Showing papers on "Charged particle published in 1991"

Particle-in-cell charged-particle simulations, plus Monte Carlo collisions with neutral atoms, PIC-MCC

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

The plasma physics of shock acceleration

Abstract: The notion that plasma shocks in astrophysical settings can and do accelerate charged particles to high energies is not a new one However, in recent years considerable progress has been achieved in understanding the role particle acceleration plays both in astrophysics and in the shock process itself In this paper we briefly review the history and theory of shock acceleration, paying particular attention to theories of parallel shocks which include the backreaction of accelerated particles on the shock structure We discuss in detail the work that computer simulations, both plasma and Monte Carlo, are playing in revealing how thermal ions interact with shocks and how particle acceleration appears to be an inevitable and necessary part of the basic plasma physics that governs collisionless shocks We briefly describe some of the outstanding problems that still confront theorists and observers in this field

Physics of Nonneutral Plasmas

Abstract: Theoretical Models of Nonneutral Plasma Fundamental Properties of Nonneutral Plasma Kinetic Equilibrium and Stability Properties Macroscopic Equilibrium and Stability Properties The Diocotron Instability Coherent Electromagnetic Wave Generation by the Cyclotron Maser and Free Electron Laser Instabilities Equilibrium and Stability Properties of Intense Nonneutral Flow in High-Power Diodes Propagation and Stability of Intense Charged Particle Beams in a Solenoidal Focusing Field Propagation and Stability of Intense Charged Particle Beams in an Alternating-Gradient Focusing Field.

Laser ionization of noble gases by Coulomb-barrier suppression

Abstract: Laser ionization of noble gases was studied with a 1.053-μm, 1-psec Nd:glass laser. A systematic scan of intensities from mid-1013 W/cm2 to mid-1016 W/cm2 was performed, resulting in the production of charge states as high as Xe12+. Ionization occurs exclusively in the tunneling regime. We compare experimental ion production rates with those predicted by several different theories. Agreement between experimental ion-production curves and theoretical predictions is good for two theoretical models: (1) an elaboration of the Keldysh tunneling model, developed by Ammosov et al. [ Sov. Phys. JETP64, 1191 ( 1986)] and (2) a much more primitive model, based on Coulomb-barrier suppression, in which tunneling and other quantum-mechanical effects are ignored completely. The success of the more primitive model suggests that a new term, barrier-suppression ionization, rather than tunneling or multiphoton ionization, may be the most appropriate at this wavelength and in this range of intensities.

Above-surface neutralization of highly charged ions: The classical over-the-barrier model

Abstract: The neutralization of highly charged ions during interaction with metallic surfaces is accompanied by the ejection of a large number of secondary electrons Recent experiments demonstrate two main contributions to this electron ejection process: one from the region below the surface and the second from the above-surface portion of the trajectory We present a theoretical analysis of the neutralization dynamics above the surface, prior to impact, based on the classical over-the-barrier model The theory incorporates resonant multielectron capture of conduction electrons, resonant loss into unoccupied states of the conduction band, and intra-atomic Auger deexcitation The effective barrier potential includes quantum corrections to the classical image potential The effect of below-barrier (``tunneling'') transfer is investigated The solution of a coupled system of rate equations allows the approximate determination of the n-shell populations, the projectile charge state, and the total number of Auger electrons The calculation describes the transient formation of ``hollow'' atoms We find satisfactory agreement with recent data for K Auger yields by Meyer et al [Phys Rev Lett 67, 723 (1991)]

Ground-state correlation energies for two- to ten-electron atomic ions.

Abstract: Improved estimates of the nonrelativistic stationary nucleus correlation energies of the ground-state atomic ions with three to ten electrons and Z up to 20 are derived by combining experimental data and improved ab initio calculations. Unlike previous work in this area, we focus on the correlation contribution to individual ionization energies, computed by comparing experimental data with relativistic complete-valence-space Hartree-Fock energies.

Summation of Coulomb fields in computer-simulated disordered systems

Abstract: Formulae are derived for the sums over Coulomb forces exerted on a charged particle by other charged particles, the central cell system being repeated to infinity by periodic boundary conditions. Such sums are needed in molecular dynamics simulations involving either ions or neutral molecules represented as bound conglomerates of charges, and in astrophysical simulations of gravitating masses. The derived sums are rapidly convergent, being expressed in terms of Bessel functions Kr(z), which decrease exponentially with z. The force expressions are integrated analytically to give the potential function, which may be used in Monte Carlo simulations. The geometries considered are: (i) systems confined between two parallel walls, and (ii) unconfined three-dimensional systems.

Particle simulations of radio-frequency glow discharges

Abstract: Particle-in-cell (PIC) simulations are used to study the structure of radio-frequency (RF) glow discharges in helium between parallel-plate electrodes. The authors have examined a range of conditions and report on a variety of observed phenomena. Comparisons to experiment and analytical models are made, when possible. The differences between discharges in which secondary electrons play a key role in sustaining the discharge and those in which secondary electrons are unimportant are examined in three cases which illustrate the importance of the discharge-sustaining mechanisms. Electron-energy distributions are found to be, in general, non-Maxwellian, with shapes that depend in complex ways on discharge conditions. In the absence of secondary electron emission, electron heating in the sheath regions of the discharge is enhanced at higher voltages compared to ohmic heating in the bulk of the plasma. Fast electrons accelerated by the advancing sheath can carry a substantial fraction of the conduction current in the bulk of the discharge, reducing the effective bulk ohmic heating of electrons. Ion-energy distributions at electrode surfaces have been predicted and are compared to experimental measurements. >

Negative ions in the coma of comet Halley

Abstract: The detection of negatively charged cometary ions in the inner coma of Comet Halley is reported. These ions are observed in three broad mass peaks at 7-19, 22-65, and 85-110 AMU, with densities reaching greater than about 1/cu cm, about 0.05/cu cm, and about 0.04/cu cm, respectively, at a distance of about 2300 km from the nucleus. The ion species thought to be present include O(-), OH(-), C(-), CH(-), CN(-) and heavier complex CHO molecular ions. As negative ions are easily destroyed by solar radiation at about 1 AU, an efficient production mechanism, so far unidentified, is required to account for the observed densities. The detection of negative ions in the coma near 1 AU implies that negative ions should also be present in similar neutral gas and dust environment farther away from the sun. If the negative-ion densities are large enough, they could play an important part in physical processes such as radiative transfer or charge exchange.

Resilience of all-carbon molecules C60, C70, and C84: A surface-scattering time-of-flight investigation

Abstract: Ion beam scattering experiments on the larger carbon molecules (C{sub 60}{sup {plus minus}}, C{sub 70}{sup +} C{sub 80}{sup +}) demonstrate their exceptionally high stability with respect to impact-induced fragmentation processes. The charged molecules are formed by ultraviolet laser desorption of high purity molecular samples into a pulsed helium jet. Extracted ions impact Si(100) or graphite (0001) in a high-resolution ion beam/surface collider with mass time-of-flight and angular analysis. Collisions are highly inelastic processes: a large fraction of the entire perpendicular momentum component is lost, and 60 {plus minus} 20% of the component is either lost or exchanged. No more than 10% of the incident ions are returned, which is attributed to neutralization during the collision event. In contrast to all molecular ions (benzene and naphthalene cations) and cluster (alkali-metal halides), these molecules exhibit no evidence for impact-induced fragmentation, even at impact energies exceeding 200 eV. In the case of C{sub 60}{sup {minus}}, both the intact parent ion and ejected electrons are detected, with the latter becoming dominant above 120 eV impact energy. C{sub 60}{sup +} is found to have an exceptionally low energy threshold for inducing sputtering processes of adsorbed overlayers on graphite. Some of these results may be interpretablemore » in terms of the unique structural-energetic characteristics of the fullerene family. The results are compared to recent computer simulations of the impact event, which predict high resilience for these molecules.« less

Lifetime of ring current particles due to coulomb collisions in the plasmasphere

Abstract: Ring current ions and electrons in the trapped belt are scattered and slowed down to Coulomb interactions with the thermal plasma in the plasmasphere and are eventually removed from trapped orbits. An expression for bounce-averged Coulomb lifetimes is derived using methods developed by Wentworth et al. (1959) but with the improvements of considering a more realistic representation of the thermal plasma distribution, as a Maxwellian rather than a delta function, and considering heavy ions in the ring current and thermal plasmas. Bounce-average Coulomb lifetimes for the major ring current ions are presented and are compared to Coulomb lifetimes in the literature. Coulomb lifetimes for ring current ions with energies less than {approximately} 10 keV are longer than those predicted using previously derived expressions; however, they are comparable to, or shorter than, charge exchange lifetimes at these energies. For realistic thermal plasma densities in the outer plasmasphere, Coulomb lifetimes for ring current heavy ions can be comparable to charge exchange lifetimes at energies near the peak of the ring current differential number density. These results suggest that Coulomb decay is an important ring current loss process for tens of keV ions and might explain the discrepancy noted by Kistler et al.more » (1989) between model ring current distributions and the ring current fluxes measured by AMPTE/CCE during magnetically active conditions.« less

Calibration of 192Ir high-dose-rate afterloading systems.

Abstract: A method is described for calibration of 192Ir high-dose-rate (HDR) brachytherapy afterloading systems. Since NIST does not offer calibration of ionization chambers with the gamma-ray spectrum of iridium-192, an interpolation procedure is employed, using calibrations above (137Cs, 662 keV) and below (250 kVcp, 146-keV x rays) the exposure-weighted average 192Ir energy of 397 keV. The same total wall + cap thickness must be used for both calibrations, and for the 192Ir measurements. A wall + cap thickness of 0.3 g/cm2 is recommended to assure charged particle equilibrium and to exclude secondary electrons emitted from the source encapsulation. Procedures are described for determining the corrections for source-chamber distance and room scatter during the source calibration in inverse-square-law geometry. A new well-type ionization chamber has been designed specifically for convenient routine use with the HDR afterloading system. It can be calibrated by means of a previously calibrated 192Ir source, and offers a simple means for verifying the decay rate and for calibrating 192Ir replacement sources.

Excitation and ionization of 4He clusters by electrons

Abstract: Clusters are produced by expanding high pressure (P0≤20 bar), low temperature (T0≥5 K) helium gas through a 5 μm nozzle into a vacuum. The neutral beam time‐of‐flight distribution has three peaks which we associate with distinct groups of large and small clusters, and atoms. The beam is ionized by electron impact and the resulting time resolved charged fragment mass distribution reveals in addition to previously observed anomalies (‘‘magic numbers’’) a new strong He+4 signal at high source pressures and low temperatures. The dependence of the various charged and neutral metastable fragment currents on the bombarding electron energy reveals that each has a unique appearance potential. A comparison with the calculated energy required for an electron to create various electronic excitations in the interior of a large cluster indicates that the production and dynamical evolution of metastable 3S1 atomic and a 3Σ+u molecular excitations plays a significant role in the formation of charged fragments from large ...

Electrostatic trapping of contamination particles in a process plasma environment

Abstract: A number of authors have observed, by using light scattering from a laser beam, contamination particles suspended in an rf process plasma. The region of space occupied by the particles appears finite, e.g., a ring; there is experimental evidence that the particles are negatively charged. We show, by using a tuned Langmuir probe, that the trap is electrostatic in nature. The volume of a trap is as much as 5 V larger in electrostatic potential than the surrounding plasma. This means that the volume of the trap is positively charged with the electric field being directed outward from the trap. Thus, negatively charged particles will flow into it. The electrostatic potential rises so rapidly at a trap boundary that a double layer may exist there. Finally, the plasma‐sheath interface is found to follow the topographic contour of the rf electrode surface.

Limits on particles of small electric charge

Abstract: We consider experimental limits on particles of small (not necessarily rational) electric charge. Such particles are possible within the standard model, and may be a natural consequence of extensions of the standard model incorporating an extra U(1) gauge group associated with a "mirror universe" sector. For both these cases we examine the limits from low-energy quantum electro-dynamics corrections, direct accelerator searches, stellar astrophysics, constraints on big-bang nucleosynthesis, and relic cosmological densities. The combined results exclude significant regions of the charge-mass parameter plane.

Multifragment disintegration of the 129Xe+197Au system at E/A=50 MeV.

Abstract: Multifragment disintegrations following $^{129}\mathrm{Xe}$${+}^{197}$Au collisions at E/A=50 MeV have been studied with a multidetector system covering 88% of 4\ensuremath{\pi} in solid angle. The average number of intermediate-mass fragments (Z=3--20) increases strongly as a function of charged-particle multiplicity and reaches values larger than six for the most violent collisions. The results are compared to calculations with both dynamical and statistical models.

Evolution of ion-charge-state distributions in an electron-beam ion trap

Abstract: We present measurements and calculations of the evolution of the ion-charge-state distributions in an electron-beam ion trap. Direct measurements of the charge-state distributions are made with an ion extraction system. The data are used to test our computer model, which solves the set of coupled nonlinear differential equations for the energy and charge balance for all ionic species. The model is used to assess the importance of various processes in establishing the charge-state distribution.

Electromagnetic wave scattering in dusty plasmas

Abstract: The cross section for transition scattering of electromagnetic waves on charged dust particles in a plasma is calculated, extending the results of a previous paper [J. Plasma Phys. 42, 429 (1989)] where the case of longitudinal waves has been considered. For the case of nonlinear screening of the charged dust by the plasma particles (i.e., ‖eφ0/Te‖ ≫ 1, where φ0 is the dust grain surface potential and Te is the electron plasma temperature), numerical and analytical results are presented, showing a significant enhancement, proportional to the square of the grain surface charge, in the cross section with respect to scattering by free electrons. The effect is independent of the sign of the charge for wavelengths larger than the Debye length.

Electron-energy and -angular distributions in the double photoionization of helium.

Abstract: Photoelectron spectra of helium have been measured at different angles and at various energies above the double-ionization threshold up to 120 eV to investigate the behavior of the energy and angular distributions, of shake-off electrons. Both energy and angular distributions clearly show a U-shaped profile which turns to a flat curve near threshold pointing to a uniform intensity distribution over the kinetic energy for all angles in this excitation energy regime. Our results for the angular-distribution asymmetry parameter indicate qualitative agreement with theoretical predictions but fail to provide them quantitatively.

Bipolar Charge Distributions of Aerosol Particles in High-Purity Argon and Nitrogen

Abstract: The bipolar diffusion charging has been studied for monodisperse sodium chloride and silver particles of 5–100 nm in atmospheric air, argon and nitrogen. The particles were bipolarly charged in a neutralizer by ions, produced by beta-rays from a Kr 85 source. The differently charged particle fractions were separated in a differential mobility analyzer and measured with an aerosol electrometer. The experimentally determined results in atmospheeric air are comparable with earlier measured asymmetric bipolar charge distributions. They show good agreement with the theoretically determined results based on the extended Fuchs model with four-input ion parameters: mobilities and masses of positive and negative ions. The experimentally determined bipolar charge distributions in argon and nitrogen are more asymmetric than in atmospheric air. The theoretically determined distributions, based on the extended Fuchs model, can be fitted to the experimental data. Furthermore, the extended Fuchs model is strongly depend...

Observation of laser-induced recombination in merged electron and proton beams.

Abstract: The first observation of laser-induced recombination, performed with merged beams of protons, electrons, and laser photons in an ion storage ring, is reported. The process was used to study the photorecombination spectrum with high resolution. Deviations from earlier theoretical predictions turned out to be due to a weak external electric field and could be well described by a simple extension of the field-free quantum-mechanical theory, based on properties of the classical electron trajectories in the combined external and Coulomb fields.

Resonant acceleration of alpha particles by ion cyclotron waves in the solar wind

Abstract: Preferential acceleration of alpha particles interacting with left-hand polarized ion cyclotron waves is studied. It is shown that a small positive drift velocity between alpha particles and protons can lead to alpha particle velocities well in excess of the proton bulk velocity. During the acceleration process, which is assumed to take place at heliocentric distances less than 0.3 AU, the alpha particle drift velocity should exceed the proton bulk velocity, and then the gap which exists around the alpha particle gyrofrequency should disappear. It is also shown that for proton thermal anisotropies of the order of those observed in fast solar wind, the waves either grow or are not damped excessively, so that the waves can exist and might thus lead to the observed differential speeds. However, the way in which the alpha particles exceed the proton velocity remains unexplained.

Structure and Madelung energy of spherical Coulomb crystals

Abstract: With the help of molecular-dynamics computer simulations, we study the equilibrium configurations of systems of N=2--5000 strongly correlated charged particles under the influence of a radial harmonic external confining force and their mutual Coulomb forces. The temperature is well below the crystallization point; i.e., the ratio of Coulomb to kinetic energy is as large as \ensuremath{\Gamma}=${10}^{9}$. The particles arrange in concentric spherical shells with approximately constant intershell distances. On the surfaces plane hexagonal structures are well pronounced. The calculated radii, occupation numbers, and energies per particle are compared with results of classical geometrical and shell models with homogeneously charged shells corrected for hexagonal surface occupation. The closed-shell particle numbers also agree well with those of multilayer icosahedra. From the computer simulations we extract a Madelung (excess) energy of -0.8926, which is close to the theoretical value of the shell model corrected for plane hexagonal surfaces, -0.8923, but larger than the one of the infinite geometrical lattice, -0.8944, and of the bcc value of -0.8959. Surface-energy effects are positive and of the order of ${\mathit{N}}^{\mathrm{\ensuremath{-}}1/3}$.

Measurements of the Coulomb dissociation cross section of 156 MeV 6Li projectiles at extremely low relative fragment energies of astrophysical interest

Abstract: Coulomb dissociation of light nuclear projectiles in the electric field of heavy target nuclei has been experimentally investigated as an alternative access to radiative capture cross sections at low relative energies of the fragments, which are of astrophysical interest As a pilot experiment the breakup of 156 MeV {sup 6}Li projectiles at {sup 208}Pb with small emission angles of the {alpha} particle and deuteron fragments has been studied Both fragments were coincidentally detected in the focal plane of a magnetic spectrograph at several reaction angles well below the grazing angle and with relative angles between the fragments of 0{degree}--2{degree} The experimental cross sections have been analyzed on the basis of the Coulomb breakup theory The results for the resonant breakup give evidence for the strong dominance of the Coulomb dissociation mechanism and the absence of nuclear distortions, while the cross section for the nonresonant breakup follows theoretical predictions of the astrophysical {ital S} factor and extrapolations of corresponding radiative capture reaction cross section to very low cm energies of the {alpha} particle and deuteron Various implications of the approach are discussed

Monte Carlo‐fluid hybrid model of the accumulation of dust particles at sheath edges in radio‐frequency discharges

Abstract: Particulate contamination (dust) has been observed to accumulate near the sheath‐plasma boundary in both radio‐frequency (rf) and direct‐current (dc) discharges. We have developed and applied a hybrid Monte Carlo‐fluid simulation of electron, ion, and charged dust transport in rf discharges to investigate the dynamics of particulate contamination. The processes governing the transport of charged dust in the model are drift of partially shielded particles in the electric field, collisions with the fill gases, and viscous ion drag arising from Coulomb interactions of particles with ions drifting and diffusing in the plasma. We find that negatively charged dust particles accumulate near the sheath‐plasma boundary, and that transport of the particles is dominated by ion drag.

Modeling of low-pressure microwave discharges in Ar, He, and O/sub 2/: similarity laws for the maintenance field and mean power transfer

Abstract: A kinetic study of low-pressure microwave discharges in Ar, He, and O/sub 2/ is carried out using electron-transport parameters and rate coefficients derived from solutions to the Boltzmann equation, together with the continuity and transport equations for the charged particles, taking into account stepwise-ionization processes. The Boltzmann equation is solved over a wide range of the applied frequency, omega /2 pi , but assuming that the angular frequency omega > tau /sub e//sup -1/, with tau /sub e/, denoting the characteristic time for electron-energy relaxation by collisions. The formulation provides discharge characteristics for the maintenance field and for mean absorbed power per electron in the three gases, which are shown to agree satisfactorily with experimental data obtained from surface-wave discharges. It is shown that such an agreement would not always be obtained without consideration of the role played by stepwise-ionization processes in sustaining the discharge. >

Fragmentation of gold projectiles: From evaporation to total disassembly

Abstract: We have studied the fragmentation of Au projectiles interacting with targets of C, Al and Cu at an incident energy ofE/A=600 MeV. The employed inverse kinematics allowed a nearly complete detection of projectile fragments with chargeZ≧2. The recorded fragmentation events were sorted according to three observables, the multiplicityMlp of light charged particles, the largest atomic numberZmax within an event, and a new observable,Zbound, representing the sum of the atomic numbersZ of all fragments withZ≧2. Using these observables, the impact parameter dependence of the fragmentation process was investigated. For all three targets, a maximum mean multiplicity of 3 to 4 intermediate mass fragments (IMFs) is observed. The corresponding impact parameters range from central collisions for theC target to increasingly peripheral collisions for the heavier targets. It is found that the correlation between the IMF multiplicity andZbound, extending from evaporation type processes (largeZbound) to the total disassembly of the projectile (smallZbound), is independent of the target nucleus. This universal behaviour may suggest an — at least partial — equilibration of the projectile fragment prior to its decay.