Showing papers on "Charged particle published in 1999"

SIMNRA, a simulation program for the analysis of NRA, RBS and ERDA

Abstract: SIMNRA is a Microsoft Windows 95/Windows NT program with fully graphical user interface for the simulation of non-Rutherford backscattering, nuclear reaction analysis and elastic recoil detection analysis with MeV ions. About 300 different non-Rutherford and nuclear reactions cross-sections are included. SIMNRA can calculate any ion-target combination including incident heavy ions and any geometry including transmission geometry. Arbitrary multi-layered foils in front of the detector can be used. Energy loss straggling includes the corrections by Chu to Bohr’s straggling theory, propagation of straggling in thick layers, geometrical straggling and straggling due to multiple small angle scattering. The effects of plural large angle scattering can be calculated approximately. Typical computing times are in the range of several seconds.

Creation of an Ultracold Neutral Plasma

Abstract: The study of ionized gases in neutral plasma physics spans temperatures ranging from 10 16 K in the magnetosphere of a pulsar to 300 K in the earth’s ionosphere [1]. At lower temperatures, the properties of plasmas are expected to differ significantly. For instance, three-body recombination, which is prevalent in high temperature plasmas, should be suppressed [2]. If the thermal energy of the particles is less than the Coulomb interaction energy, the plasma becomes strongly coupled, and the usual hydrodynamic equations of motion and collective mode dispersion relations are no longer valid [3]. Strongly coupled plasmas are difficult to produce in the laboratory and only a handful of examples exist [4], but such plasmas do occur naturally in astrophysical systems. In this work, we create an ultracold neutral plasma with an electron temperature as low as Te 100 mK, an ion temperature as low as Ti 10 mK, and densities as high as n 2 3 10 9 cm 23 . We obtain this novel plasma by photoionization of laser-cooled xenon atoms. Within the experimentally accessible ranges of temperatures and densities, both components can be simultaneously strongly coupled. A simple model describes the evolution of the plasma in terms of the competition between the kinetic energy of the electrons and the Coulomb attraction between electrons and ions. A numerical calculation accurately reproduces the data. Photoionization and laser cooling have been used before in plasma experiments. Photoionization in a 600 K Cs vapor cell produced a plasma with Te $ 2000 K [5], and a strongly coupled non-neutral plasma was created by laser cooling magnetically trapped Be 1 ions [6]. A plasma is often defined as an ionized gas in which the charged particles exhibit collective effects [7]. The length scale which divides individual particle behavior and collective behavior is the Debye screening length lD. It is the distance over which an electric field is screened by redistribution of electrons in the plasma, and is given by lD p

Collisional Breakup in a Quantum System of Three Charged Particles

Abstract: Since the invention of quantum mechanics, even the simplest example of the collisional breakup of a system of charged particles, e(-) + H --> H(+) + e(-) + e(-) (where e(-) is an electron and H is hydrogen), has resisted solution and is now one of the last unsolved fundamental problems in atomic physics. A complete solution requires calculation of the energies and directions for a final state in which all three particles are moving away from each other. Even with supercomputers, the correct mathematical description of this state has proved difficult to apply. A framework for solving ionization problems in many areas of chemistry and physics is finally provided by a mathematical transformation of the Schrodinger equation that makes the final state tractable, providing the key to a numerical solution of this problem that reveals its full dynamics.

Gold nanoparticles : Production, reshaping, and thermal charging

Abstract: Gold nanoparticles are of great interest for various nanoelectronic applications, e.g., for making single electron transistors or very fine leads to molecular size entities. For this and other applications, it is important that all particles have controllable size and shape. In this paper, we describe the production of size-selected gold aerosol particles in the 20 nm range made by evaporation in a high-temperature tube furnace and subsequent size selection. To obtain spherical particles, it was necessary to reshape the particles at high temperature, which was investigated for temperatures between 25°C and 1200°C. High-resolution transmission electron microscopy showed that the degree of crystallinity became higher for higher reshaping temperature. During reshaping at high temperature, an anomalous charging behavior was discovered, whereby negatively as well as positively charged particles became multiply negatively charged. Possible mechanisms for explaining this thermally activated phenomenon are discussed.

Beyond Poisson-Boltzmann: Fluctuations and Correlations

Abstract: We formulate the non-linear field theory for a fluctuating counter-ion distribution in the presence of a fixed, arbitrary charge distribution. The Poisson-Boltzmann equation is obtained as the saddle-point, and the effects of fluctuations and correlations are included by a loop-wise expansion around this saddle point. We show that the Poisson equation is obeyed at each order in the loop expansion and explicitly give the expansion of the Gibbs potential up to two loops. We then apply our formalism to the case of an impenetrable, charged wall, and obtain the fluctuation corrections to the electrostatic potential and counter-ion density to one-loop order without further approximations. The relative importance of fluctuation corrections is controlled by a single parameter, which is proportional to the cube of the counter-ion valency and to the surface charge density. We also calculate effective interactions between charged particles, which reflect counter-ion correlation effects.

Transition from Attractive to Repulsive Forces between Dust Molecules in a Plasma Sheath

Abstract: The electrostatic interaction of a system of two single dust particles in a plasma-sheath environment with flowing ions has been investigated quantitatively It is shown that attractive net ``binding'' forces between the negatively charged particles exist, leading to the formation of a dust molecule By laser manipulation of the dust particles, it is demonstrated that the attraction is asymmetric in such a way that it acts only on one of the particles Moreover, the net forces between the particles can be reversibly changed between attraction and repulsion

Modeling of particulate coagulation in low pressure plasmas.

Abstract: In this paper we study the growth of nanometer particles in low pressure plasmas due to coagulation. We describe results of a model which involves the self-consistent determination of plasma properties, the description of particle charging, as well as the description of the particle size distribution via solution of the general dynamic equation for an aerosol. Our results show that particle coagulation in the low pressure plasma is enhanced compared to coagulation in neutral aerosols due to the attraction of oppositely charged particles. The temporal behavior of the coagulation follows the same laws as coagulation of neutral particles as long as the density of nanometer particles is larger than the positive ion density in the plasma. The positive ion density can be considered as the critical density for coagulation to occur. We also show that the details of the particle charging mechanism are only of minor importance for the coagulation dynamics but of great importance for the accurate prediction of plasma parameters.

Fundamentals of Radiation Chemistry

Abstract: Introduction. Interaction of Radiation with Matter: Energy Transfer from Fast Charged Particles. Structure of Charged Particle Tracks in Condensed Media. Ionization and Excitation Phenomena. Radiation Chemistry of Gases. The Solvated Electron. Spur Theory of Radiation Chemical Yields: Diffusion and Stochastic Models. Electron Thermalization and Related Phenomena. Electron Escape: The Free-Ion Yield. Electron Mobility in Liquid Hydrocarbons. Radiation Chemical Applications in Science and Industry.

Electrostatic Repulsion of Positively Charged Vesicles and Negatively Charged Objects

Abstract: A positively charged, mixed bilayer vesicle in the presence of negatively charged surfaces (for example, colloidal particles) can spontaneously partition into an adhesion zone of definite area and another zone that repels additional negative objects. Although the membrane itself has nonnegative charge in the repulsive zone, negative counterions on the interior of the vesicle spontaneously aggregate there and present a net negative charge to the exterior. Beyond the fundamental result that oppositely charged objects can repel, this mechanism helps to explain recent experiments on surfactant vesicles.

Must: a silicon strip detector array for radioactive beam experiments

Abstract: A new and innovative array, MUST, based on silicon strip technology and dedicated to the study of reactions induced by radioactive beams on light particles is described. The detector consists of 8 silicon strip – Si(Li) telescopes used to identify recoiling light charged particles through time of flight, energy loss and energy measurements and to determine precisely their scattering angle through X, Y position measurements. Each 60×60 mm 2 double sided silicon strip detector with 60 vertical and 60 horizontal strips yields an X – Y position resolution of 1 mm, an energy resolution of 50 keV, a time resolution of around 1 ns and a 500 keV energy threshold for protons. The backing Si(Li) detectors stop protons up to 25 MeV with a resolution of approximately 50 keV. CsI crystals read out by photo-diodes which stop protons up to 70 MeV are added to the telescopes for applications where higher energy particles need to be detected. The dedicated electronics in VXIbus standard allow us to house the 968 logic and analog channels of the array in one crate placed adjacent to the reaction chamber and fully remote controlled, including pulse visualization on oscilloscopes. A stand alone data acquisition system devoted to the MUST array has been developed. Isotope identification of light charged particles over the full energy range has been achieved, and the capability of the system to measure angular distributions of states populated in inverse kinematics reactions has been demonstrated.

The discharge current induced by the motion of charged particles in time-dependent electric fields; Sato's equation extended

Abstract: The formula derived by Nobuyasu Sato (1980 J. Phys. D: Appl. Phys. 13 L3-L6), for the current flowing in an external circuit due to the motion of charged particles in a gap is extended to include a time-dependent applied voltage. The effects of negative ions, and electron, positive ion and negative ion diffusion are also included. It is found that Sato's original equation still describes the contribution of charged particle motion to the external circuit current. The total circuit current is determined by adding the displacement current that effectively flows across the gap to the current determined by Sato's equation. Some comments are also made about the interpretation of Sato's equation in three dimensions.

Debye-Hückel theory for interfacial geometries.

Abstract: The Debye-H\"uckel theory for bulk electrolyte solutions is generalized to planar interfacial geometries, including screening effects due to mobile salt ions which are confined to the interface and solutions with in general different salt concentrations and dielectric constants on the two sides of the interface. We calculate the general Debye-H\"uckel interaction between fixed test charges, and analyze a number of relevant special cases as applicable to charged colloids and charged polymers. Salty interfaces, which are experimentally realized by monolayers or bilayers made of cationic and anionic surfactants or lipids, exert a strong attraction on charged particles of either sign at large separations from the interface; at short distances image-charge repulsion sets in. Likewise, the effective interactions between charged particles are strongly modified in the neighborhood of such a salty interface. On the other hand, charged particles which are immersed in a salt solution are repelled from the air (or a substrate) interface, and the interaction between two charges decays algebraically close to such an interface. These general results have experimentally measurable consequences for the adsorption of charged colloids or charged polymers at monolayers, solid substrates, and interfaces.

Field theory for charged fluids and colloids

Abstract: Charged systems are considered using field theoretic methods which include fluctuations and correlations via a systematic expansion in powers of the local electrostatic potential. At the Gaussian level we recover the classical Debye-Huckel (DH) theory, which is found to exhibit the full hierarchy of multibody correlations determined by pair-distribution functions given by the screened DH potential. Higher-order corrections can lead to attractive pair interactions between similarly charged particles in asymmetric ionic environments. The phase diagram for an ionic fluid shows a critical point of demixing. The critical temperature and critical density exhibit pronounced deviations from DH theory for highly asymmetric ionic systems.

Dynamic Field Measurements of Submicron Particles from Diesel Engines

Abstract: The authors present new tools and a complete setup with which the submicron particles, emitted from combustion processes, can be measured and classified according to their chemical nature even in the field. Diesel engines are the predominant source of these particles in the industrialized countries. The setup comprises an exhaust conditioning part including means for precipitation of the coarse particle fraction, dilution, and removal of volatile material. The submicron particles are detected with three different sensors: one for particle number concentration (condensation particle counter, CPC), one for the particle Fuchs-surface, and one that responds specifically to the carbonaceous particles. These sensors possess a short response time so that transients such as those occurring in the free acceleration mode may be observed as well. Experiments on a conventional engine test bench demonstrate the tools. The test results are highly reassuring. Correct handling of the volatile fraction is essential to avoid severe artifacts from gas to particle reactions particularly in exhaust from engines equipped with particle traps.

Relativistic charged spheres: II. Regularity and stability

Abstract: We present new results concerning the existence of static, electrically charged, perfect fluid spheres that have a regular interior and are arbitrarily close to a maximally charged black-hole state. These configurations are described by exact solutions of Einstein's field equations. A family of these solutions had already been found (de Felice et al 1995 Mon. Not. R. Astron. Soc. 277 L17) but here we generalize that result to cases with different charge distributions within the spheres and show, in an appropriate parameter space, that the set of such physically reasonable solutions has a non-zero measure. We also perform a perturbation analysis and identify the solutions which are stable against adiabatic radial perturbations. We then suggest that the stable configurations can be considered as classic models of charged particles. Finally, our results are used to show that a conjecture of Kristiansson et al (1998 Gen. Rel. Grav. 30 275) is incorrect.

Photon emission by an ultra-relativistic particle channeling in a periodically bent crystal

Abstract: This paper is devoted to a detailed analysis of the new type of undulator radiation generated by an ultra-relativistic charged particle channeling along a crystal plane, which is periodically bent by a transverse acoustic wave, as well as to the conditions limiting the observation of this phenomenon. This mechanism makes feasible the generation of electromagnetic radiation, both spontaneous and stimulated, emitted in a wide range of the photon energies, from X- up to γ-rays.

Three-dimensional calculations for a 4 kA, 3.5 MV, 2 microsecond injector with asymmetric power feed

Abstract: The DARHT-2 accelerator under construction at Los Alamos National Laboratory requires a long flattop ($2\ensuremath{\mu}\mathrm{s}$) 2--4 kA, 3.5 MV, low-emittance electron beam source. The injector is being constructed at Lawrence Berkeley National Laboratory and consists of a large-area thermionic cathode mounted atop a vertical column. The 90\ifmmode^\circ\else\textdegree\fi{} bend between the horizontally emitted beam and the column produces dipole and higher-pole fields which must be corrected. In addition, the fast rise of the current flowing into the vacuum tank excites rf modes which cause transverse oscillations of the beam centroid. We have modeled these effects with the 3D electromagnetic code LSP. The code has models for pulsed power transmission lines, space-charge-limited emission and transport of charged particles, externally applied magnetic fields, and frequency-dependent absorption of rf. We calculate the transverse displacement of the beam as a function of time during the current pulse, and the positioning and thickness of ferrite absorber needed to damp the rf modes. The numerical results are compared to analytic calculations.

Stability of alpha particle driven Alfvén eigenmodes in high performance JET DT plasmas

Abstract: The stability of alpha particle driven Alfven eigenmodes (AEs) is analysed in high fusion power DT discharges on JET. Both hot ion H mode and shear optimized discharges are considered. Unstable AEs are not observed in hot ion H mode DT discharges even at the highest fusion power with alpha particle beta βα (0) ≈ 0.7%. Theoretical analysis shows that the AE stabilization is caused by the large plasma pressure, which prevents the existence of core localized AEs at peak fusion performance. Kinetic toroidal AEs (KTAEs), which persist at high plasma pressure, are found to be radially extended and subject to strong damping. The stability analysis based on the CASTOR-K code confirms that AEs cannot be driven unstable by alpha particles in high performance hot ion H mode discharges performed at JET. Alfven eigenmodes in shear optimized regimes are more unstable than those in the hot ion H mode mainly due to the elevated central safety factor q, which increases the efficiency of AE interaction with energetic ions. As a consequence, AEs are observed in shear optimized DT discharges when ion cyclotron heating as low as 1 MW is applied.

Role of Ar2+ and Ar2+ ions in a direct current argon glow discharge: A numerical description

Abstract: A two-dimensional model has been developed for calculating the behavior of Ar2+ and Ar2+ ions in a direct current argon glow discharge, by the use of balance equations describing the various production and loss processes for these species, as well as their transport by diffusion and migration. These balance equations are coupled to the equations for the Ar+ ions and electrons and solved simultaneously with Poisson’s equation, to obtain a self-consistent description of the charged particles behavior and the electrical characteristics in the glow discharge. Moreover, this model is combined with the other models that we have developed previously for the Ar atoms in various excited levels and the Cu atomic and ionic species, to obtain an overall description of the direct current argon glow discharge. The model is applied to typical conditions used for glow discharge mass spectrometry (pressure of 50–100 Pa, voltage of 600–1400 V, and current of 0.4–15 mA). Typical calculation results include the densities and...

Populating of the cusp and boundary layers by energetic (hundreds of keV) equatorial particles

Abstract: We examine the dynamics of charged particles in the frontside magnetosphere We show that the existence of a magnetic field minimum in the outer cusp region has significant implications for the large-scale transport of energetic (a few hundreds of keV) particles that are trapped near the equator Upon approach (within a few Earth radii) of the magnetopause in the day side sector, these particles are subjected to a mirror force pointing away from the equator and may escape toward high latitudes We demonstrate that via this process, energetic electrons and ions from the outer radiation belts and ring current may leak into the outer cusp and be scattered back into the nightside plasma sheet During transport, the particle behavior critically depends upon gradient drift timescale as compared with convection timescale We show that by diverting energetic equatorial particles toward the frontside magnetopause, dynamical reconfiguration of the magnetospheric field lines during substorms may favor such injections toward high latitudes and loading of the outer cusp At a given energy, equatorial particles with higher charge state (ie, larger drift timescale) reside longer in the dayside magnetosphere and are thus more susceptible to transport toward the field minimum

Numerical modeling of the constriction of the dc positive column in rare gases

Abstract: The constriction of the positive column of a dc glow discharge in argon at high pressures is analyzed using the continuity equations for the charged particles and the gas thermal balance equation coupled with the local electron Boltzmann equation and a detailed collisional-radiative model for the atomic and ionic species. Contrary to the other existing models of the constriction in inert gas, the present model is self-consistent and fully detailed, and provides a quantitative description of all the discharge properties. The numerical techniques used to solve the boundary value problem corresponding to our set of equations are discussed in detail. The transition from the diffuse to the constricted state and the properties of this latter state are investigated. The model predicts the existence of multimodal solutions for the discharge parameters as a function of the discharge specific power, within a limited range of values of the latter above a critical value, which explains the observed abrupt changes in the discharge parameters and the hysteresis associated with constriction. The radial distributions of the gas temperature and of the densities of all neutral and charged species considered are determined along with various other discharge characteristics, such as the steady-state discharge maintenance electric field, as a function of the discharge operating parameters. The results for argon show satisfactory agreement with data from experiments. A few model simulations are further presented that enable one to gain physical insight on the relevant kinetic processes of constriction in argon. Such simulations are instrumental to understanding also the mechanisms of constriction in the other inert gases.

The possible role of organics in the formation and evolution of ultrafine aircraft particles

Abstract: We investigate the potential contribution of organic species emitted by aircraft engines to the formation of ultrafine volatile particles detected in jet wakes The model employed here includes both acidic and organic aerosols—in their “pure” states and mixed with soot—as well as small ion clusters and electrically charged particles The microphysical treatment accounts for the effects of charge, within the framework of a multicomponent size-resolved kinetically controlled vapor-aerosol system Owing to their high proton affinities, water/acid solubility, and/or reactivities with liquid sulfuric acid and other inorganic compounds, certain organic constituents known to be generated by aircraft engines are likely to condense onto ions and charged particles and/or to be taken up by liquid sulfuric acid particles, where these particles are expected to act as efficient reaction sites The present simulations, which are constrained by in situ measurements, indicate that in the case of very low sulfur emissions, organic species in the exhaust stream can dominate the mass of volatile particles detected at very early times In the case of fuels with medium to high sulfur contents, the organic emissions may still contribute a significant fraction of the total aerosol mass and alter the composition of the exhaust particles

The glass transition of charged and hard sphere silica colloids

Abstract: Dynamic and static light scattering is applied to concentrated suspensions of silica nanoparticles with surface functionalizations causing highly charged or hard sphere interaction potentials, respectively. The index of refraction of the dispersion medium was matched to that of the particles using a mixture of water/glycerol for the charged particles and toluene/ethanol for the hard spheres. The static structure factors correspond to the appropriate theoretical models, Percus–Yevick and rescaled mean spherical approximation. At volume fractions φ=0.18 a glass transition for the charged systems and at φ=0.53 for the hard spheres can be observed, as evident from the nondecaying components of the intermediate scattering functions. In the glassy state the experimental correlation functions agree with the predictions of the mode-coupling theory over several orders of magnitude in time. Using the fitted experimental structure factors as input for the mode-coupling theory we find good agreement between the theor...

Imaging of fusion reaction zone in plasma focus

Abstract: In a low energy (2.3 kJ) Mather-type deuterium plasma focus, neutron and charged particle emission is investigated by using time-resolved neutron detectors and time-integrated charged particle pinhole imaging camera. The time-integrated charged particle pinhole images demonstrate the varying influence of magnetohydrodynamic (MHD) instabilities vis-a-vis filling pressure. The neutron production mechanism at play strongly depends upon the pressure. At lower pressure, the plasma column is highly unstable due to MHD instabilities and the neutron emission is found to be low with fluence anisotropy exceeding 3.5. At optimum pressure (2.5 mbar for this system), an almost stable dense plasma of about 17 mm3 volume is formed about 5 mm away from the anode, with neutron emission at its highest and the fluence anisotropy lowest. At higher pressure, the plasma column is stable, although it moves away from the anode like a jet and may then be called a moving boiler. In this case, the neutron emission is lowered compared to its optimum value and fluence anisotropy is increased. The data suggest beam-target mechanism at low pressure, trapped gyrating particles at optimum pressure and a jetlike moving boiler at higher pressure.In a low energy (2.3 kJ) Mather-type deuterium plasma focus, neutron and charged particle emission is investigated by using time-resolved neutron detectors and time-integrated charged particle pinhole imaging camera. The time-integrated charged particle pinhole images demonstrate the varying influence of magnetohydrodynamic (MHD) instabilities vis-a-vis filling pressure. The neutron production mechanism at play strongly depends upon the pressure. At lower pressure, the plasma column is highly unstable due to MHD instabilities and the neutron emission is found to be low with fluence anisotropy exceeding 3.5. At optimum pressure (2.5 mbar for this system), an almost stable dense plasma of about 17 mm3 volume is formed about 5 mm away from the anode, with neutron emission at its highest and the fluence anisotropy lowest. At higher pressure, the plasma column is stable, although it moves away from the anode like a jet and may then be called a moving boiler. In this case, the neutron emission is lowered compar...