current outside the tube. Retracting the electrode into the tube, the power spectrum of the signal displays an exponential decay that is different for different frequencies. Assuming a mainly diffusive behaviour of the plasma inside the tube, the spectrum decay length can be used to derive the value of the diffusion coefficient. The measurements were performed at several radial positions in the CASTOR edge region for three different values of Btor. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Intermittency and extended self-similarity in space and fusion plasma: boundary effects

A comparative study of fluctuation features in the edge plasma of fusion devices and in turbulent boundary layers (TBLs) of the Earth's magnetosphere has demonstrated similar statistical characteristics including scalings of structure functions and multifractal spectra. The detected intermittency and anomalous transport of mass and momentum is carried by sporadic plasma flux bursts with nonGaussian probability of flux magnitude. The turbulence exhibits a generalized (extended) self-similarity in an extended scale range. The experimental scalings of the structure functions are rather well fitted by the log-Poisson model considering quasi-1D singular dissipative structures. It appears that the turbulence in the edge plasma of fusion devices and in the TBL of the Earth's magnetosphere is governed by cross-field motions similar to hydrodynamic turbulence. Here the experimental scalings from the plasma are available for a comparison with experimental results from neutral fluids. The plasma scalings display universal properties of intermittent turbulence. A statistical approach permits us to evaluate turbulent transport scalings. The time dependence of an average squared particle displacements ⟨δx2⟩ ∝ τα infers superdiffusion with α ≈ 1.4–1.87 > 1.

A probe-based method for measuring the transport coefficient in the tokamak edge region

A new method for measuring the diffusion coefficient in the edge plasma of fusion devices is presented. The method is based on studying the decay of the plasma fluctuation spectrum inside a small ceramic tube having its mouth flush with a magnetic surface and its axis aligned along the radial direction. The plasma fluctuations are detected by an electrode, radially movable inside the tube.

Spectral line shapes modeling in turbulent plasmas

In this work we investigate the influence of low frequency turbulence on Doppler spectral line shapes in magnetized plasmas. Low frequency refers here to fluctuations whose typical time scale is much larger than those characterizing the atomic processes, such as radiative decay, collisions and charge exchange. This ordering is in particular relevant for drift wave turbulence, ubiquitous in edge plasmas of fusion devices. Turbulent fluctuations are found to affect line shapes through both the spatial and time averages introduced by the measurement process. The profile is expressed in terms of the fluid fields describing the plasma. Assuming the spectrometer acquisition time to be much larger than the turbulent time scale, an ordering generally fulfilled in experiments, allows to develop a statistical formalism. We proceed by successively investigating the effects of density, fluid velocity and temperature fluctuations on the Doppler profile of a spectral line emitted by a charge exchange population of neutrals. Line shapes, and especially line wings are found to be affected by ion temperature or fluid velocity fluctuations, and can in some cases exhibit a power-law behavior. These effects are shown to be measurable with existing techniques, and their interpretation in each particular case would rely on already existing tools. From a fundamental point of view, this study gives some insights in the appearance of non-Boltzmann statistics, such as Levy statistics, when dealing with averaged experimental data.

Multifractal analysis of plasma turbulence in biasing experiments on Castor tokamak

Experiments as well as some stochastic models have shown that behaviour of the transport in the edge plasma in fusion devices should be considered as a multi-scale phenomenon. The multifractal statistics describes processes as a multiplicative cascade with a complex self-similarity. The multifractal analysis, which enables to clearly demonstrate the turbulent properties of plasma, was applied to fluctuating signals in the biasing experiments. The influence of the biasing parameters is shown. A comparison with other types of the analysis is illustrated, too.

Two-dimensional density and density fluctuation diagnostic for the edge plasma in fusion devices

A technique is described for the two-dimensional measurement of electron density profile and fluctuations in edge regions of magnetically confined fusion plasmas. The method is based on existing lithium beam beam emission spectroscopy technique, two-dimensional resolution is achieved by electrostatically scanning the beam. If scanning is performed faster than the lifetime of the turbulent structures in the plasma, the diagnostic is capable of measuring the structure of electron density fluctuations as well. The beam strength of currently available beams makes the detection of single fluctuation events impossible, but the full two-dimensional spatial structure of correlations can still be determined. The article describes the technique and fast beam deflection tests up to 250kHz. The capabilities of such a diagnostic for fluctuation measurement are explored by simulating measurement signals. Measurement of both the two-dimensional density profile, fluctuation correlation function and poloidal flow velocity are demonstrated at the Wendelstein 7-AS stellarator. The shape of the density profile, the radial and poloidal correlation lengths and the flow velocity are in agreement with expectations and previous Langmuir probe measurement.

Understanding of edge plasmas in magnetic fusion energy devices

A limited overview is given of the theoretical understanding of edge plasmas in fusion devices. This plasma occupies the thin region between the hot core plasma and material walls in magnetic confinement devices. The region is typically formed by a change in magnetic topology from close magnetic field lines (i.e. the core region) and open field lines that contact material surfaces (i.e. the scrape-off layer (SOL)), with the most common example being magnetically diverted tokamaks. The physics of this region is determined by the complex interaction of plasma with neutral species in the presence of plasma turbulence, and impurity radiation is often an important component. Recent advances in modelling strong, intermittent micro-turbulent edge-plasma transport are given, as is the closely coupled self-consistent evolution of the edge-plasma profiles in tokamaks. In addition, selected new results are given for the characterization of edge-plasmas behaviour in the areas of edge-pedestal relaxation and SOL transport via edge-localized modes, impurity formation including dust and magnetic field-line stochasticity at the edge of tokamaks.

Non-linear collective phenomena in dusty plasmas

Dusty plasmas are unusual states of matter where the interactions between the dust grains can be collective and are not a sum of all pair particle interactions. This state of matter is appropriate to form non-linear dissipative collective self-organized structures. It is found that the potential around the grains can be over-screened leading to a new phenomenon—collective attraction of pairs of large charge grains of equal sign. The grain clouds can self-contract and their collapse is terminated at distances where the interaction becomes repulsive. The homogeneous dusty plasma distribution is universally unstable to form structures. The potential of the collective attraction is proportional to the square of the dimensionless parameter P = ndZd/ni, where nd and ni are the average dust and ion densities, respectively, and Zd is the dust charge in units of electron charge. The collective attraction is determined by finite grain size and by the presence of absorption of plasma flux on grains. The physics of attraction is related to the space charge accumulation caused by collective flux disturbances. The collective attraction operates for systems with size larger than the mean free path for ion–dust absorption, the condition met in many existing low temperature dusty plasma experiments, in edge plasmas of fusion devices and in space dusty plasmas. The collective attraction exceeds the previously known non-collective attraction such as shadow attraction or wake attraction. The collective attraction can be responsible for pairing of dust grains (this process is completely classical in contrast to the known pairing in superconductivity) and can serve as the main process for the formation of more complicated dust complexes up to dust-plasma crystals. The equilibrium structures formed by collective attraction have universal properties and can exist in a limited domain of parameters (similar to the equilibrium balance known for stars). The balance conditions for dust structures include not only the pressure effects but also the ion drag, grain charging and collective plasma flux effects. The non-linear effects in collective attraction are relatively small for astrophysical conditions and for edge fusion plasmas. For plasmas used in plasma etching and in plasma crystal experiments the non-linear effects in attraction are important. The distance between the grains for the first attraction well is in rough agreement with the observed separation of grains in dust-plasma crystals. The results of the numerical simulations of the final stationary configurations with appropriate wall boundary conditions are in rough agreement with recent results of structure observation in the experiments on the International Space Station.

Proposal of Flexible Atomic and Molecular Process Management for Monte Carlo Impurity Transport Code Based on Object Oriented Method

A Monte Carlo impurity transport code based on object-oriented method has been developed. Edge plasma in fusion devices has a lot of impurity atoms and molecules such as hydrocarbon species, C x H y due to low electron temperature in the edge. Various atomic/molecular processes have been considered in the edge plasma region and we need to determine which process is essential in edge plasma conditions. To take such complicated processes into account in a systematic way, Monte Carlo impurity simulation, which has a strong flexibility in dealing with various atomic and molecular processes, would be useful. By employing the object-oriented method in the simulation code, we can handle arbitrary atomic and molecular processes.

Recent Developments in Monte Carlo Codes for Edge Plasma Studies

In recent years, the rapid evolution of computing power, available computer memory and parallel processing methods have brought the self-consistent modelling of edge plasmas in fusion devices within reach. The use of orbit-following Monte Carlo simulation has been validated in numerous challenging problems of neoclassical physics, and molecular dynamics simulations of plasma-wall interactions can be utilized to obtain impurity yields for use in edge plasma and neutral codes. A combination of such methods may facilitate the self-consistent description of edge plasma transport in fusion devices.

Recent Developments in Monte Carlo Codes for Edge Plasma Studies

In recent years, the rapid evolution of computing power, available computer memory and parallel processing methods have brought the self-consistent modelling of edge plasmas in fusion devices within reach. The use of orbit-following Monte Carlo simulation has been validated in numerous challenging problems of neoclassical physics, and molecular dynamics simulations of plasma-wall interactions can be utilized to obtain impurity yields for use in edge plasma and neutral codes. A combination of such methods may facilitate the self-consistent description of edge plasma transport in fusion devices.

Secondary electron emission from plasma facing materials and its impact on sheath voltage

A combined Monte Carlo simulation of secondary electron emission from solids and transport of secondary electrons in a plasma sheath in front of the surface is performed for analyzing a relationship between the secondary electron yield and sheath voltage formed on the wall in magnetic fusion devices. In spite of the sheath acceleration, due to an oblique magnetic field, the prompt return of secondary electrons appears within first gyration in the sheath and this causes a decrease in the secondary electron yield of the wall, and as a result, it suppresses the reduction of the sheath voltage due to the secondary electrons. The difference between the secondary electron yield of low- Z (Be, C) and high- Z (Mo, W) metals is suppressed as well due to the prompt return. The secondary electron yield and sheath voltage depend on the electron temperature and density in the plasma, magnetic field strength and inclination angle of magnetic field lines to the wall surface. Therefore, these dependences are calculated for the relevant ranges to edge plasmas in fusion devices.

The disturbing effect of probes inserted into edge plasmas of fusion devices

With the exception of small, simple Langmuir probes, other probes used to diagnose the scrape-off plasma behind the limiter radius or separatrix in tokamaks are usually sufficiently large that they disturb the plasma they are measuring. A theory is presented for recovering the undisturbed plasma parameters from those measured by a large probe in a collisionless scrape-off layer.