Showing papers on "Charged particle published in 1988"

The physics of charged-particle beams

Abstract: Introduction Beam optics and focusing systems Laminar beams with self-fields Non-Laminar beams without collisions Beams with scattering or dissipation Waves and instabilities in beams

Fermi-bose transmutations induced by gauge fields

Abstract: We show that in (2+1)-dimensional abelian gauge theory with the Chern-Simons term in the action, charged particles reverse their statistics.

The equation of state for stellar envelopes. II - Algorithm and selected results

Abstract: A free-energy-minimization method for computing the dissociation and ionization equilibrium of a multicomponent gas is discussed. The adopted free energy includes terms representing the translational free energy of atoms, ions, and molecules; the internal free energy of particles with excited states; the free energy of a partially degenerate electron gas; and the configurational free energy from shielded Coulomb interactions among charged particles. Internal partition functions are truncated using an occupation probability formalism that accounts for perturbations of bound states by both neutral and charged perturbers. The entire theory is analytical and differentiable to all orders, so it is possible to write explicit analytical formulas for all derivatives required in a Newton-Raphson iteration; these are presented to facilitate future work. Some representative results for both Saha and free-energy-minimization equilibria are presented for a hydrogen-helium plasma with N(He)/N(H) = 0.10. These illustrate nicely the phenomena of pressure dissociation and ionization, and also demonstrate vividly the importance of choosing a reliable cutoff procedure for internal partition functions.

Straggling in thin silicon detectors

Abstract: Experimental and theoretical data for dielectric functions, x-ray absorption coefficients, and generalized oscillator strengths needed for a description of the energy-loss spectrum of fast charged particles in solid silicon are given. Theories used to calculate spectra of total energy loss ("straggling spectra") are described. The convolution method is used to calculate straggling functions for thin silicon absorbers. They are compared with those obtained from other theories (Landau). For relativistic particles ($\ensuremath{\gamma}g100$), the Vavilov-Shulek theories give incorrect functions for absorbers of thicknesses $tl1$ mm. The conversion of energy-loss spectra into ionization spectra is discussed, and the latter are compared with experimental functions. Good agreement is found between calculated and observed functions for electrons, mesons, protons, and their antiparticles and for $\ensuremath{\alpha}$ particles. From this agreement, the error ($1\ensuremath{\sigma}$) of the theoretical values of the most probable energy loss ${\ensuremath{\Delta}}_{p}$ and the full width at half maximum, $w$, is estimated to be less than \ifmmode\pm\else\textpm\fi{}1%.

Generation of photon energy deposition kernels using the EGS Monte Carlo code

Abstract: The EGS Monte Carlo code was used to generate photon energy deposition kernels which describe the energy deposited by charged particles set in motion by primary, first scattered, second scattered, multiple scattered and bremsstrahlung plus annihilation photons. These were calculated for a water medium irradiated with monoenergetic photons with energies in the range 0.1-50 MeV. In addition to the primary energy deposition kernels, primary charged particle transport was further characterised by computing the effective centre of the voxels, and the effective penetration depth, effective radius and effective lateral distance travelled by these particles. The dose per unit collision kerma for parallel monoenergetic primary photons beta ' was calculated. Additional applications of the energy deposition kernels are discussed.

The CDF Detector: An Overview

Abstract: The Collider Detector at Fermilab (CDF) is a 5000 t magnetic detector built to study 2 TeV p p collisions at the Fermilab Tevatron. Event analysis is based on charged particle tracking, magnetic momentum analysis and fine-grained calorimetry. The combined electromagnetic and hadron calorimetry has approximately uniform granularity in rapidity-azimuthal angle and extends down to 2° from the beam direction. Various tracking chambers cover the calorimeter acceptance and extend charged particle tracking down to 2 mrad from the beam direction. Charged particle momenta are analyzed in a 1.5 T solenoidal magnetic field, generated by a superconducting coil which is 3 m in diameter and 5 m in length. The central tracking chamber measures particle momenta with a resolution better then δpT/pT2 = 2 × 10−3 (GeV/c)−1 in the region 40°

Ion plasma waves in dusty plasmas: Halley's comet

Abstract: We investigate ion waves in a plasma in the presence of massive charged dust particles, a common space-plasma component now known to exist also in planetary rings and comets. We derive an equation describing low-frequency electrostatic perturbations on a non-homogeneous background, where the inhomogeneity is due to a distribution of charged grains, each surrounded by an equilibrium statistical distribution of plasma particles. This model is then applied to propose an interpretation of some recent data from the Vega and Giotto space probes to Halley's comet the increase of the low-frequency electrostatic noise (ion-acoustic waves) in the region of increased dust.

Determination of auroral electrostatic potentials using high- and low-altitude particle distributions

Abstract: The auroral electrostatic potential differences were determined from the particle distribution functions obtained nearly simultaneously above and below the auroral acceleration region by DE-1 at altitudes 9000-15,000 km and DE-2 at 400-800 km. Three independent techniques were used: (1) the peak energies of precipitating electrons observed by DE-2, (2) the widening of loss cones for upward traveling electrons observed by DE-1, and (3) the energies of upgoing ions observed by DE-1. The assumed parallel electrostatic potential difference calculated by the three methods was nearly the same. The results confirmed the hypothesis that parallel electrostatic fields of 1-10 kV potential drop at 1-2 earth radii altitude are an important source for auroral particle acceleration.

Computer modeling of test particle acceleration at oblique shocks

Abstract: The present evaluation of the basic techniques and illustrative results of charged particle-modeling numerical codes suitable for particle acceleration at oblique, fast-mode collisionless shocks emphasizes the treatment of ions as test particles, calculating particle dynamics through numerical integration along exact phase-space orbits. Attention is given to the acceleration of particles at planar, infinitessimally thin shocks, as well as to plasma simulations in which low-energy ions are injected and accelerated at quasi-perpendicular shocks with internal structure.

Transverse-momentum distributions of charged particles produced in p-barp interactions at sqrt s-bar=630 and 1800 GeV.

Abstract: Measurements of inclusive transverse-momentum spectra for charged particles produced in proton-antiproton collisions at ..sqrt..2 of 630 and 1800 GeV are presented and compared with data taken at lower energies.

Apparatus for acceleration and application of negative ions and electrons

Abstract: An apparatus for generating X-rays from electron synchrotron radiation or beams of accelerated ions for ion radiography or ion therapy includes a source of electrons and a source of ions, both of which are connectable to preaccelerators. The preaccelerators supply the appropriate type of charged particle to a synchrotron accelerator which accelerates ions to an energy level that is appropriate for radiography or therapy and which accelerates electrons to a level that generates X-rays by synchrotron radiation in a useful frequency range. The accelerator system also includes a storage ring into which particles are switched and circulated for later use. Electrons are extracted from the synchrotron and injected into the storage ring by fast extraction using a kicker magnet and a septum magnet. They then circulate in the storage ring for periods of hours generating X-rays which may be used for lithography of computer chips with submicron resolution. The energy loss because of this radiation is continuously replaced by a radio-frequency acceleration system. During the period that electrons are circulating in the storage ring, the synchrotron may be utilized to accelerate ions for ion radiography or ion therapy with beam extracted from the synchrotron by stripping extraction through thin foils. Other simultaneous uses for the ions or electrons from the preaccelerator may prove advantageous.

New display‐type analyzer for the energy and the angular distribution of charged particles

Abstract: A new principle for analyzing the kinetic energy as well as the angular distribution of charged particles has been discovered. A new display type analyzer was constructed according to theory. It consists of an inner hemispherical grid and an outer hemispherical electrode which has a radius two times larger than that of the inner grid. Charged particles with the same kinetic energy radiate from one point to all directions inside the inner grid, are repelled by the electric field between the two spheres, and converge at the point which is symmetric to the center of the hemispheres. When an aperture is put at the point, the angular distribution of the charged particles can be observed using a two‐dimensional detector which is positioned out of the aperture. The characteristics of the analyzer are (1) the acceptance cone is ultimately wide, for instance, 2π sr for the charged particles emitted from a flat surface, (2) the pattern obtained is not distorted, and (3) the two electrodes are very simple and easy to construct.

Determination of static and dynamic interactions between monodisperse, charged silica spheres in an optically matching, organic solvent

Abstract: A light scattering study is presented on static and dynamic interactions between monodisperse, charged silica spheres suspended in an optically matching, salt free mixture of ethanol and toluene up to volume fractions of 10%. The static structure factor S(K), well described by calculations in the RMSA approximation, is combined with the wave vector (K) dependent (short‐time) diffusion coefficient De(K) to give the function H(K) which represents the hydrodynamic interactions. From H(K), obtained for the first time for charged particles, we conclude that the long‐range electrostatic repulsion between the spheres has a pronounced influence on the hydrodynamics of large‐scale, collective particle motions, whereas small‐scale single‐particle diffusion is relatively unaffected.

A comparative study of (BI)polaronic (super)conductivity in high- and low-Tc superconducting oxides

Abstract: A comparative study is made on the conductivity and superconductivity mechanisms in the series of superconducting oxides SrTiO3-δTc⋍0.3 K), BaPb1-xBixO3(Tc⋍12 K), Li1+xTi2-xO4(Tc⋍11 K), La2-xSrxCuO4(Tc⋍40 K), and YBa2Cu3O7-δ (Tc⋍60 K for δ⋍0.3−0.4; Tc⋍90 K for δ⋍0). We claim that all these materials can be viewed as doped semiconductors, where the conductivity is due to doping or self-doping. This procedures mixed-valence conditions for the metal-ions, leading to the formation of small-polarons, namely the combination of the metal atom with its extra charge plus its deformed oxygen coordination. The mobility of the small-polarons is ensured through the mixed-valence charge transfer mechanism. They may be viewed as a dilute gas of charged particles on a lattice, moving in the random fluctuating potential due to the impurities. By varying the concentration of these, small-polarons metal-insulator transitions (probably of the Anderson type) occur in all these materials, which explaines the observed behaviour of the conductivity and points to a universal “minimum conductivity”, σmin⋍102 (Ω cm)−1. The special non-linear (solitonic) features of these mixed-valence small-polarons are pointed out. It is further argued that under special conditions these small-polarons, may combine to form small-bipolarons, and the possible binding mechanism for each of the materials is investigated. The superconductivity may then be explained in terms of a Bose-Einstein condensation of these bipolarons, for which the theory has been developed in the last few years. To substantiate our arguments, a short overview of these theoretical results is given. In addition quite a number of relevant experimental data available in the literature on these compounds is reanalysed in terms of the above concepts.

Quasi-neutral theory of positive columns in electronegative gases

Abstract: A theory of the positive column in electronegative gases based on fluid-type momentum equations to describe charged particle motion is presented. It is assumed that quasi-neutrality conditions prevail and the ion inertial terms are neglected. The positive ions are assumed to be created by electron collisions with neutral molecules and the negative ions to be formed by dissociative electron attachment and destroyed by detachment in reactions with neutral species. The mathematical formulation consists of a two-point boundary value problem involving two independent parameters, functions of collisional and transport data, and two eigenvalues. One of these is the central ratio of the negative ion density to the electron density, while the other is related to the ionisation-loss balance and embodies a discharge characteristic for the maintenance field. These eigenvalues and the radial density distributions of the charged species were calculated for a wide range of variation of the independent parameters. An application of the theory to a positive column in oxygen is given as an illustrative example.

Vacuum polarization in a strong external field

Abstract: Vacuum polarization in the field of a high-Z finite-size nucleus is examined, and the polarization charge density in coordinate space of order alpha(Z alpha)exp n not less than 3 is calculated. Energy-level shifts of K- and L-shell electrons in hydrogenlike systems are given.

On the Dynamical Coupling between the Superfluid Interior and the Crust of a Neutron Star

Abstract: Some implications of earlier work on the coupling of a neutron star's superfluid core to its crust are clarified. It is shown that the very short time, tau/sub v/, for the relaxation of relative velocity between the plasma interior (protons and electrons) and the neutron vortices implies a time scale tau sub d = tau/sub v/ (rhorho/sub c/) for the coupling of the neutron star's core superfluid to the crust where rho(rho/sub c/) is the total (charged) fluid density. One consequence of such a short coupling time is that free precession of a neutron star, as is suggested for the 35-day mode of Her X-1, must be continuously excited, as it would otherwise be damped out in 400-10,000 precession periods. 9 references.

Thermal coupling of ions and electrons by collective effects in two-temperature accretion flows

Abstract: A plasma instability which may lead to the thermal coupling of ions and electrons in two-temperature accretion flows on a time scale shorter than the Coulomb coupling time has been found. The processes involved with the instability are described. It is shown that the instability has time to grow only in regions of large-amplitude, small-scale MHD turbulence.

Molecular-orbital description of the states of two-electron systems

Abstract: The molecular-orbital (MO) method introduced previously by us [Phys. Rev. Lett. 57, 984 (1986)] to treat two-electron atoms is developed further. The states of systems consisting of two electrons and one positively charged particle are analyzed with use of the interelectronic distance as an adiabatic coordinate in analogy to the interprotonic distance in ${\mathrm{H}}_{2}^{+}$. The motion of two electrons then separates into rotational, vibrational, and internal motion, the latter being described by MO, exactly as in molecules. Indeed, adiabatic MO potential curves for atomic systems are obtained by scaling the corresponding curves for ${\mathrm{H}}_{2}^{+}$. Approximate quantum numbers for two-electron states, derived previously by empirical methods or from an ad hoc rovibrational model, arise naturally since the two-center Coulomb problem is exactly separable in MO coordinates and the corresponding nodal surfaces are conserved for all interelectronic separations. In addition, the gerade-ungerade symmetry of MO is exactly preserved and appears as a fundamental symmetry of two-electron states.

The negative ion states of sulfur hexafluoride

Abstract: The reaction of SF6 with thermal electrons has been studied in a Ni‐63 atmospheric pressure ionization source for a quadrupole mass spectrometer (API/MS). The major ions that are observed are the parent negative ion (SF−6) and the parent minus a fluorine atom (SF−5). The ratio of [SF−5]/[SF−6] is highly temperature dependent above 500 K. The dissociation energy of the ground state negative ion into SF−5 and F has been determined to be 1.35±0.1 eV. This gives values of 3.8±0.15 eV for the electron affinity of SF5 and 1.15±0.15 eV for the electron affinity of SF6. The negative ion states of sulfur hexafluoride have been described by ‘‘pseudo‐two‐dimensional’’ Morse potentials calculated using experimental data.

Charged particle production inK + p, π p andpp interactions at 250 GeV/c

Abstract: New results are reported on inclusive charged particle production inK + p,π + p andpp collisions at 250 GeV/c. Inclusive longitudinal and transverse momentum spectra of positively and negatively charged particles are presented. Scaling in the fragmentation regions and scaling violation in the central c.m. region is investigated in detail. The topological pseudo-rapidity densities are shown to scale in the c.m. energy range from 22 to 900 GeV.

X-ray transitions in highly charged neonlike ions

Abstract: Wavelength measurements of n=3 to n=2 transitions in neonlike ${\mathrm{Xe}}^{44+}$, ${\mathrm{La}}^{47+}$, ${\mathrm{Nd}}^{50+}$, and ${\mathrm{Eu}}^{53+}$ have been made using a high-resolution Bragg-crystal spectrometer on the Princeton Large Torus tokamak. The measurements cover the wavelength regions 2.00--3.00 A\r{} and include the electric dipole and the electric and magnetic quadrupole transitions. The measured wavelengths are compared to energy levels obtained from a multiconfigurational Dirac-Fock calculation. Systematic differences between the experimental and theoretical values are found, which vary smoothly with atomic number. The magnitude of the differences depends on the particular type of transition and ranges from -2.8 eV to +2.2 eV. Inclusion of electron correlation corrections due to ground-state correlations and (super-) Coster-Kronig-type fluctuations in the theoretical energies is shown to reduce the differences for some but not all types of transitions.

Hydrogenic radiative recombination at low temperature and density

Abstract: The radiative recombination of hydrogenic ions is calculated in the limit of low density by the nl-method, extending the results of Pengelly (1964) to much lower temperatures (both case A and case B). A strong motivation is the detection of the emission-line spectrum of the old shell of Nova DQ Her 1934 reported by Williams et al. (1978), which revealed an electron temperature T(e) of about 500 K. The results are compared with the 1959 result of Seaton using the n-method, with those of Hummer and Storey (1987), which allow for the effects of collisions at finite density, and with the data for three cold old nova shells. Another motivation is the detection of relatively strong optical radiative recombination lines of C, N, and O in these shells. The abundances of these elements (recombining ions of charge Z) can be derived using a hydrogenic approximation to the effective radiative recombination coefficients, requiring the recombination coefficients of H(+) at even lower temperatures. 27 references.

Particle acceleration by superposition of frequency-controlled laser pulses

Abstract: As the energies of particle accelerators are extended to the TeV range and beyond, the decrease of interaction cross-sections with increasing energy will require that luminosities be increased1. This will increase the cost of conventional accelerators, and has prompted an examination of alternative designs, incorporating very-high-intensity lasers2,3. One example is the beat-wave accelerator, in which a longitudinal resonance field4 produced in a plasma by the beating of two laser beams can accelerate electrons by5 ≈1GeV m−1. Unfortunately, this and other types6 of laser accelerator involving plasmas encounter problems such as instabilities, self-focusing7,8, de-tuning9,10 and the appearance of internal electric fields and double layers11,12, which appear to limit the energy to a few MeV. To overcome these difficulties, I propose here a laser accelerator system that does not require a plasma, in which charged particles are accelerated by nonlinear forces13,14 towards minima in a field created by two collinear laser beams, and the laser intensity gradients are moved in phase with the accelerated particles by electro-optical modulation of the frequency and/or phase of the beams. With very high laser powers, electrons from conventional accelerators could be further accelerated by as much as 600 GeV cm−1.

Resonant ac-dc magnetic fields: calculated response.

Abstract: An elementary model consisting of one charged particle in a viscous medium exposed to weak ac-dc low-frequency magnetic fields is analyzed to identify and explain the fundamental characteristics of the physical mechanisms that result in a resonance response, which is similar to the familiar cyclotron resonance. The model predicts both frequency and amplitude windows, which are explained in terms of synchronization of the particle with electric fields. Although extrapolation of model results to biological systems is limited by the elementary nature of the model, the model results indicate that observed resonant responses by others of biological systems to ac-dc magnetic fields are probably not due to resonant response of ions in solution, since the model predicts that no resonant response is possible unless the viscous damping is very low, many orders of magnitude lower than the viscous damping of ions in solution.