119158164
Averaging Robertson-Walker cosmologies
The cosmological backreaction arises when one directly averages the Einstein equations to recover an effective Robertson-Walker cosmology, rather than assuming a background a priori. While usually discussed in the context of dark energy, strictly speaking any cosmological model should be recovered from such a procedure. We apply the scalar spatial averaging formalism for the first time to linear Robertson-Walker universes containing matter, radiation and dark energy. The formalism employed is general and incorporates systems of multiple fluids with ease, allowing us to consider quantitatively the universe from deep radiation domination up to the present day in a natural, unified manner. Employing modified Boltzmann codes we evaluate numerically the discrepancies between the assumed and the averaged behaviour arising from the quadratic terms, finding the largest deviations for an Einstein-de Sitter universe, increasing rapidly with Hubble rate to a 0.01% effect for h = 0.701. For the ΛCDM concordance model, the backreaction is of the order of Ωeff0 ≈ 4 × 10−6, with those for dark energy models being within a factor of two or three. The impacts at recombination are of the order of 10−8 and those in deep radiation domination asymptote to a constant value. While the effective equations of state of the backreactions in Einstein-de Sitter, concordance and quintessence models are generally dust-like, a backreaction with an equation of state weff < −1/3 can be found for strongly phantom models.
119047522
Cosmological backreaction from perturbations
We write the averaged Einstein equations in a form suitable for use with Newtonian gauge linear perturbation theory and track the size of the modifications to standard Robertson–Walker evolution on the largest scales as a function of redshift for both Einstein–de Sitter and ΛCDM cosmologies. In both cases the effective energy density arising from linear perturbations is of the order of 10−5 times the matter density, as would be expected, with an effective equation of state weff≈−1/19. Employing a modified Halofit code to extend our results to quasilinear scales, we find that, while larger, the deviations from Robertson–Walker behaviour remain of the order of 10−5.
116966118
Concerning the statistics of cosmic magnetism
Magnetic fields appear to be a generic feature of the early universe and are a natural source of secondary CMB non-Gaussianity. In recent years the statistical nature of the stresses of a primordial magnetic field has been well studied. In this paper we confirm and extend these studies at one- and two-point level, and present analytical results for a wide range of power-law spectra. We also consider two non-power law cases of interest: a blue spectrum with an extended damping tail on small scales, which could be generated by the non-linear mixing of density and vorticity; and a red spectrum with a damping tail on large scales. We then briefly consider the CMB impacts that result from such fields. While this paper focuses on the one- and two-point moments, the techniques we employ are designed to ease the analysis of the full bispectra induced by primordial magnetic fields.
119284118
Primordial Magnetic Fields in Cosmology
Magnetic fields have been observed in galaxies, clusters of galaxies and probably in superclusters. While mechanisms exist to generate these in the late universe, it is possible that magnetic fields have existed since very early times. This thesis is concerned with methods to predict the form of such imprints. We review in detail a standard, linearised cosmology before introducing an electromagnetic field. We then consider the intrinsic statistics of the magnetic stresses in two ways, analytically and via static realisations. We construct the power spectra, some of which we present for the first time. At the one- and three-point level we find significant intrinsic non-Gaussianities. Finally we turn to the observable impacts a primordial magnetic field. Assuming coherence, the statistics of the source can be mapped onto the CMB in a simple manner. We demonstrate that our approach is valid by reproducing the signals for Gaussian power law fields on the microwave sky. [ABRIDGED]
97258429
High energy implantation with high-charge-state ions in a vacuum arc ion implanter
Ion implantation energy can in principal be increased by increasing the charge states of the ions produced by the ion source rather than by increasing the implanter operating voltage, providing an important savings in cost and size of the implanter. In some recent work the authors have shown that the charge states of metal ions produced in a vacuum arc ion source can be elevated by a strong magnetic field. In general, the effect of both high arc current and high magnetic field is to push the distribution to higher charge states--the mean ion charge state is increased and new high charge states are formed. The effect is significant for implantation application--the mean ion energy can be about doubled without change in extraction voltage. Here they describe the ion source modifications, the results of time-of-flight measurements of ion charge state distributions, and discuss the use and implications of this technique as a means for doing metal iron implantation in the multi-hundreds of keV ion energy range.
95332975
MULTIPLY STRIPPED ION GENERATION IN THE METAL VAPOR VACUUM ARC
We consider the charge state distribution of ions produced in the metal vapor vacuum arc plasma discharge. A high current metal ion source, the MEVVA ion source, in which the ion beam is extracted from a metal vapor vacuum arc plasma, has been used to obtain the spectra of multiple charged ions produced within the cathode spots. A computer calculation of the charge state distribution that evolves within the spots via stepwide ionization of ions by electron impact provides a theoretical basis for comparison of the data. In this paper we report on the measured charge state distributions for a wide variety of metallic species and compare these results with the predictions of this theory. 55 refs.
122942161
Charge state distribution studies of the metal vapor vacuum arc ion source
We have studied the charge state distribution of the ion beam produced by the MEVVA (metal vapor vacuum arc) high current metal ion source. Beams produced from a wide range of cathode materials have been examined and the charge state distributions have been measured as a function of many operational parameters. In this article we review the charge state data we have accumulated, with particular emphasis on the time history of the distribution throughout the arc current pulse duration. We find that, in general, the spectra remain quite constant throughout most of the beam pulse, as long as the arc current is constant. There is an interesting early‐time transient behavior when the arc is first initiated and the arc current is still rising, during which time the ion charge states produced are observed to be significantly higher than during the steady current region that follows.
38209657
On the Current Intensity Limit of a Vacuum-Arc Ion Source
The maximum ion current that can be extracted as a high-energy beam from a metal-vapor vacuum-arc ion source is considered. Results are presented of measurements of the plasma ion current in the metal-vapor vacuum-arc II (MEVVA II) ion source. It is shown that this source is an efficient generator of metal ions, an intense flux of which is efficiently transported to the beam extractor. The maximum metal-ion current that is available for extraction at the extractor location is 5 percent of the arc current. The limitation to the intensity of the metal-ion beam that can be produced by this kind of ion source is found to be in the extractor design.
32998739
High-current heavy ion beams in the electrostatic plasma lens
We describe applications of the electrostatic plasma lens for manipulating and focusing moderate-energy, high-current, broad, heavy ion beams. Use of a plasma lens in this way has been successfully demonstrated in a series of experiments carried out collaboratively between the Institute of Physics, National Academy of Sciences, Kiev, Ukraine, and the Lawrence Berkeley National Laboratory, Berkeley, CA, in recent years. Here, we briefly review the plasma lens fundamentals, peculiarities of focusing heavy ion beams, and summarize some recent developments (experiments, computer simulations, theory). We show that there is a very narrow range of low magnetic field for which the optical properties of the lens improve markedly. This opens up some attractive possibilities for the development of a new-generation compact lens based on permanent magnets. Preliminary experimental results obtained at Kiev and Berkeley on the operation of a permanent magnet plasma lens for manipulating wide aperture high-current heavy ion beams are presented and summarized.