This discussion is concerned primarily with communications blackout during spacecraft entry into a planetary atmosphere. The gas in the shock layer, between shock wave and vehicle surface, ionizes from the intense heating which takes place in the bow shock wave and a viscous region of high gas enthalpy. This ionization may persist throughout the subsequent flow over the vehicle and into the wake, thus completely engulfing the vehicle and its communications elements. The problem will be to simulate a plasma model that will be of interest for hypervelocity reentry vehicles and to provide meaningful expressions for the various plasma parameters of interest (electron density, electron temperature, collision frequency, etc.) in terms of the microwave measurables (amplitude, phase shifts, frequency shifts, polarization, etc.)

No description available.

No description available.

by Wong Tong Pak. / Bibliography: leaves 188-189 / Thesis (M.Ph.)--Chinese University of Hong Kong, 1986

by Cheung Chin Tao. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1989. / Bibliography: leaf [67].

No description available.

The theoretical basis for using electron mirrors as correctors of chromatic aberration is presented and an experimental verification of correction of chromatic aberration is demonstrated. A hyperbolic electrostatic electron mirror operating in its converging range and at unity magnification was used as a corrector. A novel separating system with deflections taking place at image planes was developed to implement the mirror without impairing the resolution. Correction was demonstrated in an electron optical probe system. The chromatic aberration was measured by means of the shadows cast by a fine mesh placed near the final image. The experimental method and equipment are described. The experiment serves as a verification of the theory as well as a successful test of the method of separating the electron beams traveling to and from the mirror.

This thesis describes an experimental study of superelastic electron scattering from the 6^2P_3/2 state of caesium. The present status of electron-atom collision studies is initially reviewed and the motivation behind the current work is then presented. A description of the theoretical framework is subsequently provided in the context of the present experimental study, followed by an overview of the several theoretical approaches for describing electron-atom interactions which are currently available. The apparatus and experimental setup used throughout the project are also described in detail. Technical specifications and data are provided, including diagrams (where appropriate) for a laser frequency locking system, electron gun and spectrometer, atomic beam source and data acquisition system. The experimental procedures are explained and discussed, including a detailed analysis of the optical pumping process required to excite the atomic target. A substantial component of this project was to address several potential sources of systematic error and to reduce these wherever possible. All of the errors and uncertainties relevant to the experiment are discussed in chapter 5. In chapter 6 the results of the present superelastic electron scattering experiments are reported for incident electron energies of 5.5eV, 8.5eV and 13.5eV, corresponding to superelastic electron energies of 7eV, 10eV and 15eV. These results are presented as three reduced Stokes parameters, P1, P2, P3 and a coherence parameter, P+ . For comparison, predictions from a number of currently available theories are presented alongside the experimental results. Finally, conclusions are drawn on this work in the context of the current status of electron-atom scattering from alkali-metals.

In this thesis we develop Bragg scattering as a tool for probing and manipulating ultra-cold atoms. Our approach is based on a mean-field treatment of degenerate quantum gases. Bose-Einstein condensates are described by the Gross-Pitaevskii equation and degenerate Fermi gases are described by the Bogoliubov-de-Gennes equations. Our work is presented in three inter-related topics. In Part I we investigate Bose-Einstein condensation in a time-averaged orbiting potential trap by deriving solitary-wave dynamical eigenstates of the system. We invoke the quadratic average approximation in which the dynamic effects of the time-dependent potential can be described simply, even when accounting for atomic collisions. By deriving the transformation to the translating frame, dynamical eigenstates of the system are defined and those states are solitary-wave solutions in the laboratory frame, with a particular circular centre-of-mass motion independent of the strength of the collisional interactions. Our treatment in the translating frame is more general than previous treatments that use the rotating frame to define system eigenstates, as the use of the rotating frame restricts eigenstates to those that are cylindrically symmetric about their centre of mass. In Part II we describe Bragg spectroscopy of a condensate with solitary-wave motion. Our approach is based on a momentum space two-bin approximation, derived by Blakie et al. [Journal of Physics B 33:3961, 2000] to describe Bragg scattering of a stationary condensate. To provide an analytic treatment of Bragg scattering of a solitary-wave condensate we use the translating frame, in which the time dependence of the system is described entirely by a time-dependent optical potential. We derive a simplified treatment of the two-bin approximation that provides a physical interpretation of the Bragg spectrum of a solitary-wave condensate. Our methods are applied to Bragg spectroscopy of a condensate in a time-averaged orbiting potential trap, which accelerates as a solitary wave as derived in Part I. The time-averaged orbiting potential trap system is ideal for testing our approximate analytic methods because the micromotion velocity is large compared to the condensate momentum width. In Part III we present a theoretical treatment of Bragg scattering of an ultra-cold Fermi gas. We give the first non-perturbative numerical calculations of the dynamic behaviour of a degenerate Fermi gas subjected to an optical Bragg grating. We observe first order Bragg scattering, familiar from Bragg scattering of stationary Bose-Einstein condensates, and at lower Bragg frequencies we predict scattering of Cooper pairs into a correlated spherical shell of atoms. Correlated-pair scattering is associated with formation of a grating in the pair potential. We give an analytic treatment of Bragg scattering of a homogeneous Fermi gas, and develop a model that reproduces the key features of the correlated-pair Bragg scattering. We discuss the effect of either a trapping potential or finite temperature on the correlated-pair Bragg scattering.

Thesis (M. Phil.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.