Downstream Particle Research Articles

Steady State Electrostatic Layers from Weibel Instability in Relativistic Collisionless Shocks

It is generally accepted that magnetic fields generated in the nonlinear development of the transverse Weibel instability provide effective collisionality in unmagnetized collisionless shocks. Recently, extensive two- and three-dimensional simulations improved our understanding of the growth and saturation of the instability in colliding plasma shells. However, the steady state structure of the shock wave transition layers remains poorly understood. We use basic physical considerations and order-of-magnitude arguments to study the steady state structure in relativistic unmagnetized collisionless shocks in pair plasmas. The shock contains an electrostatic layer resulting from the formation of stationary, magnetically focused current filaments. The filaments form where the cold upstream plasma and the counterstreaming thermal plasma interpenetrate. The filaments are not entirely neutral, and strong electrostatic fields are present. Most of the downstream particles cannot cross this layer into the upstream medium because they are trapped by the electrostatic field. We identify the critical location in the shock transition layer where the electromagnetic field ceases to be static. At this location, the degree of charge separation in the filaments reaches a maximum value and the current inside the filaments comes close to the Alfven limit. We argue that the radius of the current filaments upstream of the critical location is about equal to the upstream plasma skin depth. Finally, we show that some downstream particles cross the electrostatic layer and run ahead of the shock into the preshock medium without causing instability. These particles may play an important role in diffusive particle acceleration.

Particle characteristics following cloud‐modified transport from Asia to North America

Fast response measurements of particle size distributions, bulk submicron particle composition, and single particle composition were made aboard the NOAA WP‐3D research aircraft in the free troposphere over the eastern Pacific Ocean and the western coast of North America. Measurements of gas‐phase compounds and meteorological analysis show evidence of long‐range transport of layers of aerosol particles from anthropogenic and biomass‐burning sources in eastern Asia. Layers of crustal particles with no evident sources were also encountered. Measurements of substantially enhanced particulate sulfate mass and gas‐phase H2SO4 encountered during one transport event were interpreted with the aid of a numerical model of particle formation and growth and trajectory analysis. The observed particle size distributions and sulfate mass concentration were consistent with the simulation, indicating that the particles were formed over the mid‐Pacific from gas‐to‐particle conversion following long‐range transport of SO2 through a midlatitude cyclonic system. Such cloud systems appear to effectively scavenge most pre‐existing particle mass, but can allow transport of gas‐phase precursors which substantially alter downstream particle microphysical and chemical properties.

Analysis of additive metals in fuel and emission aerosols of diesel vehicles with and without particle traps.

Fuel additives used in particle traps have to comply with environmental directives and should not support the formation of additional toxic substances. The emission of metal additives from diesel engines with downstream particle traps has been studied. Aspects of the optimisation of sampling procedure, sample preparation and analysis are described. Exemplary results in form of a mass balance calculation are presented. The results demonstrate the high retention rate of the studied filter system but also possible deposition of additive metals in the engine.

Relating quality, capacity and energy consumption of oil filters

In the theoretical part of the paper a simple theory is presented relating filtrate clarity, filter capacity and energy consumption. The particle size distribution is subdivided in coarse, intermediate and fine particles. The key role play the intermediate particles who can enter and clog the filter medium. This combined with a layered cake model selects complete blocking as the filtration mechanism for the intermediate particles. In the experimental part results are given on the Multipass flat cell filtration of SAE 5–80 test dust in oil, monitoring upstream and downstream pressure and particle concentration as a function of time. For the chosen filter media and within the experimental range of flow and concentration the simple theory can be applied successfully to these SAE oil filtration experiments.

Pair distribution functions of colloidal particles on a quartz collector in a parallel plate and stagnation point flow chamber

Pair distribution functions of polystyrene particles adhering on a quartz collector surface are compared for a parallel plate (PP) and stagnation point (SP) flow chamber at a common Peclet number and identical surface coverage. Radial pair distribution functions of deposition patterns around the stagnation point in the SP flow chamber typically shows dampened oscillations at larger interparticle distances. Radial pair distributions in the PP flow chamber, however, do not indicate such oscillations. Furthermore, in the PP flow chamber, local distribution functions calculated in the direction of flow are asymmetrical with a depletion zone downstream adhering particles. Local pair distribution functions perpendicular to the flow are symmetrical. The comparison of pair distribution functions obtained in two different flow chambers shows how deposition patterns can be influenced by flow and collisions between flowing and adhering particles.

Injection and acceleration of H<sup>+</sup> and He<sup>2+</sup> at Earth's bow shock

Abstract. We have performed a number of one-dimensional hybrid simulations (particle ions, massless electron fluid) of quasi-parallel collisionless shocks in order to investigate the injection and subsequent acceleration of part of the solar wind ions at the Earth's bow shock. The shocks propagate into a medium containing magnetic fluctuations, which are initially superimposed on the background field, as well as generated or enhanced by the electromagnetic ion/ion beam instability between the solar wind and backstreaming ions. In order to study the mass (M) and charge (Q) dependence of the acceleration process He2+ is included self-consistently. The upstream differential intensity spectra of H+ and He2+ can be well represented by exponentials in energy. The e-folding energy Ec is a function of time: Ec increases with time. Furthermore the e-folding energy (normalized to the shock ramming energy Ep) increases with increasing Alfvén Mach number of the shock and with increasing fluctuation level of the initially superimposed turbulence. When backstreaming ions leave the shock after their first encounter they exhibit already a spectrum which extends to more than ten times the shock ramming energy and which is ordered in energy per charge. From the injection spectrum it is concluded that leakage of heated downstream particles does not contribute to ion injection. Acceleration models that permit thermal particles to scatter like the non-thermal population do not describe the correct physics.Key words. Interplanetary physics (planetary bow shocks) · Space plasma physics (charged particle motion and acceleration; numerical simulation studies)

A Study of Resuspended Particles from a Line Bed at the Wall of a Fluid Flow. Statistical Aspects

A statistical analysis of the dynamic behaviour of a bed of particles on the wall of a fluid flow is presented. By applying the different forces acting on each particle, i.e. fluid forces, adhesion to the wall and autohesion forces, conditions needed to obtain possible motions are determined and motions of the last downstream particle are described. It is shown that, above a critical value, increasing flow rate will promote the re‐entrainment process along the wall, whereas increasing adhesion and autohesion forces will maintain the line bed stationary. Moreover, in some circumstances, starting particles execute some small jumps before rolling and/or sliding along the wall, which reduces the residence time of the particle on the wall. These results confirms some published hypotheses.

Entropy generation at the multi-fluid solar wind termination shock producing anomalous cosmic ray particles

Different from other work the solar wind termination shock is described here as a multi-fluid phenomenon taking into account the wavefield-induced, magnetohydrodynamic self-interaction of a multi-species plasma, consisting of solar wind ions, pick-up ions and shock-generated anomalous cosmic ray particles. Caused by the diffusive interaction of the low- and high-energy plasmas, an extended transition structure of the shock is formed consisting of a precursor region and a gas dynamic sub-shock. The innerheliospheric pick-up ions, when convected with the solar wind plasma towards the sub-shock, will serve there as a local source, and all over the precursor region as a continuous source, of high energy particles via first-order Fermi acceleration in the shock-induced wave turbulences, thereby being energized from keV up to MeV energies and thus will operate as an entropy generator. The spatial diffusion of these high energy particles relative to the resulting, pressure-modified solar wind flow structure is described by a coupled system of differential equations describing mass-, momentum-, and energy-flow continuities for all plasma components. The energy loss due to escape of energetic particles (MeV) from the precursor into the inner heliosphere is taken into account. The hydrodynamic properties of the anomalous cosmic ray gas and the low-energy solar wind are determined in a consistent manner both for solutions with existing sub-shocks and for shock-free solutions. Also the variation of the compression ratio within the shock structure is quantitatively determined and is related to the pick-up ion energization efficiency and to the mean energy of the downstream anomalous cosmic ray particles. The variation of the resulting shock structure and of the extent of solar wind sheath plasma beyond the shock is discussed with respect to its consequences for the LISM neutral gas filtration and to the three-dimensional shape of the heliosphere.

Particles in C2F6-CHF3 and CF4-CHF3 etching plasmas

Particulate generation has been studied during reactive-ion etching of oxide wafers in C2F6-CHF3 and CF4-CHF3 plasmas using both a commercial etch tool and the GEC reference cell modified to resemble the commercial tool. Under certain discharge process conditions, copious amounts of submicrometre-sized particles are shown to form due to plasma interactions with the oxide substrate. In the commercial tool, particles were detected only by a downstream particle flux monitor, whereas in the reference cell, particles were observed by both in situ laser light scattering and downstream monitoring. In the commercial tool, wafers etched to end-point were shown by post-process surface analysis to be contaminated by submicrometre-sized columnar structures. Previous reports of similar such columnar structures formed during reactive-ion etching of oxide films have attributed the phenomenon to polymer micromasking. However, the results of this study clearly contradict this conclusion and suggest that the presence of columnar oxide etch residues is linked to process-induced particulate contamination. Laser light scattering measurements were made in the reference cell during reactive-ion etching of blanket oxide wafers and used to help clarify the complex processes of particulate nucleation, growth and deposition during oxide etching. Polarization coagulation of spherical particles formed in the reference cell is shown to occur, presumably in the high-field regions of the sheath, forming filamentous rod-like particle aggregates. The implications of this observation for wafer contamination are explored.

Relativistic particle acceleration in plerions

We discuss recent research on the structure and particle acceleration properties of relativistic shock waves in which the magnetic field is transverse to the flow direction in the upstream medium, and whose composition is either pure electrons and positrons or primarily electrons and positrons with an admixture of heavy ions. Particle-in-cell simulation techniques as well as analytic theory have been used to show that such shocks in pure pair plasmas are fully thermalized -- the downstream particle spectra are relativistic Maxwellians at the temperature expected from the jump conditions. On the other hand, shocks containing heavy ions which are a minority constituent by number but which carry most of the energy density in the upstream medium do put approximately 20% of the flow energy into a nonthermal population of pairs downstream, whose distribution in energy space is N(E) varies as E(exp -2), where N(E)dE is the number of particles with energy between E and E+dE. The mechanism of thermalization and particle acceleration is found to be synchrotron maser activity in the shock front, stimulated by the quasi-coherent gyration of the whole particle population as the plasma flowing into the shock reflects from the magnetic field in the shock front. The synchrotron maser modes radiated by the heavy ions are absorbed by the pairs at their (relativistic) cyclotron frequencies, allowing the maximum energy achievable by the pairs to be gamma(sub +/-)m(sub +/-)c squared = m(sub i)c squared gamma(sub 1)/Z(sub i), where gamma(sub 1) is the Lorentz factor of the upstream flow and Z(sub i) is the atomic number of the ions. The shock's spatial structure is shown to contain a series of 'overshoots' in the magnetic field, regions where the gyrating heavy ions compress the magnetic field to levels in excess of the eventual downstream value. This shock model is applied to an interpretation of the structure of the inner regions of the Crab Nebula, in particular to the 'wisps,' surface brightness enhancements near the pulsar. We argue that these surface brightness enhancements are the regions of magnetic overshoot, which appear brighter because the small Larmor radius pairs are compressed and radiate more efficiently in the regions of more intense magnetic field. This interpretation suggests that the structure of the shock terminating the pulsar's wind in the Crab Nebula is spatially resolved, and allows one to measure gamma(sub 1) and a number of other properties of the pulsar's wind. We also discuss applications of the shock theory to the termination shocks of the winds from rotation-powered pulsars embedded in compact binaries. We show that this model adequately accounts for (and indeed predicted) the recently discovered X-ray flux from PSR 1957+20, and we discuss several other applications to other examples of these systems.

Relativistic Particle Acceleration in Plerions

AbstractWe discuss recent research on the structure and particle acceleration properties of relativistic shock waves in which the magnetic field is transverse to the flow direction in the upstream medium, and whose composition is either pure electrons and positrons or primarily electrons and positrons with an admixture of heavy ions. Particle-in-cell simulation techniques as well as analytic theory have been used to show that such shocks in pure pair plasmas are fully thermalized—the downstream particle spectra are relativistic Maxwellians at the temperature expected from the jump conditions. On the other hand, shocks containing heavy ions which are a minority constituent by number but which carry most of the energy density in the upstream medium do put ~20% of the flow energy into a nonthermal population of pairs downstream, whose distribution in energy space is N(E) ∝ E−2, where N(E)dE is the number of particles with energy between E and E + dE.The mechanism of thermalization and particle acceleration is found to be synchrotron maser activity in the shock front, stimulated by the quasi-coherent gyration of the whole particle population as the plasma flowing into the shock reflects from the magnetic field in the shock front. The synchrotron maser modes radiated by the heavy ions are absorbed by the pairs at their (relativistic) cyclotron frequencies, allowing the maximum energy achievable by the pairs to be γ±m±c2 = mic2γ1/Zi, where γ1 is the Lorentz factor of the upstream flow and Zi, is the atomic number of the ions. The shock’s spatial structure is shown to contain a series of “overshoots” in the magnetic field, regions where the gyrating heavy ions compress the magnetic field to levels in excess of the eventual downstream value.This shock model is applied to an interpretation of the structure of the inner regions of the Crab Nebula, in particular to the “wisps,” surface brightness enhancements near the pulsar. We argue that these surface brightness enhancements are the regions of magnetic overshoot, which appear brighter because the small Larmor radius pairs are compressed and radiate more efficiently in the regions of more intense magnetic field. This interpretation suggests that the structure of the shock terminating the pulsar’s wind in the Crab Nebula is spatially resolved, and allows one to measure γ1, and a number of other properties of the pulsar’s wind. We also discuss applications of the shock theory to the termination shocks of the winds from rotation-powered pulsars embedded in compact binaries. We show that this model adequately accounts for (and indeed predicted) the recently discovered X-ray flux from PSR 1957+20, and we discuss several other applications to other examples of these systems.Subject headings: acceleration of particles — ISM: individual (Crab Nebula) — relativity — shock waves

Chemistry of SiO2 Plasma Deposition

The chemistry of deposition from undiluted and He‐diluted mixtures was studied by line‐of‐sight mass spectrometry of plasma species coupled with analysis of film IR absorption and dielectric properties. It was found that if RF power is sufficient to generate an O atom supply well in excess of that needed to convert all of the to , then clean IR spectra and high dielectric strength are obtained independent of dilution. Under these plasma conditions, many gas‐phase products of the form were detected. Their concentration increased steeply with the partial pressure of the reactants , and they are the likely source of the downstream particles and deposition rate loss which were seen at 130 Pa of reactant pressure. Reduction of reactant partial pressure to 13 Pa either by He dilution or by undiluted total pressure reduction eliminated these problems. No other effects of He dilution could be detected either in the plasma chemistry or in the film properties. Both concentration and electron trapping rate in the films were much higher than in thermal oxide and were unaffected by He dilution. There is some evidence that He plasma treatment does improve Si interface quality.

Influence of particle diameter distribution on the downstream particle turbulence in a two‐phase, turbulent, round jet

AbstractA numerical algorithm is used to study the ‘fan‐spreading’ mechanism of axial particle turbulence acquisition and the effects on it of the inlet particle diameter distribution, downstream of a particle‐laden, turbulent, round jet. The algorithm is based on the particle‐source‐in‐cell numerical technique for two‐phase flows. Eulerian equations are used for the description of the gas‐phase dynamics, whilst the solid particles are treated within the Lagrangian framework. The turbulent particle dispersion is simulated with the aid of the stochastic separated flow model. It was found that the ‘fan‐spreading’ turbulence is very much affected by the mean and standard deviation of the particle size distribution; in particular, it was found that the ‘fan‐spreading’ mechanism becomes weaker when the standard deviation of the particle diameter distribution is higher and the mean particle diameter smaller. An analytical expression describing the axial particle turbulence due to ‘fan‐spreading’ is proposed and tested, along with a similarity profile valid for the particles of bigger size.

Latex Sphere Retention by Microporous Membranes in Liquid Filtration

An experimental study of particulate matter retention by microporous membranes during liquid filtration has been conducted using 0.1, 0.22, 0.45, and 0.65-μm-rated hydrophilic and hydrophobic membrane filters. Retention measurements have been made with polystyrene latex spheres using an automated filter test system and a laser particle counter to measure the upstream and downstream particle concentration. Particle filtration during loading tests was found to begin with a sieving dominant regime followed by a transition regime and a cake filtration regime as particles accumulate inside the filter pores and on the filter surface. For latex sphere sizes equal to the nominal pore size of the filter, the initial filter efficiencies ranged from 97 to 99.9 percent. Complete retention (>99.9999999 percent) was achieved for a range of particle sizes two to three times the rated pore sizes of the filter. With the addition of a surfactant to the liquid, the retention was found to be lowered as a result of enhanced particle passage through the filter due to modified surface adsorption and steric stabilization. It was found that particle retention by sieving with the addition of surfactant provided the "worst-case" test for filter performance.

Liquid jet evolution from a gas chromatographic injector

Liquid particles from the breakup of liquid jets produced by a gas chromatographic syringe autoinjection system were observed and measured experimentally. The particle size depended on the motion of the syringe plunger. Very large, fast-moving particles were observed during accelerating periods of the liquid jet. The injection test chamber temperature affected the downstream particle size by heating the liquid before it left the needle and by evaporation into the hot gas. The particles produced by the water and heptane samples used were large and did not lend themselves to rapid vaporization within the gas stream of the gas chromatographic inlet port. Under such conditions, total sample vaporization must be considered either on the hot inlet port wall or on inserts within the inlet port.

A technique for measuring Lagrangian and Eulerian particle statistics in a turbulent flow

An experimental technique is described which has been developed to study particle dispersion in a round turbulent jet. Droplets are injected on the jet axis, and a laser sheet and position sensitive photomultiplier tube are used to track their radial displacement. Data processing is greatly simplified compared to video or photo imaging techniques which provide similar measurements. Statistically large samples are used to calculate dispersion and axial velocity as a function of axial downstream distance or particle time-of-flight. Dispersion and velocity statistics can be computed which are Lagrangian or Eulerian in nature. The technique has been demonstrated with 69 μm droplets of hexadecane in a jet of air with a Reynolds number of 15,000; in principle it could be used to study the motion of very small, quasi-fluid particles.

2-D simulation of ultra-fine particle capture by a single-wire magnetic collector

A two-dimensional theoretical model for the capture of ultrafine particles on a single-wire HGMS (high-gradient magnetic separator) collector has been developed. A full set of boundary conditions is derived and, in particular, dynamic boundary conditions at the wire surface are formulated. The computational results, showing areas of depletion, accumulation, and saturation as a function of time, predict the behavior of colloidal suspensions under the combined influence of diffusion, magnetic traction, and fluid flow. It is shown that diffusion, not the fluid flow, is responsible for the downstream particle capture.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Downstream energetic proton and alpha particles during quasi‐parallel interplanetary shock events

We have studied the energetic proton and alpha particle populations during the extended downstream region of three quasi‐parallel interplanetary shock events. The observations were carried out using the University of Maryland/Max‐Planck‐Institut ISEE 3 ULECA (Ultra Low Energy Charge Analyzer) sensor, which unambiguously identifies protons and alphas using the electrostatic deflection versus residual energy technique. The downstream particles exhibit anisotropies due largely, but not entirely, to convection in the solar wind. Using a detailed technique to find the reference frame in which the particle distribution functions are isotropic, we find that this frame is moving with respect to the solar wind at speeds from tens to hundreds of kilometers/second, is usually within ∼20° of the magnetic field direction, and is directed toward the Sun. We identify this reference frame as the rest frame of the scattering centers which isotropize the particle distribution functions. In the solar wind frame the scattering center frame sometimes moves at the Alfvén velocity, but just as often it has significantly higher or lower velocity. The spectral indices of the proton and alpha distribution functions are found to be remarkably constant during the downstream period, being generally insensitive to changes in particle flux levels, magnetic field direction, and solar wind densities. In two of the three events the proton and alpha spectra are the same throughout the entire downstream period, in agreement with predictions of diffusive shock acceleration theory. We find that diffusive acceleration theory cannot predict the particle spectra using only solar wind data. However, using the scattering center velocities, we are able to correctly predict the particle spectral indices for these three events.

Movement of solids in air and water by raindrop impact. effects of drop-size and water-depth variations

Transportation of 200 pm sand grains by impacting raindrops in both air and water was experimentally investigated through the water depth range 0-75 mm, employing four drop sizes. Airsplash movement, greatest for large drops impacting bare sand, fell rapidly with decreasing dropsize and increasing water depth. Shallow surface water inhibited airsplash strongly, yields falling by large factors through the depth range 0-2 mm. Hence, airsplashing of solids, even if randomly directed from any one point, must produce net migration from bare to water-covered surfaces. Drops over about 1.5 mm in diameter disrupt on impacting water at terminal velocity, generating disturbances which can, if water is shallow, suspend soil particles. Smaller drops evidently penetrate water surfaces without disruption, disturbing soil particles more directly and efficiently. If target water flows, even very slowly on low slopes, net downstream particle migration (rain-flow transportation) results. Rain-flow transportation rates rose from zero at zero depth to near-maximum values at 2 mm. Actual maxima were reached at depths of 2-3 drop diameters, transport rates declining rapidly with further depth increases. The combination of airsplash and rain-flow transportation extends transportation to areas where overland flow is either absent or too weak, acting alone, to move solids significantly.

Cosmic rays and gamma-rays from OB stars

view Abstract Citations (80) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Cosmic rays and gamma-rays from OB stars Voelk, H. J. ; Forman, M. Abstract The possible acceleration of cosmic rays at the terminal shocks of OB star winds is investigated. Interest is focused on the interaction region downstream of the shock, its acceleration, and its transmission properties. Particles to be accelerated can come from the star (stellar energetic particles), from the shock itself through injection out of the stellar wind plasma, from the interstellar medium, and possibly from the interface between shocked wind and ambient medium. It is shown that stellar energetic particles lose far too much energy in the expanding wind to be relevant. Estimates of the scattering mean free path indicate that postshock convection prevents access of downstream particles to the shock unless the postshock region collapses due to cooling in a low density ambient medium. Disregarding electrons, shock-injection of nucleons, as estimated from analogous results at the earth's bow shock in the solar wind, will yield nonrelativistic nucleons with a power law distribution in momentum of approximately p to the -4th if they can be accelerated at the shock. This is believed to be possible at least intermittently on time scales limited by the stellar rotation rate. Thus, stellar winds might be sources of very low energy (nuclear) cosmic rays. At high ambient gas densities this may lead to nuclear gamma-rays. Publication: The Astrophysical Journal Pub Date: February 1982 DOI: 10.1086/159623 Bibcode: 1982ApJ...253..188V Keywords: Cosmic Rays; Early Stars; Gamma Ray Astronomy; O Stars; Stellar Winds; Energetic Particles; Interstellar Matter; Nucleons; Particle Acceleration; Shock Wave Interaction; Astrophysics full text sources ADS |