Browsing by Author "Baring, Matthew G"
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Item Modeling of Intense Laser Driven High Energy Density Plasmas(2014-11-19) Levy, Matthew Chase; Baring, Matthew G; Wilks, Scott C; Liang, Edison P; Kono, JunichiroIlluminating matter with petawatt (one quadrillion watts) laser light creates extreme states of plasmas with temperatures exceeding ten million degrees Celsius and pressures exceeding one billion earth atmospheres. These high energy density conditions are driven at the microscopic scale by dense currents of relativistic electrons, oscillating violently in the intense laser fields, as well as the plasma processes arising when these particles lose phase coherence and are injected into bulk target material. Suitably harnessed, this setup opens the way to compact relativistic particle accelerators, laser fusion energy sources, laboratory astrophysics, ultrafast imaging systems, proton cancer therapies, anti-matter creation, high-energy radiation sources and intense high harmonic generation. In this thesis, theoretical models of the ab- sorption of high power laser light by matter are derived, applications of these models are investigated, and simulation tools supporting the diagnosis and implementation of these applications are developed. In particular, an advanced, relativistically-correct theoretical model of petawatt laser absorption by optically-thick targets is developed, accounting for both ion and electron beam aspects of the interaction. Predictions of the model for the energetic properties of these beams, as well as the dynamical motion of the laser-matter inter- face, are elucidated. Results from high resolution, relativistic, kinetic particle-in-cell simulations using the LSP code are shown to be in good agreement with the model. The theoretical maximum and minimum absorption values in laser-solid interactions are derived from the model in a general fashion, constraining nonlinear absorption processes across the petawatt regime, spanning 10^18 < I_l λ^2_l < 10^23 W μm^2 cm^−2 for intensity I_l and wavelength λ_l. These results are shown to bound several dozens of published experimental and simulation data points, underlining the usefulness of the model. These results are extended to include effects related to heterogeneous plasmas, including relativistically-underdense plasmas relevant to ‘pre-plasma’ situations, and realistic laser spatio-temporal profiles. Our dynamic considerations of absorption processes are then extended to the 10-petawatt scale. In a manner that could support reaching the QED-plasma regime, a mechanism of focusing high power laser light to higher intensities is elucidated. Supporting the measurement and validation of these models, the development of a new simulation tool for understanding high energy density plasmas, based on the proton radiography technique, is detailed.Item Pair Creation Transparency in Gamma-Ray Pulsars(2014-07-30) Story, Sarah A; Baring, Matthew G; Chan, Anthony A; Damjanovic, DanijelaPulsars are rapidly rotating, highly magnetized neutron stars that produce photon pulses in energies from radio to gamma-rays. The population of known gamma-ray pulsars has been increased nearly twenty-fold in the past six years since the launch of the Fermi Gamma-Ray Space Telescope; it now exceeds 145 sources and has defined an important part of Fermi's science legacy. In order to understand the detectability of pulsars in gamma-rays, it is important to consider not only the radiative mechanisms that produce gamma-rays, but the processes that can attenuate photons before they can leave the pulsar magnetosphere. Here I explore two such processes, one-photon magnetic pair creation and two-photon pair creation. Magnetic pair creation has been at the core of radio pulsar paradigms and central to polar cap models of gamma-ray pulsars for over three decades. Among the population characteristics well established for Fermi pulsars is the common occurrence of exponential turnovers in the spectra in the 1-10 GeV range. These turnovers are too gradual to arise from magnetic pair creation in the strong magnetic fields of pulsar inner magnetospheres. By demanding insignificant photon attenuation precipitated by such single-photon pair creation, the energies of these turnovers for Fermi pulsars can be used to compute lower bounds for the typical altitude of GeV band emission. In this thesis, I explore such pair transparency constraints below the turnover energy and update earlier altitude bound determinations that have been deployed in various gamma-ray pulsar papers by the Fermi-LAT collaboration. For low altitude emission locales, general relativistic influences are found to be important, increasing cumulative opacity, shortening the photon attenuation lengths, and also reducing the maximum energy that permits escape of photons from a neutron star magnetosphere. Rotational aberration influences are also explored, and are found to be small at low altitudes, except near the magnetic pole. Our analysis clearly demonstrates that including near-threshold physics in the pair creation rate is essential to deriving accurate attenuation lengths and escape energies. The altitude bounds we compute for Fermi pulsars are typically in the range of 2-7 stellar radii and provide key information on the emission altitude in radio quiet pulsars that do not possess double peaked pulse profiles. The bound for the Crab pulsar is at a much higher altitude, with the detection by the atmospheric Cherenkov telescope MAGIC out to 350-400 GeV implying a lower bound of 310 km to the emission region, i.e., approximately 20% of the light cylinder radius. These results are also extended to the super-critical field domain, where it is found that emission in magnetars originating below around 10 stellar radii will not appear in the Fermi-LAT band. Two-photon pair creation becomes important at high altitudes and for photons produced by curvature radiation from charges flowing downward along magnetic field lines. Because the efficiency of two-photon pair creation does not depend on the local magnetic field strength, it can continue to be active in the weak-field regions far from the neutron star. It is found that two-photon pair creation can strongly attenuate photons emitted from downward-traveling charges except at very high altitudes of emission, but in the absence of rotational aberration, it is unable to produce significant opacity for upward-traveling charges unless unrealistically high neutron star surface temperatures are assumed.Item Radiative Transfer of Polarized X-rays: Magnetized Thomson Scattering in Neutron Stars(2017-09-29) Barchas, Joseph; Baring, Matthew GThis thesis is a focused study of the polarization characteristics of radiative transfer in a strong magnetic field. The main process examined here is magnetized Compton scattering in a non-relativistic regime (i.e.\ magnetized Thomson scattering), and we focus on applying this study to predict polarization properties of the X-ray emission from magnetars. Magnetars are a highly magnetic sub-class of neutron stars, characterized by their extremely high surface magnetic fields, comparable to or exceeding the quantum critical field ($B_{\rm cr}\simeq4.41\times10^{13}$ Gauss) at which an electron's cyclotron energy and rest mass energy are equal. There are 29 known/candidate magnetars at this time, and they commonly exhibit persistent quasi-thermal surface emission in soft X-rays with flat tails extending into the hard X-rays up to around 150 keV, as well as transient bursting activity in hard X-rays attributed to magnetospheric flares. Magnetized Thomson scattering refers to electron-photon scattering in a background magnetic field. The field introduces anisotropy to the problem, giving it a more complicated angular dependence. It also produces a strong frequency dependence to the cross section: it is resonant at the cyclotron frequency $\omega_B=eB/mc$. Additionally, electron motion perpendicular to the field becomes increasingly suppressed at higher field strengths, leading to a reduction in the cross section for certain incoming photon angles when $\omega\ll\omega_B$. There are complicated polarization characteristics for the process as well, with the differential cross section depending on the initial and final polarization state of the photon. An important distinction occurs between photons that have a component of linear polarization parallel to the field and those that are fully orthogonal to it. We explore this process in detail using a Monte Carlo simulation model, treating the transfer primarily in slab geometries, a common simplification. This allows a direct comparison with previous work and is an important step towards achieving more complicated geometries and scattering regimes. We fully map the frequency and angular dependence of this process in the optically thick regime, capturing both resonant and non-resonant properties of the scattering. We present results for a model of magnetar persistent surface emission, as well as a simple magnetar flare model. In both cases the transfer is purely due to magnetic Thomson scattering, and we superimpose the emission from regions of different temperature, density, and magnetic field. For magnetar surface emission, we see a phase-dependent linear polarization, forming either a single- or double-peaked pattern with a maximum level of roughly $\sim25\%$, depending on the angle between the observer and the spin axis. This could have important implications for polarimetric determination of the effect of vacuum birefringence as polarized X-rays transfer through the magnetosphere to infinity. For the flare model we see much stronger polarization signals as the emission is coming from more localized regions, and it is highly dependent on viewing geometry and frequency. A secondary process is also examined due to its importance in magnetized plasmas: the so-called generalized Faraday effect. This is analogous to the ordinary Faraday effect, where the phase lag caused by birefringence of circular eigenmodes of electromagnetic waves produces a constant rotation of the plane of linear polarization for a propagating wave. The generalized effect occurs when the eigenmodes are no longer circular, and it produces a very similar rotation when viewed in terms of the Poincar\'e sphere. When this effect is assumed to be prolific, the transfer can be reformulated in terms of the normal modes in what is called the normal mode description. We explore the parameter space in which this description is valid, and develop a method to handle transfer in certain regimes where it is invalid. This prepares the way for incorporating such nuances in future developments of the magnetized radiative transfer problem.