Many-particle charged-particle plasma simulations using spatial meshes for the electromagnetic field solutions, particle-in-cell (PIC) merged with Monte Carlo collision (MCC) calculations, are coming into wide use for application to partially ionized gases. The author emphasizes the development of PIC computer experiments since the 1950s starting with one-dimensional (1-D) charged-sheet models, the addition of the mesh, and fast direct Poisson equation solvers for 2-D and 3-D. Details are provided for adding the collisions between the charged particles and neutral atoms. The result is many-particle simulations with many of the features met in low-temperature collision plasmas; for example, with applications to plasma-assisted materials processing, but also related to warmer plasmas at the edges of magnetized fusion plasmas. >

Particle-in-Cell Charged Particle Simulations, plus Monte Carlo Collisions with Neutral Atoms, PIC-MCC

A blob-filament (or simply “blob”) is a magnetic-field-aligned plasma structure which is considerably denser than the surrounding background plasma and highly localized in the directions perpendicular to the equilibrium magnetic field B. In experiments and simulations, these intermittent filaments are often formed near the boundary between open and closed field lines, and seem to arise in theory from the saturation process for the dominant edge instabilities and turbulence. Blobs become charge-polarized under the action of an external force which causes unequal drifts on ions and electrons; the resulting polarization-induced E × B drift moves the blobs radially outwards across the scrape-off-layer (SOL). Since confined plasmas generally are subject to radial or outwards expansion forces (e.g., curvature and ∇B forces in toroidal plasmas), blob transport is a general phenomenon occurring in nearly all plasmas. This paper reviews the relationship between the experimental and theoretical results on blob form...

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Convective transport by intermittent blob-filaments: Comparison of theory and experiment

Strong correlations—cooperative behavior due to many-particle interactions—are omnipresent in nature. They occur in electrolytic solutions, dense plasmas, ultracold ions and atomic gases in traps, complex (dusty) plasmas, electrons and excitons in quantum dots and the quark–gluon plasma. Correlation effects include the emergence of long-range order, of liquid-like or crystalline structures and collective dynamic properties (collective modes). The observation and experimental analysis of strong correlations are often difficult, requiring, in many cases, extreme conditions such as very low temperatures or high densities. An exception is complex plasmas where strong coupling can be easily achieved, even at room temperature. These systems feature the strongest correlations reported so far and experiments allow for an unprecedented precision and full single-particle resolution of the stationary and time-dependent many-particle behavior. The governing role of the interactions in strongly correlated systems gives rise to many universal properties observed in all of them. This makes the analysis of one particular system interesting for many others. This motivates the goal of this paper which is to give an overview on recent experimental and theoretical results in complex plasmas including liquid-like behavior, crystal formation, structural and dynamic properties. It is expected that many of these effects will be of interest also to researchers in other fields where strong correlations play a prominent role. (Some figures in this article are in colour only in the electronic version) This article was invited by Gordon Baym.

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Complex plasmas: a laboratory for strong correlations

The present status of zonal flow experiments is reviewed with the historical process to attain the concept of zonal flows, which provides a new framework for understanding turbulence and transport in toroidal plasmas. The existence of zonal flows is experimentally confirmed to present a new paradigm of plasma turbulence. The paper presents contemporary experiments on zonal flows as major topics with a brief presentation of the zonal flow theories, the diagnostics and data processing techniques for turbulence and zonal flows and the peripheral issues of zonal flow physics. The accumulated experimental results introduced in this review include identification of zonal flows (both stationary zonal flows and geodesic acoustic modes), nonlinear interactions between zonal flows and turbulence, quantification of turbulent Reynolds stress, flow dynamics, energy transfer dynamics between turbulent wave components and the effects of zonal flows on plasma transport. These results have given rise to a new paradigm, namely, that the plasma turbulence is a system of zonal flows and drift waves, with an emphasis on the interaction between the disparate scale structures, e.g. zonal flows (mesoscale) and turbulence (micro-scale).

https://iopscience.iop.org/article/10.1088/0029-5515/49/1/013001/pdf

A review of zonal flow experiments

This paper reviews measurements of edge plasma turbulence in toroidal magnetic fusion devices with an emphasis on recent results in tokamaks. The dominant feature of edge turbulence is a high level of broadband density fluctuations with a relative amplitude δn/n ~ 5–100%, accompanied by large potential and electron temperature fluctuations. The frequency range of this turbulence is ~10 kHz–1 MHz, and the size scale is typically ~0.1–10 cm perpendicular to the magnetic field but many metres along the magnetic field, i.e. the structure is nearly that of 2D 'filaments'. Large intermittent bursts or 'blobs' are usually observed in the scrape-off layer. Diagnostic and data analysis techniques are reviewed and the main experimental results are summarized. Recent comparisons of experimental results with edge turbulence theory are discussed, and some directions for future experiments are suggested.

http://iopscience.iop.org/article/10.1088/0741-3335/49/7/S01/pdf

Edge turbulence measurements in toroidal fusion devices

Experiments are carried out to investigate the rotation of dust clusters in a radio-frequency plasma sheath with a vertical magnetic field. Our observations are in disagreement with the standard model, in which it was assumed that the neutral gas is at rest and that a steady rotation is attained when the ion-drag force is balanced by neutral friction. Here, we re-examine this basic assumption by carefully designed experiments. Our results suggest that the neutral gas is set into rotation by E×B induced ion flow through ion-neutral collisions and that the dust particles are advected by this flow. A hydrodynamic model is proposed to describe the rotation of the neutral gas and it can explain our observations.

Effect of neutral gas motion on the rotation of dust clusters in an axial magnetic field

Comparative studies between a toroidal low-temperature plasma and drift-Alfven-wave simulations were carried out in order to investigate the microscopic structure of turbulence. The dimensionless plasma parameters in the TJ-K torsatron [N. Krause et al., Rev. Sci. Instrum. 73, 3474 (2002)] are similar to those in the edge of a fusion plasma. At the same time the fluctuations can be fully diagnosed by probe arrays. Fluctuation spectra are analyzed by wavelet techniques indicating a large amount of intermittency in both numerical and experimental data. Since in both cases no critical gradient is present, the intermittency is not due to a state in self-organized criticality (SOC). The spectral density P(ω,k) of the turbulence was measured with a 64-tip Langmuir probe array. A broad spectrum indicates fully developed turbulence. The wave-number spectrum of the density fluctuations decays with a power law with an exponent of −3. The experiments confirm predictions from the turbulence code. The cross-phase betw...

Study of edge turbulence in dimensionally similar laboratory plasmas

We propose and demonstrate a concept that mimics the magnetization of the heavy dust particles in a complex plasma while leaving the properties of the light species practically unaffected. It makes use of the frictional coupling between a complex plasma and the neutral gas, which allows us to transfer angular momentum from a rotating gas column to a well-controlled rotation of the dust cloud. This induces a Coriolis force that acts exactly as the Lorentz force in a magnetic field. Experimental normal mode measurements for a small dust cluster with four particles show excellent agreement with theoretical predictions for a magnetized plasma.

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Magnetizing a complex plasma without a magnetic field.

For the in situ analysis of nano-sized particles in a laboratory plasma, Mie ellipsometry is a well established technique. We present a simple setup with two CCD cameras to gain online spatiotempor ...

https://iopscience.iop.org/article/10.1088/0963-0252/21/6/065005/pdf

Imaging Mie ellipsometry: dynamics of nanodust clouds in an argon-acetylene plasma

The influence of a plasma environment on melamine formaldehyde particles is studied. High-precision measurements of the vertical confinement frequency with a phase-resolved resonance method indicate that the particle mass is affected in two ways: the deposition of sputtered material at the particle leads to a mass gain, whereas the outgassing of water causes a mass loss.

Franko Greiner

Papers

Effect of neutral gas motion on the rotation of dust clusters in an axial magnetic field

Study of edge turbulence in dimensionally similar laboratory plasmas

Magnetizing a complex plasma without a magnetic field.

Imaging Mie ellipsometry: dynamics of nanodust clouds in an argon-acetylene plasma

Mass changes of microparticles in a plasma observed by a phase-resolved resonance method