Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

A theory of cosmic-ray transport in multiphase diffusive media is developed, with the specific application to cases in which the cosmic-ray diffusion coefficient has large spatial fluctuations that may be inherently multiscale. We demonstrate that the resulting transport of cosmic rays is diffusive in the long-time limit, with an average diffusion coefficient equal to the harmonic mean of the spatially varying diffusion coefficient. Thus, cosmic-ray transport is dominated by areas of low diffusion even if these areas occupy a relatively small, but not infinitesimal, fraction of the volume. On intermediate time-scales, the cosmic rays experience transient effective subdiffusion, as a result of low-diffusion regions interrupting long flights through high-diffusion regions. In the simplified case of a two-phase medium, we show that the extent and extremity of the subdiffusivity of cosmic-ray transport is controlled by the spectral exponent of the distribution of patch sizes of each of the phases. We finally show that, despite strongly influencing the confinement times, the multiphase medium is only capable of altering the energy dependence of cosmic-ray transport when there is a moderate (but not excessive) level of perpendicular diffusion across magnetic-field lines.

Abstract:

We study a novel device for generating a high speed rotating plasma. The device weakly ionises and accelerates a hydrogen gas in a co-axial cylindrical chamber via the perpendicular configuration of electrodes with a magnetic field generated by a superconducting magnetic. It has been hypothesised that extreme velocities and plasma particle compression could be achieved under this configuration1 . This work develops a rigorous theoretical model of the bulk plasma dynamics under steady state centrifugal operation. By exploiting the axisymmetry of the system, and from application of problem-specific governing assumptions, a steady state 1D model for the rotational dynamics of the bulk plasma is derived. From here, we present fully analytical solutions for the radial profiles of the MHD model: [azimuthal velocity, particle densities, pressure] and a semi-analytical solution for electric potential. Tables of selfconsistent plasma parameters are computed to provide a comprehensive characterisation of the bulk plasma state. The model is able to determine the peak velocities and plasma compression, and permits parametric studies to elucidate the complex and non-linear relationships between operational device settings and the achieved steady state plasma state condition. The new theoretical solutions therefore provide necessary insights into the viability of the novel device for high energy-density plasma applications.

Abstract:

The generation of dense electron-positron pair beams in the laboratory can enable direct tests of theoretical models of γ-ray bursts and active galactic nuclei. We have successfully achieved this using ultrarelativistic protons accelerated by the Super Proton Synchrotron at (CERN). In the first application of this experimental platform, the stability of the pair beam is studied as it propagates through a meter-length plasma, analogous to TeV γ-ray-induced pair cascades in the intergalactic medium. It has been argued that pair beam instabilities disrupt the cascade, thus accounting for the observed lack of reprocessed GeV emission from TeV blazars. If true, this would remove the need for a moderate strength intergalactic magnetic field to explain the observations. We find that the pair beam instability is suppressed if the beam is not perfectly collimated or monochromatic, hence the lower limit to the intergalactic magnetic field inferred from γ-ray observations of blazars is robust.

Abstract:

The axion is a hypothetical particle associated with a possible solution to the strong CP problem and is a leading candidate for dark matter. In this paper we investigate the emission of axions by accelerated electrons. We find the emission probability and energy within the WKB approximation for an electron accelerated by an electromagnetic field. As an application, we estimate the number of axions produced by electrons accelerated using two counter-propagating high-intensity lasers and discuss how they would be converted to photons to be detected. We find that, under realistic experimental conditions, competitive model-independent bounds on the coupling between the axion and the electron could be achieved in such an experiment.