Monte Carlo modeling of low-energy electron-induced secondary electron emission yields in micro-architected boron nitride surfaces

Surface erosion and secondary electron emission (SEE) have been identified as the most critical life-limiting factors in channel walls of Hall-effect thrusters for space propulsion. Recent wall concepts based on micro-architected surfaces have been proposed to mitigate surface erosion and SEE. The idea behind these designs is to take advantage of very-high surface-to-volume ratios to reduce SEE and ion erosion by internal trapping and redeposition. This has resulted in renewed interest to study electron-electron processes in relevant thruster wall materials. In this work, we present calculations of SEE yields in micro-porous hexagonal BN surfaces using stochastic simulations of electron-material interactions in discretized surface geometries. Our model consists of two complementary parts. First we study SEE as a function of primary electron energy and incidence angle in flat surfaces using Monte Carlo simulations of electron multi-scattering processes. The results are then used to represent the response function of discrete surface elements to individual electron rays generated using a ray-tracing Monte Carlo model. We find that micro-porous surfaces result in SEE yield reductions of over 50% in the energy range experienced in Hall thrusters. This points to the suitability of these micro-architected surface concepts to mitigate SEE-related issues in compact electric propulsion devices

Design, fabrication, and operation of two broadband force balance seismometers

The measurement of ground motion is important for a wide range of fields. In physics, advanced experiments can involve precise positioning of components. In civil engineering, engineers need to know the characteristics of ground motion to better design large scale structures, and the study of ground motion form distant earth quakes help geologists understand the structure and dynamics of the earth. Each application requires instruments of different specifications. In this thesis I describe the design, fabrication, assembly, and operation of two broad band force balance seismometers and the associated control software. The design, control elements, and methods used in this project can be extended to other applications were specific criteria are needed in the development of custom seismic sensors. A proportional, integral, and derivative (PID) control scheme was written for the negative feedback loop. Along with the control software, I include a user interface to control the feedback and assist in loop tuning. Closed loop operation of each seismometer was successfully accomplished and the step responses were compared to the step response of an ideal model of the seismometers developed in software. Three parameters are useful in the description of a step response: the settling time, overshoot, and deadtime. The ideal model step response has a settling time of 0.09 seconds and an overshoot of less than 30%. The seismometers exhibit settling times of 1 second and 0.5 seconds and overshoots of 20% and 10%. The ideal model does not exhibit a deadtime but the actual seismometer deadtime was just 30 ms

Sliver Solar Cells

Sliver solar cells are thin, mono-crystalline silicon solar cells, fabricated using micro-machining techniques combined with standard solar cell fabrication technology. Sliver solar modules can be efficient, low cost, bifacial, transparent, flexible, shadow-tolerant, and lightweight. Sliver modules require only 5 to 10% of the pure silicon and less than 5% of the wafer starts per MWp of factory output when compared with conventional photovoltaic modules. At ANU, we have produced 20% efficient Sliver solar cells using a robust, optimised cell fabrication process described in this paper. We have devised a rapid, reliable and simple method for extracting Sliver cells from a Sliver wafer, and methods for assembling modularised Sliver cell sub-modules. The method for forming these Sliver sub-modules, along with a low-cost method for rapidly forming reliable electrical interconnections, are presented. Using the sub-module approach, we describe low-cost methods for assembling and encapsulating Sliver cells into a range of module designs

Algorithms for Kullback-Leibler Approximation of Probability Measures in Infinite Dimensions

In this paper we study algorithms to find a Gaussian approximation to a target measure defined on a Hilbert space of functions; the target measure itself is defined via its density with respect to a reference Gaussian measure. We employ the Kullback-Leibler divergence as a distance and find the best Gaussian approximation by minimizing this distance. It then follows that the approximate Gaussian must be equivalent to the Gaussian reference measure, defining a natural function space setting for the underlying calculus of variations problem. We introduce a computational algorithm which is well-adapted to the required minimization, seeking to find the mean as a function, and parameterizing the covariance in two different ways: through low rank perturbations of the reference covariance; and through Schr\"odinger potential perturbations of the inverse reference covariance. Two applications are shown: to a nonlinear inverse problem in elliptic PDEs, and to a conditioned diffusion process. We also show how the Gaussian approximations we obtain may be used to produce improved pCN-MCMC methods which are not only well-adapted to the high-dimensional setting, but also behave well with respect to small observational noise (resp. small temperatures) in the inverse problem (resp. conditioned diffusion).Comment: 28 page

Autopoiesis: Self-Creation in Nietzsche

A recurrent theme in the philosophy of Friedrich Nietzsche is his imperative that we must create ourselves. Though this theme of self-creation runs throughout the entirety of his published works, Nietzsche neither fully articulates in one place the processes and guidelines by which such self-creation could occur, nor does he fully resolve the paradoxes inherent in this concept. This paper attempts to distill from these fragments a coherent interpretation of both how we can and why we should, despite (or, paradoxically, because of) our many external and internal constraints, fashion ourselves the way an artist shapes a work of art

The Effects of Changes in Water Content on Uranium(VI) Leaching in Sediment Mixtures Containing Gravel

This study is aimed at understanding the physical and chemical effects that changes in water content have on uranium leaching in sediment containing gravel. It was hypothesized that leaching will be more efficient under unsaturated conditions because flow will be restricted to the smallest pores and will have the most contact with the uranium contaminated sediment. Under saturated conditions, a large portion of the flow will bypass the \u3c 2 mm material, and in turn not come into contact with uranium contaminated material. Batch adsorption and desorption experiments were performed on \u3c 2 mm ERDF sediment to determine the linearity and reversibility of sorption processes and to aid in the interpretation of the leaching experiments. Results of the desorption experiments on aged, contaminated sediments show that the mass percent of sorbed U(VI) released to solution decreased as the sorbed concentration of U(VI) decreased. The opposite trend was observed on freshly contaminated sediments. This indicated that aging increased U(VI) affinity for the solid phase and was attributed to either the crystallization of calcite, which incorporated a portion of the sorbed U(VI) as it crystallized, or the presence of voids in basaltic lithic fragments accessed by diffusion. Column leaching experiments were performed at two water contents on artificially contaminated sediment collected from the Department of Energy’s (DOE) Hanford Site, Washington state. The sediment contained 81.3% gravel (\u3e 2 mm) by mass. Non-reactive tracers were well fit with the convection-dispersion equation (CDE) at both high and low water contents indicating physical equilibrium. The column experimental data were fitted to an analytical solution to the CDE; the results of the modeling show an increase in the distribution coefficient (Kdeffective) with decreasing water content. Several potential explanations for this trend were proposed; one is based on a physical effect in which solute exposure to reactive surfaces changes as a function of water content and the others are based on results of the batch desorption experiments. This work has important implications for the Hanford Site where there is ongoing research regarding the persistence of U(VI) in the vadose zone and underlying aquifer