[en] Above-Threshold Ionization (ATI) of atoms in a new phenomenon in intense field laser-atom interactions. The emergence of this new phenomenon challenges the established lowest-order perturbation theory in the theoretical study of multiphoton processes. In this thesis, a dressed-state perturbation theory is presented, which treats the dominant terms in the calculation of ATI cross sections non-perturbatively. The field influence on atomic energy levels is studied to give a quantitative measure of the validity of the lowest-order perturbation theory. The field influence on the measurement of the atomic ionization threshold and the photoelectron energy spectrum is discussed. In addition, a statistical model is proposed to study the photoelectronenergy spectrum

[en] Accurate and efficient simulation of chemical transport processes in the unsaturated zone of Yucca Mountain is important to evaluate the performance of the potential repository. The scale of the unsaturated zone model domain for Yucca Mountain (50 km² area with a 600 meter depth to the water table) requires a large gridblock approach to efficiently analyze complex flow and transport processes. The conventional schemes based on finite element or finite difference methods perform well for dispersion-dominated transport, but are subject to considerable numerical dilution/dispersion for advection-dominated transport, especially when a large gridblock size is used. Numerical dispersion is an artificial, grid-dependent chemical spreading, especially for otherwise steep concentration fronts. One effective scheme to deal with numerical dispersion is the random walk particle method (RWPM). While significant progress has been made in developing RWPM algorithms and codes for single continuum systems, a random walk particle tracker, which can handle chemical transport in dual-continua (fractured porous media) associated with irregular grid systems, is still absent (to our knowledge) in the public domain. This is largely due to the lacking of rigorous schemes to deal with particle transfer between the continua, and efficient schemes to track particles in irregular grid systems. The main objectives of this study are (1) to develop approaches to extend RWPM from a single continuum to a dual-continua system; (2) to develop an efficient algorithm for tracking particles in 3D irregular grids; and (3) to integrate these approaches into an efficient and user-friendly software, DCPT, for simulating chemical transport in fractured porous media

[en] This work presents modeling studies investigating the effects of capillary barriers on fluid-flow and tracer-transport processes in the unsaturated zone of Yucca Mountain, Nevada, a potential site for storing high-level radioactive waste. These studies are designed to identify factors controlling the formation of capillary barriers and to estimate their effects on the extent of possible large-scale lateral flow in unsaturated fracture rocks. The modeling approach is based on a continuum formulation of coupled multiphase fluid and tracer transport through fractured porous rock. Flow processes in fractured porous rock are described using a dual-continuum concept. In addition, approximate analytical solutions are developed and used for assessing capillary-barrier effects in fractured rocks. This study indicates that under the current hydrogeologic conceptualization of Yucca Mountain, strong capillary-barrier effects exist for significantly diverting moisture flow

[en] During the outage of some CANDU steam generators, eddy current inspection detected volumetric indications in the tubes in the top-of-tubesheet region (TTS). The majority of the indications were found to be pit-type defects based on a signal-to-signal analysis of previous inspection data. This investigation studies the pit growth by collecting individual pit size change from data collected during two successive outages and calculating the growth rates and their confidence intervals using statistical methods. The pits at the TTS are divided into separate populations according to their locations and statistical test of homogeneity. The mean growth rate, 95% upper bound growth rate, and their confidence intervals of each population are evaluated using the Bootstrap resampling method. A method is also proposed to lower the 95% upper bound growth rate by adjusting for the effect of the eddy current measurement error with a Gamma growth model. (author)

[en] Photoconductive detectors fabricated from natural lla diamonds have been used to measure the x-ray power emitted from laser produced plasmas. The detector was operated without any absorbing filters to distort the x-ray power measurement. The 5.5 eV bandgap of the detector material practically eliminates its sensitivity to scattered laser radiation thus permitting filterless operation. The detector response time or carrier life time was 90 ps. Excellent agreement was achieved between a diamond PCD and a multichannel photoemissive diode array in the measurement of radiated x-ray power and energy. 4 figs

No abstract available

[en] This work is a part of the study originally developed by a group at Brown Univ. The main purpose of the present paper is to give a completed example to illustrate the geometric topological analysis of the solutions of two-phase flow by the use of homogeneous model. Through the geometric topological analysis it is possible for us to predict the solutions of the differential equations and determine the location and type of the singular points thus figure out the global structure of the family of the solution curves before solving out the equations. The structure of the solution curves is topologically similar to the original non-linear model. The advantage of such an analysis is fully demonstrated in this paper

[en] Using a Sturmian function expansion we have computed the nth order coefficients E/sub n/(ω), for 2≤≤2, in the perturbation expansion of the ac Stark effect in the hydrogen 1s state. An ''effective convergence'' similar to that in the dc case is observed. A parametrization of these coefficients, based upon the analytic structure of the Coulomb Green's function, separates the rapid oscillatory resonant behavior and the smooth background variation with respect to ω

[en] Fretting wear is a potentially significant degradation mechanism in nuclear steam generators and other shell and tube heat transfer equipment as well. This paper presents an overview of the recently developed code FIVDYNA which is used for the non-linear flow-induced vibration and fretting wear analysis for operating steam generators (OTSG and RSG) and shell-and-tube heat exchangers. FIVDYNA is a non-linear time-history Flow-Induced Vibration (FIV) analysis computer program that has been developed by Babcock and Wilcox Canada to advance the understanding of tube vibration and tube to tube-support interaction. In addition to the dynamic fluid induced forces the program takes into account other tube static forces due to axial and lateral tube preload and thermal interaction loads. The program is capable of predicting the location where the fretting wear is most likely to occur and its magnitude taking into account the support geometry including gaps. FIVDYNA uses the general purpose finite element computer code ABAQUS as its solver. Using ABAQUS gives the user the flexibility to add additional forces to the tube ranging from tube preloads and the support offsets to thermal loads. The forces currently being modeled in FIVDYNA are the random turbulence, steady drag force, fluid-elastic forces, support offset and pre-strain force (axial loads). This program models the vibration of tubes and calculates the structural dynamic characteristics, and interaction forces between the tube and the tube supports. These interaction forces are then used to calculate the work rate at the support and eventually the predicted depth of wear scar on the tube. A very good agreement is found with experiments and also other computer codes. (author)

[en] Photoconductive detectors (PCDs) fabricated from natural IIa diamonds have been used to measure the x-ray power emitted from laser-produced plasmas. The detector was operated without any absorbing filters which distort the x-ray power measurement. The 5.5 eV band gap of the detector material practically eliminates its sensitivity to scattered laser radiation thus permitting filterless operation. Excellent agreement was achieved between a diamond PCD and a multichannel photoemissive diode array in the measurement of radiated x-ray power and energy