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.
53709253
Non-Gaussianity from cosmic magnetic fields
Magnetic fields in the early Universe could have played an important role in sourcing cosmological perturbations. While not the dominant source, even a small contribution might be traceable through its intrinsic non-Gaussianity. Here we calculate analytically the one-, two-, and three-point statistics of the magnetic stress energy resulting from tangled Gaussian fields, and confirm these with numerical realizations of the fields. We find significant non-Gaussianity, and importantly predict higher order moments that will appear between the scalar, vector, and tensor parts of the stress energy (e.g., scalar-tensor-tensor moments). Such higher order cross correlations are a generic feature of nonlinear theories and could prove to be an important probe of the early Universe.
119227429
INTRINSIC BISPECTRA OF COSMIC MAGNETIC FIELDS
Forthcoming data sets from the Planck experiment and others are in a position to probe the cosmic microwave background (CMB) non-Gaussianity with higher accuracy than has yet been possible, and potentially open a new window into the physics of the very early universe. However, a signal need not necessarily be inflationary in origin, and possible contaminants should be examined in detail. One such is provided by early universe magnetic fields, which can be produced by a variety of models including during an inflationary phase, at phase transitions, or seeded by cosmic defects. Should such fields have been extant in the early universe, they would provide a natural source of CMB non-Gaussianity. Knowledge of the CMB angular bispectrum requires the complete Fourier-space (or "intrinsic") bispectrum. In this paper, I consider in detail the intrinsic bispectra of an early-universe magnetic field for a range of power-law magnetic spectra.
118824929
Accelerating the Universe with Gravitational Waves
Inflation generically produces primordial gravitational waves with a red spectral tilt. In this paper we calculate the backreaction produced by these gravitational waves on the expansion of the universe. We find that in radiation domination the backreaction acts as a relativistic fluid, while in matter domination a small dark energy emerges with an equation of state weff ? ?8/9.
118519718
Gauges and cosmological backreaction
We present a formalism for spatial averaging in cosmology applicable to general spacetimes and coordinates, and allowing the easy incorporation of a wide variety of matter sources. We apply this formalism to a Friedmann-Lemaȋtre-Robertson-Walker universe perturbed to second-order and present the corrections to the background in an unfixed gauge. We then present the corrections that arise in uniform curvature and conformal Newtonian gauges.
119209860
The magnitude of the non-adiabatic pressure in the cosmic fluid
Understanding the non-adiabatic pressure, or relative entropy, perturbation is crucial for studies of early-Universe vorticity and cosmic microwave background observations. We calculate the evolution of the linear non-adiabatic pressure perturbation from radiation domination to late times, numerically solving the linear governing equations for a wide range of wavenumbers. Using adiabatic initial conditions consistent with Wilkinson Microwave Anisotropy Probe seven-year data, we find nevertheless that the non-adiabatic pressure perturbation is non-zero and grows at early times, peaking around the epoch of matter/radiation equality and decaying in matter domination. At early times or large redshifts (z= 10 000) its power spectrum peaks at a comoving wavenumber k≈ 0.2 h Mpc−1, while at late times (z= 500) it peaks at k≈ 0.02 h Mpc−1.
101292551
Comparison of methods of producing very highly stripped uranium beams
A comparison is made between the production of high intensity beams of helium-like uranium ions, U/sup 90 +/, by conventional and exotic ion sources, and by the foil stripping of highly accelerated ions output from the Bevalac. The parameter requirements are specified and compared to the parameters achievable by present day ion source technology. The EBIS (Electron Beam Ion Source) comes closest to satisfying the necessary parameters, and this possibility is considered in some detail. We conclude that existing and near-future ion source technology does not provide a means of production of high intensity U/sup 90 +/ beams. Foil stripping of lower charge state species that have been accelerated through the Bevalac provides a convenient approach.
96220391
High dose uranium ion implantation into silicon
Abstract Implantation of uranium ions into silicon to a dose of 6 × 1016 atoms/cm2, with a maximum concentration of 6 × 1021 atoms/cm3, has been carried out at an ion mean energy of 157 keV. The implanted uranium content was measured by Rutherford backscattering and confirmed by a measurement of the alpha-particle activity of the buried uranium layer. The range and straggling of the uranium implant, and sputtering of the silicon target by uranium, were measured and are compared with theoretical estimates.
98447994
AN INTENSE METAL ION BEAM SOURCE FOR HEAVY ION FUSION
We have developed an ion source which can produce high current beams of metal ions. The source uses a metal vapor vacuum arc discharge as the plasma medium from which the ions are extracted, so we have called this source the MEVVA ion source. The metal plasma is created simply and efficiently and no carrier gas is required. Beams have been produced from metallic elements spanning the periodic table from lithium through uranium, at extraction voltages from 10 to 60 kV and with beam currents as high as 1.1 Amperes (electrical current in all charge states). A brief description of the source is given and its possible application as an ion source for heavy ion fusion is considered. Beams such as C/sup +/ (greater than or equal to99% of the beam in this species and charge state), Cr/sup 2 +/ (80%), and Ta/sup 3+,4+,5+/ (mixed charge states) have been produced. Beam emittance measurements and ways of increasing the source brightness are discussed.
120372071
A NEW METHOD FOR ION CHARGE STATE ANALYSIS
We present results showing charge‐state separation of an ion beam in a purely cylindrical geometry, using the azimuthal magnetic field of a straight conductor. This geometry, which we refer to as a current coaxial lens, has advantages for the analysis of intense ion beams where it is important to minimize space‐charge blowup of the beam due to loss of space‐charge neutralization.
121406807
Measurements of vacuum arc ion charge-state distributions
The charge-state distribution of ions generated in the metal vapor vacuum arc under a wide range of experimental conditions was measured. The experiments were carried out using an ion source in which the metal vapor vacuum arc is used as the method of plasma production and by which a high-quality, high-current beam of metal ions is produced. Charge-state spectra were measured using a time-of-flight diagnostic; arc voltages were also measured. Parameters that were varied include cathode material, arc current, axial magnetic field strength, neutral gas pressure, and arc geometry. >