Flow Of Charged Particles Research Articles

Results from FOPI on strangeness production and propagation at SIS energies

Heavy ion collisions at SIS energies (1–2 A GeV) offer a unique tool to probe the properties of hot and dense nuclear matter. In particular, the partial restoration of chiral symmetry is predicted to lead in this energy range to in-medium modifications of hadron properties. Strange particle production below or close to the threshold energy is a useful probe to investigate these in-medium effects. The FOPI collaboration has recently measured the production and the propagation of charged and neutral strange particles. The K+ production probability is investigated as a function of the system size at a beam energy of 1.5 A GeV. Results on K0 production as well as K−/K+ ratio as a function of rapidity in Ru+Ru collisions at 1.69 A GeV are presented. In addition, the sideward flow of charged and neutral strange particles has been measured. Results are compared with predictions of transport calculations (BUU and IQMD).

Charged particle flow measurement for | n|

The measurement of elliptic flow for charged particles in Au-Au collisions at $\sqrt{s_{_{NN}}} = 130$ GeV using the PHOBOS detector at the Relativistic Heavy Ion Collider (RHIC) is presented. Charge particle distributions over a wide pseudo-rapidity region ($|\eta|<5.3$) are measured by the PHOBOS silicon multiplicity detectors. These distributions are used to extract the final-state azimuthal anisotropy. The centrality and pseudo-rapidity dependences of elliptic flow, averaged over momenta on an event-by-event basis and particle species, are discussed.

Simulation of Traveling Wave Toner Transport

A numerical computation of the dynamic flow of charged toner particles on a traveling electrostatic wave device is presented. In contrast to simple single particle models, the simulation is performed for a large ensemble of toner particles and takes into account mechanical and electrical interactions. This more realistic computation is based on a two-dimensional finite element field calculation of the imposed traveling electrostatic wave. The particles are modeled as spheres of different sizes, each carrying a point charge inside. Boundary conditions at the substrate surface are met by the method of image charges. Collective electrostatic effects that tend to force the particles towards the conveyor surface are described by an additional electric field accounting for the space charge in the toner cloud. Besides the mutual electrical interactions, collisions between the particles and impact forces at the conveyor surface are included in the calculations as well. Simulations with an ensemble of 250 particles are shown and discussed relative to experimental results.

On Transition Radiation of Surface Polaritons by a Modulated Flow of Charged Particles

On Transition Radiation of Surface Polaritons by a Modulated Flow of Charged Particles

Randomly layered active medium as a broadband amplifier

A flat-layered active medium in which two types of layers with different refractive indices alternate is considered. The thickness of a layer is assumed to be random and large as compared to the wavelength of propagating radiation. A wave propagating along the normal to the layers in such a medium is exponentially enhanced over lengths of the order of many layer thicknesses. In contrast to the familiar case of a periodic flat-layered active medium, waves with any (not necessarily definite resonant) frequency are amplified identically in a wide frequency range. By way of an example, convective instability of space-charge waves in a flow of charged particles moving through a randomly layered medium is considered. The predicted effect can be regarded as an analogue of Anderson’s localization, when increasing solutions rather than exponentially ecreasing ones are selected in view of the activity of the medium.

On Interaction of Charged Particles Flow with 2D Electron Gas

On Interaction of Charged Particles Flow with 2D Electron Gas

Relativistic consideration of a cylindrical mirror field focusing for a distant charged particle source

The analytical study of the focusing properties of a cylindrical mirror field in the nonrelativistic and relativistic energy areas for a flow of charged particles from a distant source is presented. The first-order focusing over all energy area and the expressions for focusing length and dispersion were calculated. The relativistic features of the focusing properties, such as the decreasing of both focusing length and dispersion by energy increase into deep relativistic region were obtained. The properties found are highly suitable for nuclear measurements, space and plasma investigations.

Centrality dependence of multiplicity, transverse energy, and elliptic flow from hydrodynamics

The centrality dependence of the charged multiplicity, transverse energy, and elliptic flow coefficient is studied in a hydrodynamic model, using a variety of different initializations which model the initial energy or entropy production process as a hard or soft process, respectively. While the charged multiplicity depends strongly on the chosen initialization, the p T-integrated elliptic flow for charged particles as a function of charged particle multiplicity and the p T-differential elliptic flow for charged particles in minimum bias events turn out to be almost independent of the initial energy density profile.

Gamma‐Ray Emission from an Outer Gap Accelerator: Constraints on Magnetospheric Current, Magnetic Inclination, and Distance for PSR B1055−52

We investigate a stationary pair production cascade in the outer magnetosphere of a spinning neutron star. The charge depletion due to global flows of charged particles, causes a large electric field along the magnetic field lines. Migratory electrons and/or positrons are accelerated by this field to radiate curvature gamma-rays, some of which collide with the X-rays to materialize as pairs in the gap. The replenished charges partially screen the electric field, which is self-consistently solved, together with the distribution functions of particles and gamma-rays. By solving the Vlasov equations describing this pair production cascade, we demonstrate the existence of a stationary gap in the outer magnetosphere of PSR B1055-52 for a wide range of current densities flowing in the accelerator: From sub to super Goldreich-Julian values. However, we find that the expected GeV spectrum becomes very soft if the current density exceeds the Goldreich-Julian value. We also demonstrate that the GeV spectrum softens with decreasing magnetic inclination and with increasing distance to this pulsar. We thus conclude that a sub-Goldreich-Julian current, a large magnetic inclination, and a small distance (500 pc, say) are plausible to account for EGRET observations. Furthermore, it is found that the TeV flux due to inverse Compton scatterings of infrared photons whose spectrum is inferred from the Rayleigh-Jeans side of the soft blackbody spectrum is much less than the observational upper limit.

Electrodynamic Structure of an Outer Gap Accelerator: Location of the Gap and the Gamma‐Ray Emission from the Crab Pulsar

We investigate a stationary pair-production cascade in the outer magnetosphere of a spinning neutron star. The charge depletion due to global flows of charged particles causes a large electric field along the magnetic field lines. Migratory electrons and/or positrons are accelerated by this field to radiate curvature gamma rays, some of which collide with the X-rays to materialize as pairs in the gap. The replenished charges partially screen the electric field, which is self-consistently solved together with the distribution functions of particles and gamma rays. If no current is injected at either of the boundaries of the accelerator, the gap is located around the conventional null surface, where the local Goldreich-Julian charge density vanishes. However, we first find that the gap position shifts outward (or inward) when particles are injected at the inner (or outer) boundary. Applying the theory to the Crab pulsar, we demonstrate that the pulsed TeV flux does not exceed the observational upper limit for moderate infrared photon density and that the gap should be located near to or outside of the conventional null surface so that the observed spectrum of pulsed GeV fluxes may be emitted via a curvature process. Some implications of the existence of a solution for a super Goldreich-Julian current are discussed.

Simulation of flows within an electrostatic ion thruster for space missions

AbstractIon thrusters are well suited for near-Earth and deep space missions because of their exceptionally high specific impulse. Those already flying are also proving to be very reliable. The flow of neutral and charged particles within the different components of these devices is very complex and although they have been studied for several years, little of the detail of their internal operation is properly understood. Numerical simulations of these flows can potentially yield an improved understanding of the physical phenomena involved and in turn this should assist in the optimising of new designs and in achieving higher specific impulses. The external plume, which can potentially cause damage to other spacecraft components such as solar panels, has been the object of some studies but the flow within the internal chambers where many complex phenomena are suspected to occur has been seriously neglected. For this reason the present fully kinetic neutral and charged particle simulations, which take account of the ionisation processes and the applied magnetic field, have been conducted. The studies have been made on one specific electrostatic ion thruster - the T5 model produced by the Space Department at the Defence and Evaluation Research Agency (DERA, Farnborough, UK). Two regions within the thruster have been concentrated on, namely the exit area of the hollow cathode and the main chamber.

Evolution of the density profiles and flows of charged particles during the diffusive decay of an electronegative gas plasma

Different scenarios of the spatiotemporal evolution of the parameters of the diffusive decay of a pulsed electronegative gas plasma in the absence of plasma chemical processes are studied. It is shown that nonlinear diffusion in a plasma with negative ions occurs in several stages. The rate of electron density decay increases with time and, in the beginning of the second stage, almost all the electrons escape from the discharge volume. On the other hand, the ion density profile is smoothed out due to ion-ion ambipolar diffusion and the flow of negative ions toward the wall is absent in the first stage of decay. In the second stage, the main diffusion mode is first established and then the ion-ion (electronless) plasma decays exponentially with a characteristic time determined by ion-ion ambipolar diffusion.

Elliptic flow in Au+Au collisions at square root(S)NN = 130 GeV.

Elliptic flow from nuclear collisions is a hadronic observable sensitive to the early stages of system evolution. We report first results on elliptic flow of charged particles at midrapidity in Au+Au collisions at square root(S)NN = 130 GeV using the STAR Time Projection Chamber at the Relativistic Heavy Ion Collider. The elliptic flow signal, v2, averaged over transverse momentum, reaches values of about 6% for relatively peripheral collisions and decreases for the more central collisions. This can be interpreted as the observation of a higher degree of thermalization than at lower collision energies. Pseudorapidity and transverse momentum dependence of elliptic flow are also presented.

Distribution of electric field strength around a large-scale charged particle cloud

To investigate an electrical discharge occurring from or in a space-charge cloud, a large-scale charged particle cloud was formed by using a cloud generator consisting of a blower and corona charger. The distribution of the electric field strength around a charged particle cloud has been investigated to determine the behavior of charged particles. The soil-conditioning particles were charged by corona charging and blown by high-speed air flow in a test room, 5 m wide, 10 m long, and 3 m high. The average charge-to-mass ratio of the particles blown by this method was 170 /spl mu/C/kg. The space-charge density of the cloud was calculated at the order of 10 /spl mu/C/m/sup 3/ from the electric field strength outside of the cloud. While the electric field strength at the outside of the cloud increased up to 52 kV/m within 2 m downstream from the cloud generator, it decreased below 25 kV/m farther than 2 m away from the cloud generator due to dispersion of charged particles. The change in the electric field strength due to dispersion of charged particles can be qualitatively explained by a simple cloud model. The velocity of charged particles transported by air flow and mobility of charged particles are found to be effective factors increasing the electric field strength around the large-scale charged particle cloud.

Particle-in-cell simulations of high-power cylindrical electron beam diodes

Particle-in-cell (PIC) simulations are presented that characterize the electrical properties and charged-particle flows of cylindrical pinched-beam diodes. It is shown that there are three basic regimes of operation: A low-voltage, low-current regime characterized by space-charge-limited (SCL) flow, a high-voltage, high-current regime characterized by a strongly pinched magnetically limited (ML) flow, and an intermediate regime characterized by weakly pinched (WP) flow. The flow pattern in the SCL regime is mainly radial with a uniform current density on the anode. In the ML regime, electrons are strongly pinched by the self-magnetic field of the diode current resulting in a high-current-density pinch at the end of the anode rod. It is shown that the diode must first draw enough SCL current to reach the magnetic limit. The voltage at which this condition occurs depends strongly on the diode geometry and whether ions are produced at the anode. Analytic expressions are developed for the SCL and ML regimes and compared to simulations performed over a wide range of voltages and diode geometries. In the SCL regime, it is shown that many of the results from planar diodes provide reasonably good estimates for cylindrical diodes. In the ML regime, it is found that the critical current formula provides a better fit to the simulations than the parapotential and focused flow models. An empirical fit to the I–V characteristic was developed from the simulations that smoothly transitions from the SCL regime, through the WP regime, and into the ML regime.

About some focusing properties of a cylindrical field

Abstract Analytical calculations have been performed in order to find the focusing properties of the cylindrical field in a case where the charged particles flow has a direction parallel to the symmetry axis. It is shown in the nonrelativistic part that a cylindrical mirror can be used as the energy analyzer for parallel flow with high transmission.

Equations for the envelope of a curvilinear charged-particle beam

The necessity of developing self-consistent models for the motion of charged-particle flows in external electromagnetic fields is caused by practical problems of beam and electron-ring formation and transport. Employing the equations of an envelope is one of methods for taking into account the effect of the self-field on the beam transverse dynamics. These equations are known for rectilinear and circular beams of charged particles [1‐3]. In the present paper, a method for constructing equations of the envelope for a curvilinear charged-particle beam propagating in a nonuniform magnetic field is proposed. A specific example for the occurrence of such a configuration is provided by the electron-beam injection at a certain angle to the geomagnetic field. Such a statement of the problem is of practical significance in the context of employing the electron beam for studying the ionosphere [4]. An analytical solution to the problem of constructing a self-consistent model for a curvilinear chargedparticle beam can be found for a pencil beam, when the ratios of the beam transverse size to both the radius of curvature and the radius of torsion are small quantities. In this case, there exists an approximate solution to the Euler equation for charged-particle gas in an external magnetic field, which happens to be correct to firstorder terms with respect to the specified small parameter. Here, we imply self-similar solutions to gasdynamic equations related to the class of gas motions for charged-particles, velocities of which are proportional to the distance to the center of symmetry [5]. Particle motion at the beam axis depends solely on an external field. Therefore, the position of the beam axis is determined by the trajectory Y ( s ) of the axial particle, where s is the trajectory length measured from the beam injection point. Substituting the expression for the velocity, v = u t , into the equation of particle motion in an external magnetic field, we obtain the curvature of the trajectory Y ( s ) :

Dynamical chaos in a charged-particle flow produced by a magnetron injection gun: Numerical simulation and experiment

A theoretical and experimental study of spatial and temporal current oscillations in a magnetron injection gun is presented. Basic features of the device operation are ascertained. Complicated system dynamics, namely transitions from regular to chaotic oscillations in response to changing the system parameters, is revealed. The conclusions are drawn based on the analysis of the computed electron trajectories and output-current waveforms and spectra. Experimentally, the spectra of the beam-current oscillations, noise-intensity spectral density, etc., are obtained. Strong broadband microwave oscillations of the output current are observed for a wide range of the lengths of the emitting and the nonemitting portions of the cathode.

Effect of Pulse Flows of Charged Particles on the Structure and Mechanical Properties of Metals

We investigate the effect of pulse flows of hydrogen, helium, and hydrogen–helium plasma of a specific power of 20–30 GW/m2 on the surface structure and mechanical properties of vanadium, niobium, and Kh16N15M3B and Kh18N10T austenitic stainless steels. Plasma bunches acted for 2 μsec with an average energy of particles of ≈ 2 keV. Tests of samples made of austenitic steels for tension showed that irradiation up to doses of ≈ 1018 cm−2 strengthens them by a factor of 1.8 and decreases the relative elongation by a factor of 2.3–2.7. A layer-by-layer electron-microscopic analysis revealed that a cellular structure is formed in the surface layer ≈ 25 μm in thickness as a result of irradiation, which explains the change in mechanical characteristics of the steels.

Charge Densities above Pulsar Polar Caps

A simplified model provided the framework for our investigation into the distribution of energy and charge densities above the polar caps of a rotating neutron star. We assumed a neutron star withm= 1.4M⊙,r= 10km, dipolar field |B0| = 1012G,B||Ω and Ω = 2Π · (0.5s)−1. The effects of general relativity were disregarded. The induced accelerating electric fieldE||reachesE0= 2.5 · 1013V m−1at the surface near the magnetic poles. The current density along the field lines has an upper limitnGJ, when the electric field of the charged particle flow cancels the induced electric field: At the polesnGJ(r=rns,θ= 0) = 1.4 · 1017m−3.The work function(surface potential barrier)EWis approximated by the Fermi energyEFof magnetised matter. Following Abrahams and Shapiro (1992) one needs to revise the surface density from the canonical 1.4 · 108kg m−3down toρFe = 2.9 · 107kg m−3. Withwe obtain a value ofEF=Ew= 417eV. There are two relevant particle emission processes:Field (cold cathode) emissionby quantum-mechanical tunneling of charges through the surface potentialandthermal emissionwhich is a purely classical process. In strong electric fields it is enhanced by the lowering of the potential barrier due to the Schottky effect. The combined Dushman-Schottky equationwithtells us, thatat temperatures&amp;gt; 2 · 105K the the Goldreich-Julian current can be supplied thermal emission alone. The surface temperature however has a lower limit in the order of 105K due to the rotational braking. Therefore, in most cases a sufficient supply of charges for the Goldreich-Julian current is available and the electrical field accelerating the particles will be quenched as a result of their abundance. Otherwise a residual equilibrium electric field Eeqremains with:and hence the equilibrium density is:n=nfieid(Eeq,EW) +nDS(Eeq,EW,T) For a temperature just below the onset of thermal emission (T= 1.85 · 105K) the charge density is found to vary almost linearly with the work functionEWfor values ofEWbetween 0.3 and 2 keV. At the chosen value forEWof 417 eVthe residual electric field amounts to only 8.5% of the vacuum value. Even in the residual electric field the particles are rapidly accelerated to relativistic energies balanced by inverse Compton and curvature radiation losses.