Plasma Shell Research Articles

Ultrarelativistic Plasma Shell Collisions in γ‐Ray Burst Sources: Dimensional Effects on the Final Steady State Magnetic Field

Ultrarelativistic electron-positron plasma shell collisions as an integral part of generic γ-ray burst (GRB) fireball models are studied in the framework of self-consistent three-dimensional particle-in-cell simulations. We compare scenarios at moderately relativistic (γ0 10) and ultrarelativistic (γ0 100) energies that directly correspond to the regimes of internal and external shell collisions, respectively, in GRB synchrotron emission models. Simulated systems comprise 5 × 108 particles, applying a relativistic, fully electromagnetic, massively parallelized code. It is found that Weibel-generated, steady state magnetic equipartition ratios in external collisions reach up to B ~ 12%, exceeding the respective internal ratios by nearly a power of 10. Enhanced B yields can be explained theoretically by the effective reduction of dimensionality in the ultrarelativistic limit, i.e., the energy-dependent confinement of the three-dimensional Weibel instability within quasi-two-dimensional plasma shell slices.

Study of X-ray and Neutron Emission in Experiments with Al Wires in an MA Plasma Focus

Results are presented from experimental studies of the correlation between X-ray and neutron emissions generated in the implosion of a deuteron plasma shell onto an Al wire. The experiments were carried out on the PF-1000 facility at currents of 1.5–1.8 MA. An Al wire 80 μm in diameter and 7–9 cm in length was placed at the end of the inner electrode. During the implosion of the plasma shell, Al K-shell X-rays were first emitted at the dip of the current derivative. After the X-ray pulse, a relatively stable corona with a diameter of 2–3 mm and lifetime of a few hundred nanoseconds formed around the wire. The presence of the wire did not considerably reduce the total neutron yield (at most 1011 neutrons per shot) in comparison to discharges without a wire. As a rule, the intensity of neutron emission was maximal a few tens of nanoseconds after the peak of X-ray emission. A detailed comparison of two shots with low and high neutron yields have shown that the neutron yield depends on the configuration and dynamics of the discharge. The possible influence of the self-generated axial component of the magnetic field on the development of the plasma focus and the acceleration of fast deuterons is discussed.

Self‐consistent Diffusive Lifetimes of Weibel Magnetic Fields in Gamma‐Ray Bursts

Weibel filamentation in relativistic plasma shell collisions has been demonstrated as an efficient and fast mechanism for the generation of near-equipartition magnetic fields in self-consistent particle-in-cell (PIC) simulations. In generic γ-ray burst (GRB) models with kinetically dominated plasma outflow, sufficient strength and lifetime of magnetic fields are essential to validate synchrotron emission as the source of radiative outbursts. In this article we report on self-consistent PIC simulations of pair-plasma shell collisions in the highly relativistic regime with particle ensembles up to 5 × 108. Energy dependence of magnetic field generation in the Weibel process is discussed, and for the first time the diffusion-limited lifetime of magnetic fields is investigated self-consistently. In three different analyses the diffusion coefficient is determined self-consistently from the microphysical particle data. Cross-field diffusion is identified as Bohm-type by direct confirmation of the characteristic T/B-behavior. With the stability-limiting process pinned down, the lifetime of Weibel magnetic fields in plasma shell collisions subsumes to τωp0 ~ 109 (with ωp0 the plasma inertial timescale as characteristic of any system). In the context of typical synchrotron cooling times, we conclude that the Weibel fields are sufficiently long-lived for GRB prompt emission as well as afterglows, especially in view of the likely baryonic pollution of pair-plasma shells.

Wettability and corrosion tests of diamond films grown on Ti6Al4V alloy

Diamond thin films were successfully deposited on both sides of jetted substrates Ti6Al4V alloy, without intermediate layers deposition, by using an enhanced 2.45 GHz microwave-assisted technique. This system is based on discharge surface-wave guide, Surfatron system. It was used as a bias-enhanced nucleation (BEN) applied between the plasma shell and the substrate, reaching a nucleation density around 5×10 9 parts cm −2 in just 5 min. These films showed a good quality and a total residual stress around −2.4 GPa that was evaluated by Raman scattering spectroscopy. In spite of the high residual stress, the films adhesion on substrate was excellent, even when a load of 250 kgf was applied. Wettability of these films designed a small hydrophobicity that is very similar to those of other carbon structures used as biological implants. The surface energy value of 50.5 mJ m −2 indicated that is possible to get a good tissue adhesion on diamond films. These films were also exposed to various biological fluids, including isotonic NaCl and Ringer's solution for 1 month and acid–water solution for 2 months. Diamond surface chemical stability was analyzed from films micrographs by scanning electron microscopy (SEM). The results revealed that the diamond films surfaces were not degraded by environmental agents.

Control of the Rayleigh–Taylor instability in a staged Z pinch

Z-pinch experiments and computer simulations provide evidence for enhanced stability and current transfer in a staged Z pinch, consisting of an annular krypton shell imploding onto a deuterium gas fill. Visible-streak and Schlieren imaging provide evidence for a multilayer implosion where the outer plasma shell is Rayleigh–Taylor unstable and the inner plasma column is stable. Computer simulations indicate that the discharge current diffuses through the unstable, outer Kr shell. As the discharge current layer implodes onto the deuterium, current is transferred and a stable implosion results, producing a deuterium-compression ratio of 200.

Identification of Earth-directed partial halo coronal mass ejections

The very weak storm-effectiveness of frontside partial halo coronal mass ejections (FPH CMEs) suggests that many, if not most, FPH CMEs are not Earth-directed. Based on the cone model (Zhao et al., 2002) we put forward a method to identify Earth-directed partial halo CMEs from FPH CMEs. 1. The Earthward direction of halo-type CMEs CMEs with apparent angular span greater than 120◦ are termed as halo-type CMEs in what follows. The threshold of 120◦ is selected somewhat arbitrary (see next section). The Earthward direction of a halo-type CME is a necessary condition for the halotype CME to be storm-effective. Identification of Earth-directed halo-type CMEs is thus an important task in the research of the space weather. By “Earthward direction” or “Earth-directed” here we mean that there is a component of the halo-type CME that is able to hit the Earth, though the central axis of the CME may not be aligned with the line-of-sight. Halo-type CMEs are interpreted as the result of the Thompson scattering of Sun’s white light along the line-of-sight caused by a broad shell or bubble of dense plasma which is ejected in the frontside or backside of the Sun (Howard et al., 1982). Observations show that limb CMEs propagate radially beyond the first couple of solar radii, and that many limb CMEs maintain their angular width constant. Therefore halo-type CMEs with angular span of 360◦ and having associated near surface activities, the frontside full halo (FFH) CMEs, are expected to be Earth-directed because the halos of FFH CMEs cover Sun’s disk center, the Earth’s projection in Sun’s disk. The fact that all FFH CMEs observed in 1997 have associated interplanetary counterparts near the Earth and geomagnetic storms (Zhao and Webb, 2003) confirms that FFH CMEs are indeed Earthdirected. The storm-effectiveness for both FFH and FPH CMEs significantly weaker than for FFH CMEs alone (See Table 1) suggests, however, that many, if not most, FPH CMEs are not Earth-directed. It may improve the space weather forcasting if we can identify Earth-directed partial halo CMEs from all FPH CMEs. 2. Identifying Earth-directed partial halo CMEs There are studies for identifying Earth-directed partial halo CMEs. Webb et al used the angular span of 140◦ as a threshold for selecting partial halo CMEs and found that the FPH CMEs between December 1996 and June 1997 are storm-effective when the source region in the low corona is within ∼ 0.5 radii of the Sun’s disk center. Lyons et al. selected the angular span that exceeds 100◦ and encompasses one or the other of the solar poles. Thompson et al used a similar criterion but with a higher threshold of

Optical observations of plasma formation and wire core expansion of Au, Ag, and Cu wires with 0–1 kA per wire

Explosions of Au, Ag, and Cu wires (1 cm long, 13–25 μm diameter, with/without insulating coatings) driven by a current pulser (4.5 kA peak, rise time 350 ns) were studied. Diagnostics included three channels of Schlieren imaging and two channels of shearing interferometry using the second harmonic of a Nd:YAG laser (λ=532 nm, 5 ns pulse width). The formation of a plasma shell around the neutral wire core was observed relatively late in time compared to the voltage collapse time. The corresponding plasma density was measured. From the refractive index of the wire core, some properties of the wire core material will also be discussed. The expansion rates of exploding wires were measured by interferometry and compared to the previously reported results. Two-wire experiments were also performed. The merging of the plasma at an increased plasma density was observed between two wires. The effects of the plasma shell on the wire expansion are discussed.

Roentgenographic investigations of the dynamics and spatial structure of wire-array Z-pinches

The implosion of wire arrays is studied at the Angara-5-1 facility with the help of an X-ray pinhole camera. It is shown that the drift of the plasma toward the axis occurs in the form of a “plasma rainstorm.” The data constituting a part of the experimental database are presented. Based on these data, it is established that the spatial structure of an imploding plasma is highly inhomogeneous, so that it makes no sense to talk about a plasma shell that implodes as a single entity. In this case, plasma inhomogeneities arising due to a “cold start” and prolonged plasma production have a decisive impact on the final parameters of a hyper-terawatt Z-pinch.

Measuring the Evolution of Electron Density Profile in a Small Gas-puff z-pinch

A three-frame Mach-Zehnder interferometer (TFMZI) was used for taking 3 pictures (5 ns exposure and 13 ns interpicture delay) of the imploding plasma in one z-pinch shot, which provides us a series of interferograms showing the evolution of electron density profile during z-pinch implosion stage. From these interferograms, the electron density profile ne(r , z), the velocities of plasma implosion were derived, and the plasma instabilities appearing as bubbles or fringe shearing were observed clearly. In order to study the correlation between the total energy of x-ray emission and the plasma implosion, thermopiles were also used in the experiments with TFMZI. The results of x-ray radiation diagnostics show us that a near uniform and symmetric imploding plasma shell would emit more x-ray yield than an asymmetric one, which is in good agreement with the implosion dynamics analyses.

Experimental and numerical studies of plasma production in the initial stage of implosion of a cylindrical wire array

The features are studied of plasma production in the initial stage of implosion of hollow cylindrical wire arrays at electric-field growth rates of 1012 V/(cm s). The results are presented from the analysis of both UV emission from the wire plasma and the discharge parameters in the initial stage of the formation of a Z-pinch discharge. It is found that, a few nanoseconds after applying voltage to a tungsten wire array, a plasma shell arises on the wire surface and the array becomes a heterogeneous system consisting of metal wire cores and a plasma surrounding each wire (a plasma corona). As a result, the current switches from the wires to the plasma. A further heating and ionization of the wire material are due primarily to heat transfer from the plasma corona. A model describing the primary breakdown along the wires is created with allowance for the presence of low-Z impurities on the wire surface.

Confinement and compression of magnetic flux by plasma shells

Confinement and compression of magnetic flux by plasma shells is of interest for a variety of applications associated with keV x-ray production in pulsed-power driven Z-pinch plasma radiation sources (PRS). Confinement of a Bz field with a Bθ field of the pinch current can help stabilize the implosion of a shell from a large initial radius. Compression of the azimuthal magnetic flux in the PRS (secondary) circuit with a plasma shell driven by Bθ field of the primary circuit may represent a new opportunity for using a low-cost, relatively slow pulsed power to generate large amounts of keV x rays. The magnetic field has to be compressed and/or confined by low-mass plasma shells emitting soft x-ray radiation that limits the temperature and conductivity of the shell plasma. The thickness of a plasma shell is established self-consistently during the implosion, and it is not obvious whether or not the shell becomes thick enough to confine or compress the magnetic flux. The results of analytical theory and numerical simulations demonstrate that the flux-compressing capability of a low-beta plasma shell is surprisingly good because the shell is shown to dynamically adjust its thickness so that it always remains of the order of its skin depth. The self-similar profiles of confined magnetic field predicted by the theory are consistently reproduced in the simulations.

Collisionless Slowing Down of Nova and Supernova Shells in Magnetized Interstellar Medium

The collisionless interaction of an expanding plasma cloud with a magnetized background plasma is examined in the framework of a 3D kinetic-hydrodynamic model. The slowing down of a hydrogen cloud is studied for high Alfven-Mach numbers and magneto-laminar interaction parameters. A particle-in-cell method is used to study the dynamics of the magnetic field, plasma cloud, background plasma, and collisionless shock wave generated by the intense particle flux. A numerical simulation is consistent with the nonstationary interactions between the plasma shells formed during nova and supernova explosions and the interstellar plasma medium.

Interpenetrating Plasma Shells: Near-Equipartition Magnetic Field Generation and Nonthermal Particle Acceleration

We present the first three-dimensional fully kinetic electromagnetic relativistic particle-in-cell simulations of the collision of two interpenetrating plasma shells. The highly accurate plasma-kinetic particle-in-cell (with the total of 108 particles) parallel code OSIRIS has been used. Our simulations show (1) the generation of long-lived near-equipartition (electro)magnetic fields, (2) nonthermal particle acceleration, and (3) short-scale to long-scale magnetic field evolution, in the collision region. Our results provide new insights into the magnetic field generation and particle acceleration in relativistic and subrelativistic colliding streams of particles, which are present in gamma-ray bursters, supernova remnants, relativistic jets, pulsar winds, etc.

X‐Ray Emission from the Hot Interstellar Medium and Southwest Radio Lobe of the Nearby Radio Galaxy Centaurus A

We present results from two Chandra/ACIS-I observations and one XMM-Newton observation of X-ray emission from the interstellar medium (ISM) and the inner radio lobes of the nearby radio galaxy Centaurus A. The ISM has an average radial surface brightness profile that is well described by a � -model profile with index � ¼ 0:40 � 0:04 and a temperature of kBTISM � 0:29 keV beyond 2 kpc from the nucleus. We find that diffuse X-ray emission is coincident with the outer half of the southwest radio lobe, and a bright X-ray enhancement is detected along the edge of the lobe. On the basis of energetic and lifetime arguments, we reject a nonthermal explanation for this emission. We model this emission as a thin, hot shell or cap of X-ray– emitting plasma surrounding the radio lobe that was created by the supersonic inflation of the lobe. This plasma shell is both hotter than (kBTSH � 2:9 keV) and greatly overpressurized relative to the ambient ISM, indicating supersonic expansion. We estimate that the lobe is expanding into the ISM at approximately Mach 8.5, or 2400 km s � 1 . We are not directly observing the bow shock, but rather the cooler, denser material that is accumulating ahead of the contact discontinuity. The thermal energy in the shell is a significant fraction of the thermal energy of the hot ISM, demonstrating the possibility that the hot ISM of early galaxies can be reenergized by outflows from nuclear activity. Interestingly, no similarly bright X-ray emission is detected in or along the edge of the northeast lobe, implying that there are differences in the dynamics and evolution of the kiloparsec-scale radio components. Subject headings: galaxies: active — galaxies: individual (Centaurus A, NGC 5128) — galaxies: ISM — galaxies: jets — radio continuum: galaxies — X-rays: galaxies On-line material: color figures

On the heliolatitudinal variation of the galactic cosmic-ray intensity. Comparison with Ulysses measurements

Abstract. We study the dependence of cosmic rays with heliolatitude using a simple method and compare the results with the actual data from Ulysses and IMP spacecraft. We reproduce the galactic cosmic-ray heliographic latitudinal intensity variations, applying a semi-empirical, 2-D diffusion-convection model for the cosmic-ray transport in the interplanetary space. This model is a modification of our previous 1-D model (Exarhos and Moussas, 2001) and includes not only the radial diffusion of the cosmic-ray particles but also the latitudinal diffusion. Dividing the interplanetary region into "spherical magnetic sectors" (a small heliolatitudinal extension of a spherical magnetized solar wind plasma shell) that travel into the interplanetary space at the solar wind velocity, we calculate the cosmic-ray intensity for different heliographic latitudes as a series of successive intensity drops that all these "spherical magnetic sectors" between the Sun and the heliospheric termination shock cause the unmodulated galactic cosmic-ray intensity. Our results are compared with the Ulysses cosmic-ray measurements obtained during the first pole-to-pole passage from mid-1994 to mid-1995.Key words. Interplanetary physics (cosmic rays; interplanetray magnetic fields; solar wind plasma)

Green-function theory of confined plasmons in coaxial cylindrical geometries: Zero magnetic field

We report on a theoretical investigation of plasmon propagation in the coaxial cylindrical geometries using Green-function (or response-function) theory in the presence of an applied axial magnetic field $(\stackrel{P\vec}{B}\ensuremath{\Vert}\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{z})$. The magnetoplasmon excitations in such multiple-interface structures are characterized by the electromagnetic (EM) fields that are localized at and decay exponentially away from the interfaces. Green-function theory, when generalized to be applicable to such quasi-one-dimensional systems, enables us to derive explicit expressions for the corresponding response functions (associated with EM fields), which can in turn be used to compute numerous physical properties of the system under consideration. A rigorous analytical diagnosis of the general results in diverse situations leads us to reproduce exactly the previously well-established results on planar systems, both in the presence and absence of $\stackrel{P\vec}{B}$, obtained within the different theoretical frameworks. As an application, we present several illustrative examples on the dispersion characteristics of the confined and extended magnetoplasmons in the single- and double-interface structures. These dispersive modes are also substantiated through the computation of local as well as total density of states. It is found that, unlike as in the zero-field case, the magnetoplasma propagation is nonreciprocal with respect to the sign of the index $m$ of the Bessel functions involved. The effects of an applied magnetic field and the aspect ratio on the dispersion of the confined magnetoplasmons are discussed. We also briefly clarify some delusive traces of the edge magnetoplasmons for a plasma shell embedded between two identical or unidentical dielectrics. The elegance of theory lies in the fact that it does not require matching of the messsy boundary conditions and it also lies in its simplicity and the compact form of the desired results. Our theoretical framework can also serve as a powerful technique for studying the intrasubband plasmons and magnetoplasmons in the emerging mutiple-walled carbon nanotubes.

Three-dimensional Weibel instability in astrophysical scenarios

Near equipartition magnetic fields are predicted by gamma ray bursters models and astronomical observations, in general associated with shocks or regions with colliding streams of particles. These scenarios require the conversion of kinetic energy in the outgoing plasma shells into B-fields. How the magnetic fields are generated and how particles are accelerated is still an open question, that can only be definitely addressed via fully kinetic three-dimensional (3D) numerical simulations. These shocks are collisionless because dissipation is dominated by wave–particle interactions, i.e., it is accomplished by particle scattering in turbulent electromagnetic fields generated at the shock front, or equivalently the mean free path is much longer than the shock front thickness (a few collisionless skin depths or a few Larmor radii, in magnetized plasmas). Plasma instabilities driven by streaming particles, such as the Weibel instability, are responsible for the excitation of these turbulent electromagnetic fields. Three-dimensional fully kinetic electromagnetic relativistic particle-in-cell simulations for the collision of two interpenetrating plasma shells were performed using the code OSIRIS [Fonseca et al., Lect. Notes Comput. Sci. 2331, 342 (2002)], showing (i) the generation of long-lived near-equipartition quasistatic (electro)magnetic fields, (ii) nonthermal particle acceleration, and (iii) short-scale to long-scale B-field evolution. These results may be important to understand magnetic field generation and particle acceleration in relativistic collisionless shock fronts, in gamma ray bursters, pulsar winds, and radio supernovae, and open the way to the full 3D kinetic modeling of relativistic shocks.

Use of emission-line intensities for a self-consistent determination of the particle densities in a transient plasma.

A method for a self-consistent determination of the time history of the electron density, electron temperature, and ionic charge-state composition in a multicomponent plasma, using time-dependent measurements and calculations of absolute emission-line intensities, is presented. The method is applied for studying the properties of an imploding gas-puff Z-pinch plasma that contains several oxygen ions up to the fifth ionization stage. Furthermore, by using intensity ratios of lines from different ion species, the electron temperature was determined with a much improved accuracy, in comparison to previous spectroscopic studies of the same plasma. The ion-density history obtained, together with the known time-dependent radial boundaries of the plasma shell, allowed for tracking the rise in time of the mass swept by the magnetic field during the implosion.

Raman analyses of residual stress in diamond thin films grown on Ti6Al4V alloy

The stress evolution in diamond films grown on Ti6Al4V was investigated in order to develop a comprehensive view of the residual stress formation. Residual stress is composed of intrinsic stress induced during diamond film growth and extrinsic stress caused by the different thermal expansion coefficients between the film and substrate. In the coalescence stage it has been observed that the residual stress is dominated by the microstructure, whereas on continuous films, the thermal stress is more important. In this work diamond thin films with small grain size and good size and good quality were obtained in a surface wave-guide microwave discharge, the Surfatron system, with a negative bias voltage applied between the plasma shell and substrate. For above of -100V applied bias, the ratio of carbon sp³/sp² bond may increase and the nucleation rate increase arising the high value at the -250V applied bias. Stress measurements and sp³ content in the film were studied by Raman scattering spectroscopy. The total residual stress is compressive and varied from -1.52 to -1.48 GPa between 0 and -200 V applied bias, respectively, and above the -200 V, the compressive residual stress increased drastically to -1.80 GPa. The diamond nucleation density was evaluated by top view SEM images.

Gamma‐Ray Burst Spectra and Light Curves as Signatures of a Relativistically Expanding Plasma

Temporal and spectral characteristics of prompt emission of gamma-ray burst (GRB) pulses are the primary observations for constraining the energizing and emission mechanisms. In spite of very complex temporal behavior of the GRBs, several patterns have been discovered in how some spectral characteristics change during the decaying phase of individual, well-defined long (greater than a few seconds) pulses. In this paper we compare these observed signatures with those expected from a relativistically expanding, shock-heated, and radiation-emitting plasma shell. Within the internal shock model and assuming a short cooling time, we show that the angular dependence in arrival time from a spherical expanding shell can explain the general characteristics of some well-defined long GRB pulses. This includes the pulse shape, with a fast rise and a slower decay, ∝(1 + t/τ)2, where τ is a time constant, and the spectral evolution, which can be described by the hardness-intensity correlation (HIC), with the intensity being proportional to the square of the hardness measured by the value of the peak, e.g., Ep of the νFν spectrum. A variation of the relevant timescales involved (the angular spreading and the dynamic) can explain the broad, observed dispersion of the HIC index. Reasonable estimates of physical parameters lead to situations where the HIC relation deviates from a pure power law, features that are indeed present in the observations. Depending on the relative values of the rise and decay times of the intrinsic light curve, the spectral/temporal behavior, as seen by an observer, will produce the hard-to-soft evolution and the so-called tracking pulses. In our model the observed spectrum is a superposition of many intrinsic spectra arriving from different parts of the fireball shell with varying spectral shifts. Therefore, it will be broader than the emitted spectrum and its spectral parameters could have complex relations with the intrinsic ones. Furthermore, we show that the softening of the low-energy power-law index, that has been observed in some pulses, can be explained by geometric effects and does not need to be an intrinsic behavior.