A general theory of electron detachment in negative ion collisions

In this thesis a general theory of electron detachment in slow collisions of negative ions with atoms is presented. The theory is based upon a semiclassical close-coupling framework, following the work of Taylor and Delos. The Schrodinger equation is reduced, under certain assumptions, to a non-denumerably infinite set of coupled equations. We develop a new method for solving these equations that is more general than the methods used by Taylor and Delos. A zero-order approximation of our solution is applied to the case of H(\u27-)(D(\u27-)) on Ne collisions, the results are compared with the experimental data, and we find good agreement between theory and experiment, particularly with regard to the isotope effect. A first-order approximation of the solution is proved to be very close to the exact solution, and it is applied to the case of H(\u27-)(D(\u27-)) on He collisions. We use quadratic and quartic approximations for the energy gap (DELTA)(t) to calculate, among other things, the survival probability and electron energy spectrum. There are some interesting results for the electron energy spectrum which have not yet been observed in experiments

An examination of the effects of neutral particles on the edge plasma in tokamaks

A detailed analysis of neutral atom recycling and pedestal fueling in a DIII-D [J. Luxon, Nucl. Fusion, 42, 614, 2002] high-confinement mode discharge is presented. Experimental data and 2D edge plasma fluid code calculations are employed to provide ion wall recycling and recombination neutral sources and background edge plasma parameters for a 2D edge neutral code calculation of detailed neutral density, ionization and charge-exchange distributions throughout the edge pedestal, scrape-off layer and surrounding halo region, divertor, and private flux regions. The effectiveness of the different neutral sources for fueling the confined plasma is evaluated.Ph.D.Committee Chair: Weston M. Stacey, Jr.; Committee Member: C.-K. Chris Wang; Committee Member: Farzad Rahnema; Committee Member: Nolan E. Hertel; Committee Member: Richard Groebner; Committee Member: Thomas Barker; Committee Member: Yunfeng Lian

Simulations of small-scale turbulent dynamo

We report an extensive numerical study of the small-scale turbulent dynamo at large magnetic Prandtl numbers Pm. A Pm scan is given for the model case of low-Reynolds-number turbulence. We concentrate on three topics: magnetic-energy spectra and saturation levels, the structure of the field lines, and the field-strength distribution. The main results are (1) the folded structure (direction reversals at the resistive scale, field lines curved at the scale of the flow) persists from the kinematic to the nonlinear regime; (2) the field distribution is self-similar and appears to be lognormal during the kinematic regime and exponential in the saturated state; and (3) the bulk of the magnetic energy is at the resistive scale in the kinematic regime and remains there after saturation, although the spectrum becomes much shallower. We propose an analytical model of saturation based on the idea of partial two-dimensionalization of the velocity gradients with respect to the local direction of the magnetic folds. The model-predicted spectra are in excellent agreement with numerical results. Comparisons with large-Re, moderate-Pm runs are carried out to confirm the relevance of these results. New features at large Re are elongation of the folds in the nonlinear regime from the viscous scale to the box scale and the presence of an intermediate nonlinear stage of slower-than-exponential magnetic-energy growth accompanied by an increase of the resistive scale and partial suppression of the kinetic-energy spectrum in the inertial range. Numerical results for the saturated state do not support scale-by-scale equipartition between magnetic and kinetic energies, with a definite excess of magnetic energy at small scales. A physical picture of the saturated state is proposed.Comment: aastex using emulateapj; 32 pages, final published version; a pdf file (4Mb) of the paper containing better-quality versions of figs. 5, 8, 12, 15, 17 is available from http://www.damtp.cam.ac.uk/user/as629 or by email upon request