PLASMAS IN THE LABORATORY AND THE UNIVERSE: Interactions, Patterns, and Turbulence
1242(2010); http://dx.doi.org/10.1063/1.3460125View Description Hide Description
The most efficient way to constrain the photon mass is related to observations of large‐scale magnetic fields in space physics and astrophysics. This approach is based on the change in the Ampere law caused by the finite In 1990s, a consistent set of MHD equations allowing for the finite has been written and later used to analyze the solar wind data from the Voyager 1 and 2 missions. This lead to an estimate the value currently recommended by the bi‐annual compendium of the Particle Data Group. The further progress in constraining the photon mass may come from considering the dynamics of large‐scale magnetic fields in astrophysics, in particular, the magnetic field of galaxies. The paper is concerned with related opportunities and challenges, including the problem posed by the simultaneous presence of large‐scale and much stronger small‐scale magnetic fields. Effects of recycling of the interstellar plasma involving dense molecular clouds, protostars and supernovae explosions are discussed. Possible approaches to pushing the upper bound to a limit well below are discussed.
Plasma Regimes in the Surroundings of Black Holes, Composite Plasma Disk Structures and Relevant Accretion Processes1242(2010); http://dx.doi.org/10.1063/1.3460115View Description Hide Description
The theory of the composite plasma disk structures and of the relevant magnetic field configurations that can surround black holes is presented, consistently with recent experimental observations indicating that highly coherent magnetic field configurations exist in the core of these structures. Concepts developed to describe the physics of magnetically confined laboratory plasmas are used. Thus the “paradox,” that arises when considering accreting plasmas in the presence of a transverse magnetic field is resolved by considering accretion as an intermittent process whereby particles are carried in steps, along a sequence of magnetic separatrices containing the formed magnetic islands, by the onset of the equivalent of “edge localized modes” (ELMs) observed in laboratory experiments. Inactive galactic black holes are suggested as being associated with older galaxies that have been subjected to collisions destroying the coherent structures needed to guide relevant accretion flows. Alternatively, tridimensional spiral structures can emerge from axisymmetric disk configurations in a region close to the black hole and guide the relevant accretion flows. The radial gradient of the rotation frequency and the vertical gradient of the plasma pressure are the excitation factors for spirals as well as for axisymmetric modes. These can produce vertical flows of thermal energy and particles in opposing directions that can be connected to the winds emanating from disks in Active Galactic Nuclei (AGNs). In the close vicinity of Binary Black Holes the existence of three characteristic plasma regions is envisioned. The intermediate of these regions exhibits three physical regimes that differ both for the magnetic field structure and the spectrum of the emitted radiation, with jets and High Frequency Periodic Oscillations (HFQPOs) produced in two of these regimes.
1242(2010); http://dx.doi.org/10.1063/1.3460130View Description Hide Description
Several physical effects, which can considerably decrease the magnetorotational instability (MRI) threshold are discussed. First, we show that MRI features are changed significantly in non‐uniform magnetic field. Second, the buoyancy effect is shown to can be destabilizing for azimuthal flow of a resistive plasma. Third, we demonstrate that the rotating fluid may be unstable with respect to non‐axisymmetric modes at much lower rotation velocities than with respect to usual symmetric modes.
1242(2010); http://dx.doi.org/10.1063/1.3460143View Description Hide Description
The last ten years have seen significant progress have seen major improvements in our ability to observe and characterize the X‐ray emission from black hole systems. In particular, the arrival of a new generation of X‐ray satellites that combine large collection area, broad energy response, and flexible monitoring ability (starting with RXTE, BeppoSax, INTEGRAL, and culminating with Suzaku and Swift) finally enables us to begin to uniquely probe the rapidly changing physical conditionsin the plasma near a black hole, on timescales from milliseconds to years. I review our current understanding of these conditions, with an eye towards stimulating discussions with laboratory colleagues who have access to (still) much better diagnostics.
1242(2010); http://dx.doi.org/10.1063/1.3460151View Description Hide Description
The new high energy data coming mainly from the Fermi and Swift satellites and from the ground based Cerenkov telescopes are making possible to study not only the energetics of blazar jets, but also their connection to the associated accretion disks. Furthermore, the black hole mass of the most powerful objects can be constrained through IR‐optical emission, originating in the accretion disks. For the first time, we can evaluate jet and accretion powers in units of the Eddington luminosity for a large number of blazars. Firsts results are intriguing. Blazar jets have powers comparable to, and often larger than the luminosity produced by their accretion disk. Blazar jets are produced at all accretion rates (in Eddington units), and their appearance depends if the accretion regime is radiatively efficient or not. The jet power is dominated by the bulk motion of matter, not by the Poynting flux, at least in the jet region where the bulk of the emission is produced, at Schwarzschild radii. The mechanism at the origin of relativistic jets must be very efficient, possibly more than accretion, even if accretion must play a crucial role. Black hole masses for the most powerful jets at redshift ∼3 exceed one billion solar masses, flagging the existence of a very large population of heavy black holes at these redshifts.
1242(2010); http://dx.doi.org/10.1063/1.3460152View Description Hide Description
Recent studies of plasma turbulence based on measurements within solar wind and laboratory plasmas has been discussed. Evidences for the presence of a turbulent energy cascade, using the Yaglom’s law for MHD turbulence, has been provided through data from the Ulysses spacecraft. This allows, for the first time, a direct estimate of the turbulent energy transfer rate, which can contribute to the in situ heating of the solar wind. The energy cascade has been evidenced also for electrostatic turbulence in laboratory magnetized plasmas using measurements of intermittent transport (bursty turbulence) at the edge of the RFX‐mod reversed field pinch plasma device. Finally the problem of the dispersive region of turbulence in solar wind above the ion‐cyclotron frequency, where a spectral break is usually observed, and the problem of dissipation in a collisionless fluid as the solar wind, are briefly discussed.
1242(2010); http://dx.doi.org/10.1063/1.3460153View Description Hide Description
In this topical review we combine results of research from the field of complex plasmas and colloidal dispersions. This research has opened the way to study strong coupling phenomena in real space and time at the most fundamental kinetic level. The physics of complex plasmas is dominated by the dynamics of slow moving and individually visible microparticles. In contrast to colloidal suspensions, where the fluid background medium results in huge overdamping, the neutral gas background medium in complex plasmas introduces only very little damping so that processes at all relevant time scales can be studied. This is of particular importance for some of the most outstanding questions in the self‐organization of matter and critical phenomena.
1242(2010); http://dx.doi.org/10.1063/1.3460154View Description Hide Description
Three‐dimensional finite systems of charged dust particles confined to concentric spherical shells in a dusty plasma, so‐called “Yukawa balls,” have been studied with respect to their static and dynamic properties. Here, we review the charging of particles in a dusty plasma discharge by computer simulations and the respective particle arrangements. The normal mode spectrum of Yukawa balls is measured from the 3D thermal Brownian motion of the dust particles around their equilibrium positions.
1242(2010); http://dx.doi.org/10.1063/1.3460155View Description Hide Description
The interplay between the Kelvin‐Helmholtz, the Rayleigh‐Taylor and the Magnetic Re‐connection instabilities in a magnetized inhomogeneous plasma with a sheared velocity field is investigated within the framework of a two‐dimensional, two fluid model. This magnetic configuration is of interest for the investigation of the mixing process between the solar wind plasma and the Earth’s magnetospheric plasma at low latitudes at the magnetospheric flank.
It is found that the combined role of the density inhomogeneity and of the in‐plane magnetic field during the development of the Kelvin Helmholtz instability is multi faceted. It leads to small scale magnetic islands through the development of induced magnetic field line reconnection but at the same time the in‐plane magnetic field preserves the global coherence of the vortex merging process (vortex pairing).
1242(2010); http://dx.doi.org/10.1063/1.3460112View Description Hide Description
The effect of a levitating cloud of microparticles on the parameters of a radiofrequency (RF) plasma has been studied by means of two experimental techniques. Axial distributions of 1s excited states of argon were measured by a self‐absorption method. A correction of a standard self‐absorption method for the extinction of the light by the levitating microparticles is proposed. In addition the electron temperature was estimated using the optical emission spectroscopy. Measurements at the same discharge conditions in a microparticle‐free discharge and discharge, containing a cloud of levitating microparticles, revealed the non‐local influence of the microparticle cloud on the discharge plasma. The most probable cause of this influence is the disturbance of the ionization balance by the levitating microparticles.
1242(2010); http://dx.doi.org/10.1063/1.3460113View Description Hide Description
The dust contamination in plasma deposition processes plays a crucial role in the quality and the yield of the products. To improve the quality and the yield of plasma processing, a favorable way is to remove the dust particles actively from the plasma reactors.
Our recent experiments in the striped electrode device show that a traveling plasma modulation allows for a systematic particle removal independent of the reactor size. Besides the rf powered electrode, the striped electrode device includes a segmented electrode that consists of 100 electrically insulated narrow stripes. A traveling potential profile is produced by the modulation of the voltage signals applied on the stripes. The dust particles are trapped in the potential wells and transported with the traveling of the potential profile.
The particle‐in‐cell (PIC) simulation on the potential above the segmented electrode indicates that the traveling potential profile can be realized either by applying low‐frequency (0.1–10 Hz) voltage signals with a fixed phase shift between adjacent stripes or high‐frequency (10 kHz âĂŞ 100 MHz) signals with the amplitudes modulated by a low‐frequency envelope. The transportation of the dust particles is simulated with a two‐dimensional molecular dynamics (MD) code with the potential profile obtained from the PIC simulation. The MD results reproduce the experimental observations sucessfully.
This technology allows for an active removal of the contaminating particles in processing plasmas and it is independent of the reactor size. The removal velocity is controllable by adjusting the parameters for the modulation.
1242(2010); http://dx.doi.org/10.1063/1.3460114View Description Hide Description
The analytical generalization of the classical dynamical friction formula (derived under the assumption that all the field particles have the same mass) to the case in which the masses of the field particles are distributed with a mass spectrum is presented. Two extreme cases are considered: in the first, energy equipartition is assumed, in the second all the field particles have the same (Maxwellian) velocity distribution. Three different mass spectra are studied in detail, namely the exponential, discrete (two components), and power‐law cases. It is found that the dynamical friction deceleration can be significantly stronger than in the equivalent classical case, with the largest differences (up to a factor of 10 or more in extreme cases) arising for test particle velocities comparable to the mass‐averaged velocity dispersion of the field particles. The present results are relevant to our understanding of the dynamical evolution of globular clusters, in particular in the modelization of mass segregation and sedimentation of Blue Straggler stars and Neutron stars, and for the study of binary black holes in galactic nuclei.
1242(2010); http://dx.doi.org/10.1063/1.3460116View Description Hide Description
Spiral structure in galaxies can be traced in the optical (reflecting recent star formation) in the near infrared (tracing the dominant stellar mass component) and in tracers of the disc gas (such as thermal emission at 8 μm). We first review the predictions (based on both analytic and numerical studies) for the spatial relationship between these features according to various models for spiral structure formation and then present the result of our recent infrared imaging study of nearby galaxies with the Spitzer Space Telescope (SINGS). We find that in the case of M81 the shock in the disc gas is azimuthally displaced from the potential minimum and that this offset increases in the upstream direction with increasing radius. This result is consistent with the predictions of a model in which the gas responds to a long lived rigidly rotating spiral pattern whose pattern speed is consistent with previous estimates in M81. On the other hand we find that the potential arms are only sufficiently regular to attempt such an analysis in a further 7 galaxies and that in these cases—although there is a general tendency towards upstream offsets at large radius—there is no consistent radial trend that would allow one to identify a pattern speed in these cases. We draw attention to the fact that the spread in pitch angles that we measure in the infrared is much smaller than in the optical and that there is no correlation between the pitch angle of potential arms and a galaxy’s Hubble type.
1242(2010); http://dx.doi.org/10.1063/1.3460117View Description Hide Description
Several spiral galaxies display a prominent coherent spiral structure in their gaseous outer disk. Here we argue that these observed coherent patterns can be naturally interpreted as the manifestation of the mechanism that excites grand‐design spiral structure in the main, star‐dominated body of the disk. While a discrete spectrum of global modes is determined by the structure of the main body of the disk, which acts as a resonant cavity, the excitation is expected to be driven by angular momentum transport to the outer regions, through trailing density waves outside the corotation circle that can penetrate beyond the Outer Lindblad Resonance in the gaseous component of the disk. Because of conservation of the density wave action, these outgoing waves are likely to become more prominent in the outer disk and eventually reach non‐linear amplitudes, much like ocean waves moving close to the shore. In this context, we will describe some interesting results on the role of finite thickness effects.
1242(2010); http://dx.doi.org/10.1063/1.3460118View Description Hide Description
We present two new families of global equilibrium solutions of the Vlasov‐Poisson equations, intended to model collisionless but quasi‐relaxed stellar systems, such as globular clusters, characterized by the presence of internal rotation. The first one is an extension to the case of ax‐isymmetric equilibria flattened by solid‐body internal rotation of the well‐known family of King models, defined by a quasi‐Maxwellian distribution function. In turn, the second family is characterized by differential rotation, designed to be rigid in the center and to vanish in the outer parts of the stellar system, where the energy truncation is effective. The physical scenario that inspired the definition of the families is discussed in the more general context of the dynamical evolution of quasi‐relaxed stellar systems and a preliminary analysis of their intrinsic properties is provided.
1242(2010); http://dx.doi.org/10.1063/1.3460119View Description Hide Description
An experimental program underway at Caltech has produced plasmas where the shape is neither fixed by the vacuum chamber nor fixed by an external coil set, but instead is determined by self‐organization. The plasma dynamics is highly reproducible and so can be studied in considerable detail even though the morphology of the plasma is both complex and time‐dependent. A surprising result has been the observation that self‐collimating MHD‐driven plasma jets are ubiquitous and play a fundamental role in the self‐organization. The jets can be considered lab‐scale simulations of astrophysical jets and in addition are intimately related to solar coronal loops. The jets are driven by the combination of the axial component of the force and the axial pressure gradient resulting from the non‐uniform pinch force associated with the flared axial current density. Behavior is consistent with a model showing that collimation results from axial non‐uniformity of the jet velocity. In particular, flow stagnation in the jet frame compresses frozen‐in azimuthal magnetic flux, squeezes together toroidal magnetic field lines, thereby amplifying the embedded toroidal magnetic field, enhancing the pinch force, and hence causing collimation of the jet.
1242(2010); http://dx.doi.org/10.1063/1.3460120View Description Hide Description
An asymptotic model based on a reductive perturbative expansion of the drift kinetic and the Maxwell equations, is used to demonstrate that, near the instability threshold, the nonlinear dynamics of mirror modes in a magnetized plasma with anisotropic ion temperatures, involves a subcritical bifurcation, leading to the formation of small‐scale structures with amplitudes comparable with the ambient magnetic field. The influence of quasilinear effects on the initial stage of formation of mirror structures is discussed.
1242(2010); http://dx.doi.org/10.1063/1.3460121View Description Hide Description
Small asymmetries of the confining electric and magnetic fields pose an upper limit on the lifetime of nonneutral plasmas trapped in Malmberg‐Penning traps. The present paper reviews the effect of magnetic and electric field errors on the equilibrium of nonneutral plasma based on a suitable parallel current constraint. Together with Poisson’s equation, this constraint provides a full set of equations for determining self‐consistent asymmetric equilibria of non‐neutral plasmas in a Malmberg‐Penning trap. Using this approach, the effect on the plasma equilibrium of weak magnetic and electric squeezes (field errors with azimuthal number tilt and quadrupole distortions is investigated. Analytical and semi‐analytical solutions for the electric potential variation inside the trap are found in a paraxial limit for various radial density profiles of the plasma, including the case of global thermal equilibrium. A direct analog of the Pfirsch‐Schlüter and Stupakov currents is also derived. The presented theory of the plasma equilibrium distorted by field errors provides the basis for elaborating a self‐consistent theory of the asymmetry induced transport in a Malmberg‐Penning trap.
1242(2010); http://dx.doi.org/10.1063/1.3460122View Description Hide Description
The diocotron mode of a nonneutral plasma column confined in a Malmberg‐Penning trap, i.e. the rotation of the plasma center‐of‐charge around the longitudinal symmetry axis, is experimentally found to be unstable. We have investigated in the ELTRAP device a control mechanism of the radial drift of the column based on a Radio Frequency drive applied on an azimuthally sectored electrode of the trap. Systematic experiments show the characteristic features of the mechanism, namely the presence of amplitude and frequency thresholds as well as the non‐resonant behavior, whose interpretation invokes the concept of dynamic stabilization.