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]
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.
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.
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.
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.
122299704
Transport of vacuum arc plasma through straight and curved magnetic ducts
We report on an experimental investigation of the transport of a metal vapor vacuum arc plasma through straight and curved magnetic solenoids for the case of an iron plasma and for magnetic field strength up to 650 G. We find that the fraction of plasma transported increases with magnetic field strength and saturates at a field at which the ion gyroradius equals the radius of the plasma channel. The magnetic field strength at the cathode location, where the plasma is injected into the solenoid, was found to be important, and for the present work the injection efficiency optimizes at a field of about 150 G.
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.
9991030
A transient relativistic radio jet from Cygnus X-1
We report the first observation of a transient relativistic jet from the canonical black hole candidate, Cygnus X-1, obtained with the Multi-Element Radio-Linked Interferometer Network (MERLIN). The jet was observed in only one of six epochs of MERLIN imaging of the source during a phase of repeated X-ray spectral transitions in 2004 Jan-Feb, and this epoch corresponded to the softest 1.5-12 keV X-ray spectrum. With only a single epoch revealing the jet, we cannot formally constrain its velocity. Nevertheless, several lines of reasoning suggest that the jet was probably launched 0.5-4.0 d before this brightening, corresponding to projected velocities of 0.2c ≤ v app ≤ 1.6c, and an intrinsic velocity of ≥0.3c. We also report the occurrence of a major radio flare from Cyg X-1, reaching a flux density of ∼120 mJy at 15 GHz, and yet not associated with any resolvable radio emission, despite a concerted effort with MERLIN. We discuss the resolved jet in terms of the recently proposed 'unified model' for the disc-jet coupling in black hole X-ray binaries, and tentatively identify the 'jet line' for Cyg X-1. The source is consistent with the model in the sense that a steady jet appears to persist initially when the X-ray spectrum starts softening, and that once the spectral softening is complete the core radio emission is suppressed and transient ejecta/shock observed. However, there are some anomalies, and Cyg X-1 clearly does not behave like a normal black hole transient in progressing to the canonical soft/thermal state once the ejection event has happened.
97512037
Miniature high current metal ion source
A small, simple ion source for the production of high brightness beams of metal ions is described. A metal vapor vacuum arc discharge is used to establish the high density plasma from which the ion beam is extracted. The source is finger sized, and can produce pulsed metal ion beams with current up to the 10‐mA range.
12225490
Effect of Resonant Microwave Power on a PIG Ion Source
We have investigated the effect of applying microwave power at the electron cyclotron frequency on the characteristics of the ion beam extracted from a hotcathode PIG ion source. No change was seen in the ion charge state distribution. A small but significant reduction in the beam noise level was seen, and it is possible that the technique may find application in situations where beam quiescence is important.