Showing papers on "Charged particle published in 1996"

Global models of pulse-power-modulated high-density, low-pressure discharges

Abstract: Global (volume-averaged) models of high-density, low-pressure electropositive and electronegative discharges are described both for continuous wave (CW) and for pulsed-power excitation. Argon and chlorine discharges are treated. The particle and energy balance equations are applied to determine the charged particle and neutral dynamics. For argon just after the power has been turned on, the analysis shows an initial very sharp rise in electron temperature , followed by a decay of and an increase in the electron density to steady state values during the pulse-on time. Just after the power has been turned off, decays rapidly and decays more slowly. The time-average can be considerably higher than that for CW discharges for the same time-average power. For chlorine, a CW discharge is highly dissociated and the negative ion density is lower than . For a pulsed discharge, the initial rise and subsequent decay of just after the power has been turned off are determined analytically. A pulsed discharge can have the same neutral radical (Cl) flux to the walls for a reduced average power. The analytical models are compared to more complete global model simulations and to experimental observations. We find that global models can provide considerable insight into the discharge dynamics.

High energy electrodynamics in matter

Abstract: This study presents an interpretation of classical and quantum theories of fast charged particle radiation in an external field. Various methods describing the process of particle interaction with substance and external fields are analyzed, such as Born approximation, the Schwinger operator method and different versions of the quasi-classical approximation. Such problems as time evolution of an electromagnetic field applied to and emitting electrons, coherence length, radiation in the simplest external fields, Vavilov-Cherenkov radiation and the correspondence of different methods describing the interaction between fast particles and matter are also considered.

Photoelectron imaging spectrometry: Principle and inversion method

Abstract: A new photoelectron spectrometer has recently been used to analyze the energy and spatial distribution of photoelectrons produced by multiphoton ionization of rare gases. It is based on the analysis of the image obtained by projecting the expanding electron cloud resulting from the ionization process onto a two‐dimensional position sensitive detector by means of a static electric field. In this article, we present the principle of this imaging spectrometer and the relevant equations of motion of the charged particle in this device, together with an inversion method that allows us to obtain the energy and angular distribution of the electrons. We present here the inversion procedure relevant to the case where the electrostatic energy acquired in the static field is large as compared to the initial kinetic energy of the charged particles. A more general procedure relevant to any regime will be described in a following article.

Adsorption of Charged Latex Particles on Mica Studied by Atomic Force Microscopy

Abstract: Atomic force microscopy (AFM) is used to study the localized adsorption of charged latex particles onto a mica substrate with varying adsorption times and solution ionic strengths. Images are obtained by employing tapping mode AFM with silane-treated probes to reduce the intrusive effect of the imaging process on physisorbed particles. The initial kinetics of the adsorption process are found to be diffusion-limited and the long-time asymptotic kinetics are found to resemble those of a random sequential adsorption process. Double-layer screening of interparticle electrostatic repulsions is shown to have a controlling effect on the surface coverage at saturation and, to a lesser degree, on the diffusion-limited rate of adsorption.In situimages of adsorbed layers are compared with those of dried samples to reveal the morphological effects of film evaporation on the microstructure of adsorbed particles. Results show that particles in adsorbed layers of high surface coverage can rearrange into two-dimensional clusters during film evaporation—a phenomenon attributed to capillary forces between particles in near contact. Ordering of particles at the solid–liquid interface is analyzed using two-dimensional radial distribution functions. A high degree of short-range order is found among adsorbed particles when the surface coverage approaches saturation, and a significant reduction in the range of ordering due to double-layer screening of interparticle repulsions is also reflected in the radial distributions.

Inactivation of V79 cells by low-energy protons, deuterons and helium-3 ions

Abstract: Previous work by ourselves and by others has demonstrated that protons with a linear energy transfer (LET) about 30 keVmum(-1)are more effective at killing cells than doubly charged particles of the same LET. In this work we show that by using deuterons, which have about twice the range of protons with the same LET, it is possible to extend measurements of the RBE of singly charged particles to higher LET (up to 50 keVmum(-1). We report the design and use of a new arrangement for irradiating V79 mammalian cells. Cell survival measurements have been made using protons in the energy range 1.0-3.7 MeV, deuterons in the energy range 0.9-3.4MeV and 3He2+ ions in the energy range 3.4-6.9 MeV. This corresponds to volume-averaged LET (within the cell nucleus) between 10 and 28 keVmum(-1) for protons, 18-50 keVmum(-1) for deuterons, and 59-106 keVmum(-1) for helium ions. Our results show no difference in the effectiveness of protons and deuterons matched for LET. However, for LET above about 30 keVmum(-1) singly charged particles are more effective at inactivating cells than doubly-charged particles of the same LET and that this difference can be understood in terms of the radial dose distribution around the primary ion track.

Shock waves in plasmas containing variable‐charge impurities

Abstract: Shock structures in plasmas containing variable‐charge macro particles are shown to exist because of an effective dissipation associated with charging of the latter.

Ion‐dust streaming instability in processing plasmas

Abstract: An ion‐dust streaming instability is investigated for conditions representative of plasma sheath interface regions where dust grains are observed to be present in processing plasmas. Kinetic theory is used, and the effects of collisions of charged particles with neutrals is retained. It is found that an instability with growth rate of the order of the dust plasma frequency may be driven by the drift of ions relative to charged dust. Possible implication for plasma etching is briefly discussed.

Pulse-time modulated plasma discharge for highly selective, highly anisotropic and charge-free etching

Abstract: Highly selective, highly anisotropic, notch-free and charge-build-up damage-free polycrystalline silicon etching is performed by using electron cyclotron resonance plasma modulated at a pulse timing of a few tens of microseconds. A large quantity of negative ions is produced in the afterglow of the pulse-time modulated plasma. The decay times of electron density, electron temperature and sheath potential are considerably reduced, which is attributable to negative ion generation. Furthermore, the pulse-time modulated plasma reduces the time-averaged sheath potential. As a result of these effects, charged particles in the sheath are strongly modified from the continuous discharge, and they should improve selective etching in the pulsed ECR plasma and elimination of charge accumulation on the substrate.

Pulse-time-modulated ECR plasma discharge for highly selective, highly anisotropic and charge-free etching

Abstract: Highly selective, highly anisotropic, notch-free, and charge-build-up damage-free silicon etching is performed by using Electron Cyclotron Resonance (ECR) Cl 2 plasma modulated at a pulse timing of a few tens of microseconds. A large quantity of negative ions are produced in the afterglow of the pulse-time-modulated plasma. The decay times of electron density, electron temperature, and sheath potential are considerably reduced, which is attributable to negative-ion generation. Furthermore, the pulse-time-modulated plasma reduces the time averaged sheath potential. As a result of these effects, charged particles in the sheath are strongly modified from the continuous discharge, and they should improve the selective etching in the pulsed ECR plasma and eliminate charge accumulation on the substrate.

Method for sweeping charged particles out of an isochronous cyclotron, and device therefor

Abstract: A method for sweeping a charged particle beam out of an isochronous cyclotron (1) comprising a solenoid forming a magnetic circuit that includes at least a number of sectors (3, 3') known as 'ridges' where the air-gap is reduced, and separated by sector-shaped spaces (4) known as 'valleys' where the air-gap is larger. According to the sweeping method, the particle beam is swept without using a sweeping device as the magnetic field has a special arrangement produced by designing the solenoid air-gap at the ridges (3, 3') of the isochronous cyclotron in such a way that the aspect ratio between the solenoid air-gap at the ridges in the region of the maximum radius, and the radius gain per turn of the particles accelerated by the cyclotron at said radius is less than 20.

Central Collisions of Au on Au at 150, 250 and 400 A MeV

Abstract: Collisions of Au on Au at incident energies of 150, 250 and 400 A MeV were studied with the FOPI-facility at GSI Darmstadt. Nuclear charge (Z < 16) and velocity of the products were detected with full azimuthal acceptance at laboratory angles of 1-30 degrees. Isotope separated light charged particles were measured with movable multiple telescopes in an angular range of 6-90 degrees. Central collisions representing about 1 % of the reaction cross section were selected by requiring high total transverse energy, but vanishing sideflow. The velocity space distributions and yields of the emitted fragments are reported. The data are analysed in terms of a thermal model including radial flow. A comparison with predictions of the Quantum Molecular Model is presented.

Momentum transfer theory of nonconservative charged particle transport in mixtures of gases: General equations and negative differential conductivity.

Abstract: In this paper we develop general equations for the momentum transfer theory (MTT) for swarms of particles in mixtures of gases including the effect of nonconservative (reactive) collisions. MTT equations for mean energy, drift velocity, diffusion tensor components, and rate coefficients are derived for different degrees of approximation including the hydrodynamic limit and small swarm to gas particle mass ratio. Specific formulas were developed for the criteria for negative differential conductivity (NDC) in mixtures of gases with elastic collisions only and for a single gas with reactions. The criteria and the numerical calculations are in excellent agreement, showing that NDC can be induced by light atomic constituents in purely elastic collisions and also by ionization and attachment.

Stochastic charge fluctuations in dusty plasmas

Abstract: We consider the stochastic properties of charge fluctuations of noninteracting particles surrounded by a stationary, undisturbed plasma of electrons and singly charged positive ions. For unscreened Coulomb interaction in the orbit–motion limit the mean particle charge and the variance of the fluctuations are proportional to the particle size. The result applies to both Maxwellian and non‐Maxwellian electrons as well as to charging by ion winds. When the polarizability of the particle is included in the electrostatic interaction, both the ion and electron currents to the particle increase and the mean particle charge becomes more negative. Except for particles in the low nanometer range, the effect of the polarizability can be neglected at the electron temperatures encountered in typical plasmas.

Circular accelerator, method of injection of charged particle thereof, and apparatus for injection of charged particle thereof

Abstract: The present invention is to provide a method and an apparatus which are able to inject large electric current to a circular accelerator. In order to inject large electric current, that is, a large number of charged particles, a means is provided for injecting a beam into other region of a vacuum duct than the region which is defined as having a height equivalent to the height of the injected beam and a width from the injected point in the vacuum duct to the symmetrical point to the injected point with respect to the geometrical center of the vacuum duct.

Charged-Particle Motion in Electromagnetic Fields Having at Least One Ignorable Spatial Coordinate.

Abstract: We give a rigorous derivation of a theorem showing that charged particles in an arbitrary electromagnetic field with at least one ignorable spatial coordinate remain forever tied to a given magnetic field line. Such a situation contrasts with the significant motions normal to the magnetic field that are expected in most real three-dimensional systems. It is pointed out that while the significance of the theorem has not been widely appreciated, it has important consequences for a number of problems and is of particular relevance for the acceleration of cosmic rays by shocks.

Electrophoresis of a colloidal sphere in a circular cylindrical pore

Abstract: The electrophoretic motion of a dielectric sphere along the centerline of a long circular cylindrical pore is studied theoretically. The imposed electric field is constant and parallel to the nonconducting pore wall, and the particle and wall surfaces are assumed uniformly charged. Electrical double layers adjacent to solid surfaces are assumed to be thinner than particle radius and gap width between surfaces. The presence of the pore wall affects particle velocity : 1. an electroosmotic flow of the suspending fluid exists due to interaction between the electric field and the charged wall ; 2. the local electric field on the particle surface is enhanced by the insulated wall, speeding up the particle ; and 3. the wall increases viscous retardation of the moving particle. To solve electrostatic and hydrodynamic governing equations, general solutions are constructed from fundamental solutions in both cylindrical and spherical coordinate systems. Boundary conditions are enforced at the pore wall by Fourier transforms and then on the particle surface by a collocation technique. Typical electric-field-line, equipotential-line and streamline patterns for the fluid phase are exhibited, and corrections to the Smoluchowski equation for particle electrophoretic velocity are presented for various relative separation distances between the particle and wall. The presence of the pore wall always reduces the electrophoretic velocity ; however, the net wall effect is quite weak, even for very small gap width between the particle and wall.

Ion‐ion correlations and effective charges in electrolyte and macroion systems

Abstract: Various ion-ion correlation mechanisms that affect double layer interactions between charged macroparticles in electrolyte solutions are reviewed and discussed. Such effects give rise to quantitative and sometimes qualitative deviation from the prediction of the Poisson-Boltzmann theory. Special attention is given to the attractive double layer interactions that can appear between equally charged surfaces at short surface separations for aqueous systems with divalent counterions. The long-range behaviour of the double layer interaction is also treated. It is efficiently described in terms of effective surface charge densities, which summarizes how the inner parts of the diffuse double layer affects the distant parts. The magnitude as well as the sign of the effective charge depends on the state of the system. Charge reversal can occur for aqueous systems with divalent counterions. A major objective of the paper is to describe and illustrate the mechanisms in a simple and graphical manner.

Electron Cooling of Protons in a Nested Penning Trap.

Abstract: Trapped protons cool via collisions with trapped electrons at 4 K.This first demonstration of sympathetic cooling by trapped species of opposite sign of charge utilizes a nested Penning trap. The demonstrated interaction of electrons and protons at very low relative velocities, where recombination is predicted to be most rapid, indicates that this may be a route towards the study of low temperature recombination. The production of cold antihydrogen is of particular interest, and electron cooling of highly stripped ions may also be possible. {copyright} {ital 1996 The American Physical Society.}

Plasma-assisted deposition at atmospheric pressure

Abstract: Most plasma-assisted deposition methods currently available use gas discharges at pressures below 1 hPa. In many cases, the process temperature can be kept low due to the fact that the energy necessary for the initiation of chemical reactions is transferred via charged particles. However, a low pressure requires a large amount of vacuum equipment. Processes at atmospheric pressure are more favourable if results similar to those of existing methods can be achieved. Barrier discharges provide the basis for a new plasma-assisted deposition method at atmospheric pressure. These discharges consist of a large number of transient microdischarges in parallel which are distributed statistically on the surface to be coated. Starting with some basic considerations on the properties of microdischarges, the deposition of thin polymeric films on glass surfaces is described using barrier discharges at atmospheric pressure and acetylene. Uniform polymeric films up to 1 μm are obtained if trains of voltage pulses are used. The parameters influencing the deposition rate and the film quality are discussed. In addition, it is estimated whether further improvements of the deposition process are possible.

Measurements of electron temperature, electron density, and neutral density in a radio‐frequency inductively coupled plasma

Abstract: Electron temperature, electron density, and neutral atom density were measured in a radio‐frequency (rf) inductively coupled plasma using Thomson and Rayleigh scattering of laser radiation. Measurements were made in an argon discharge for pressures from 1 to 20 mTorr and input rf powers from 100 to 500 W. Spatial distribution profiles were measured for discharges with different aspect ratios. Electron temperature was found to depend on pressure but only weakly on power. Electron density depended strongly on both pressure and power. The neutral density was found to be significantly depleted in the plasma center and this depletion was attributed to heating of the neutrals by charged particle collisions. These results were compared to a simple model of inductively coupled plasmas.

Isospin dependence of intermediate mass fragment production in heavy-ion collisions at E/A=55 MeV.

Abstract: The production of intermediate mass fragments (IMF's) from the four reactions 55A MeV $^{124,136}\mathrm{Xe}$ + $^{112,124}\mathrm{Sn}$ is studied with an experimental apparatus which is highly efficient for the detection of both charged particles and neutrons. The IMF's are more localized in the midvelocity region than are the light charged particles, and the detected multiplicity of IMF's depends linearly on the charge lost from the projectile and increases with the neutron excess of the system. Remnants of the projectile, with very little velocity reduction, are found for most of the reaction cross section. Isotopic and isobaric fragment yields in the projectile-velocity region indicate that charge-to-mass ratio neutralization is generally not achieved but is approached when little remains of the projectile. For all systems, the fragments found in the midvelocity region are substantially more neutron rich than those found in the velocity region dominated by the emission from the projectile. This observation can be accounted for if the midvelocity source (or sources) is either more neutron rich or smaller, with the same neutron-to-proton ratio, than the source with the velocity of the projectile. Taken together, the observations of this work suggest that the intermediate mass fragments are, to a large extent, formed by multiple neck rupture of the overlap material, a process which might enhance the neutron-to-proton ratio of the primary source material and/or limit the size of the sources. This scenario is reminiscent of low-energy ternary fission and one predicted by Boltzmann-Uehling-Uhlenbeck calculations. However, these calculations predict too much velocity damping of the projectile remnant. The calculations improve, in this regard, when the in-medium nucleon-nucleon cross sections and the cost of creating low density material are reduced. \textcopyright{} 1996 The American Physical Society.

4/3 problem in classical electrodynamics

Abstract: We evaluate the self-force acting on an extended nonrelativistic charged particle, in the framework of classical electrodynamics with a cutoff at short distances. We show that in the regularized Maxwell theory, the point particle limit is finite and well defined. As a result, the electromagnetic mass of a point particle enters the equation of motion in a form consistent with the special theory of relativity. \textcopyright{} 1996 The American Physical Society.