Analysis, Prototyping, and Design of an Ionization Profile Monitor for the Spallation Neutron Source Accumulator Ring

The Spallation Neutron Source (SNS) located in the Oak Ridge National Laboratory is comprised of a 1 GeV linear H- [H^-] accelerator followed by an accumulator ring that delivers high intensity 1 μs [microsecond] long pulses of 1.5x1014 [1.5x10^14] protons to a liquid mercury target for neutron production by spallation reaction. With its strict 0.01% total beam loss condition, planned power upgrade, and proposed second target station, SNS ring beam-profile diagnostics capable of monitoring evolving beam conditions during high-power conditions are crucial for efficient operation and improvement. By subjecting ionized electrons created during beam interactions with the residual gas to a uniform electric field perpendicular to the beam direction, a profile may be collected based on the relation between measured ionized particle current and the beam density responsible for ionization. This form of nondestructive profile beam profile diagnostic known as an Ionization Profile Monitor (IPM). Introducing a magnetic field parallel to the electric field constrains the transverse particle motion to produce spatially accurate profiles. Presented in this work is the analysis and design of an IPM for the SNS ring capable of measuring turn-by-turn profiles with a 10% spatial accuracy for a fully accumulated high intensity proton beam. A theoretical framework is developed for the IPM operational principles and estimations for system design parameters are made based on calculations and measurement data. Detailed simulations are presented which are also used to determine design details and experimental results from a proof-of-principle IPM test chamber are reported and analyzed. Finally, a complete system design is presented based on the design criteria and simulation optimization that meets the required IPM system objectives

Spent-beam refocusing analysis and multistage depressed collector design for a 75-W, 59- to 64-GHz coupled-cavity traveling-wave tube

A computational design technique for coupled-cavity tubes (TWTs) equipped with spent-beam refocusers (SBRs) and multistage depressed collectors (MDCs) is described. A large-signal multidimensional computer program was used to analyze the TWT-SBR performance and to generate the spent-beam models used for MDC design. The results of a design involving a 75-W, 59 to 64 GHz TWT are presented. The SBR and MDC designs are shown, and the computed TWT, SBR, and MDC performances are described. Collector efficiencies in excess of 94 percent led to projected overall TWT efficiencies in the 40-percent range

Computational Studies of Molecular Materials for Unconventional Energy Conversion: The Challenge of Light Emission by Thermally Activated Delayed Fluorescence

In this paper we describe the mechanism of light emission through thermally activated delayed fluorescence (TADF)—a process able to ideally achieve 100% quantum efficiencies upon fully harvesting the energy of triplet excitons, and thus minimizing the energy loss of common (i.e., fluorescence and phosphorescence) luminescence processes. If successful, this technology could be exploited for the manufacture of more efficient organic light-emitting diodes (OLEDs) made of only light elements for multiple daily applications, thus contributing to the rise of a sustainable electronic industry and energy savings worldwide. Computational and theoretical studies have fostered the design of these all-organic molecular emitters by disclosing helpful structure–property relationships and/or analyzing the physical origin of this mechanism. However, as the field advances further, some limitations have also appeared, particularly affecting TD-DFT calculations, which have prompted the use of a variety of methods at the molecular scale in recent years. Herein we try to provide a guide for beginners, after summarizing the current state-of-the-art of the most employed theoretical methods focusing on the singlet–triplet energy difference, with the additional aim of motivating complementary studies revealing the stronger and weaker aspects of computational modelling for this cutting-edge technology.Computational resources were provided by: (i) the University of Alicante under Grant No. VIGROB-108; and (ii) the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifiques de Belgique (F.R.S.-FNRS) under Grant No. 2.5020.11

Characterisation of silicon nanoparticles produced by mechanical attrition using scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray photoemission spectroscopy

Includes abstract.Includes bibliographical references.The establishment of printing technologies, using nanoparticle based inks, promises inexpensive manufacture of electronic devices. However, to produce working devices, nanoparticles have to meet requirements on size, shape, and composition. In the application of silicon nanoparticles in electronics, it is important that a network of interconnecting particles is formed through which charge transport can take place. Of further importance is that there is an absence of surface oxide in order to maintain a direct silicon-silicon connection within the network. In this work, cheap and scalable production of silicon nanoparticles is achieved efficiently with a top-down process of mechanical attrition by high energy milling

The impact of complimentary advertising strategies on sponsored search advertisement

The aim of this research was to find relationships between complimentary advertising strategies and sponsored search advertisement (SSA) in order to formulate a model to maximise return on investment achieved from online sponsored search advertisements. The results obtained from statistical analyses of SSA campaign data showed that complimentary online and offline advertisement campaigns have various different correlations to impressions, click-through rates, number of pages visited, time spent visiting a website, bounce rate of visitors to the website, cost-per-click and number of new registrations per keyword search from visitors gained through SSA campaigns. In particular, online display advertisements were found to have a slight positive correlation with new registrations made by customers gained through a simultaneously running SSA campaign. Offline radio adverts were found to have a positive correlation with impressions gained for SSA campaigns, whilst at the same time showing a negative correlation with the number of pages viewed by website visitors obtained through the SSA campaign. Some negative correlations to SSA campaign performance were also found, with the time visitors spent viewing the website decreasing, their bounce rate increasing and the cost-per-clicks for the keywords in the SSA campaign also increasing during periods when offline radio adverts were active. Offline television adverts were found to have a negative correlation with impressions gained for SSA campaigns, as well as the click-through rate for the keywords in these SSA campaigns. Offline television adverts did however also show a negative correlation with the cost-per-clicks for keywords in the SSA campaigns. Finally, a graphical model was developed to illustrate these correlations found between complimentary advertisement campaigns and SSA performance metrics.Dissertation (MBA)--University of Pretoria, 2012.Gordon Institute of Business Science (GIBS)unrestricte

Investigating the antiparasitic potential of the marine sesquiterpene avarone, its reduced form avarol, and the novel semisynthetic thiazinoquinone analogue thiazoavarone

The chemical analysis of the sponge Dysidea avara afforded the known sesquiterpene quinone avarone, along with its reduced form avarol. To further explore the role of the thiazinoquinone scaffold as an antiplasmodial, antileishmanial and antischistosomal agent, we converted the quinone avarone into the thiazinoquinone derivative thiazoavarone. The semisynthetic compound, as well as the natural metabolites avarone and avarol, were pharmacologically investigated in order to assess their antiparasitic properties against sexual and asexual stages of Plasmodium falciparum, larval and adult developmental stages of Schistosomamansoni (eggs included), and also against promastigotes and amastigotes of Leishmania infantum and Leishmania tropica. Furthermore, in depth computational studies including density functional theory (DFT) calculations were performed. A toxic semiquinone radical species which can be produced starting both from quinone- and hydroquinone-based compounds could mediate the anti-parasitic effects of the tested compounds

Preliminary aerosol generator design studies

The design and construction of a prototype vaporization generator for highly dispersed sodium chloride aerosols is described. The aerosol generating system is to be used in the Science Simulator of the Cloud Physics Laboratory Project and as part of the Cloud Physics Laboratory payload to be flown on the shuttle/spacelab

Sttructure-Function Correlations of the Antagonist Effects of Flavonoids Present In Medicinal Plants on Oxidative Reactive Molecules

Flavonoids are phenolic compounds with significant antioxidant properties. The propensity of a flavonoid to inhibit free-radical mediated events is governed by its chemical structure. Since these compounds are based on the flavan nucleus, the number, positions, and types of substitutions influence radical scavenging and chelating activity. The main objective of this thesis was to establish structure-activity relationships of flavonoids by means of experimental and computational techniques. Initially, a series of dietary flavonoids belonging to the most representative families (flavonols; myricetin, quercetin, morin, kaempferol, and 3-hydrxoyflavone, flavones; flavone and flavanones; naringenin) were studied during the reaction with DPPH radical following addition of the flavonoid by UV -Vis spectrophotometry; they revealed two distinctive steps of reaction, a first rapid and a second slower. DPPH scavenging followed a second order kinetics during the rapid step; stoichiometric factors, rate constants as well as antiradical activities were determined. The DPPH radical allowed good discrimination between the flavonoids, as demonstrated by the relatively large ranges of rate constants (k = 10-10,000 M-1S-1), total stoichiometric factors (2-5) and antiradical activities (1-84%). Since the oxidizabi1ity of flavonoids reflects their ability to scavenge free radicals, the electrochemical oxidations of the 8 flavonoids were measured in different pH solutions using cyclic voltammetry. Flavone with no hydroxyl groups showed no oxidation potentials. Myricetin, quercetin, morin and kaempferol had the lowest oxidation potentials. This is in good agreement with the DPPH radical scavenging activities. Oxidation of flavonoids appeared to be pH dependent. Experimental studies revealed that the catechol structure in quercetin scavenged the highest number of DPPH radicals (4.44 ± 0.24) and exhibited the highest antiradical activity (84%). On the other hand, pyrogallol structure in myricetin had the lowest oxidation potential. A series of density functional theory calculations using Gaussian program for 28 flavonoids belonging to the major flavonoids\u27 families were carried out to establish the structural requirements of flavonoids for appreciable radical-scavenging activity. Energy of the same number and type of nuclei were compared. On the other hand, the dipole moments were compared for flavonoids of similar structures but different substituents i.e OCH3 and/or OH. Methoxy groups introduced unfavorable steric effects and therefore decreased the dipole moments of the studied flavonoids. Calculations of HOMO-LUMO gaps were performed to give insights of flavonoids\u27 reactivity. Flavonols exhibited the lowest HOMO-LUMO gap among all other classes in this study. Since chemical potential properties of flavonoids measure their tendency to give or capture electrons and therefore their antioxidant potential, these properties which include: electronic affinity (EA), ionization potential (IP), chemical potential (µ), electronegativity (χ), hardness (η) and electrophilicity (ω) were computed for all flavonoids in each class. Again, flavonols showed the lowest values among all classes which is another proof of their antioxidant ability. Structure-activity relationships are well established from density functional calculations. Multiple hydroxyl groups confer upon the molecule substantial antioxidant activity. Methoxy groups introduce unfavorable steric effects. A double bond and carbonyl function in the heterocycle of the nuclear structure increases activity by affording a more stable flavonoid radical through electron delocalization

Modeling the Resupply, Diffusion, and Evaporation of Cesium on the Surface of Controlled Porosity Dispenser Photocathodes

High quantum efficiency (QE) photocathodes are useful for many accelerator applications requiring high brightness electron beams, but suffer from short operational lifetime due to QE decay. For most photocathodes, the decrease in QE is primarily attributed to the loss of a cesium layer at the photocathode surface during operation. The development of robust, long life, high QE photoemitters is critically needed for applications demanding high brightness electron sources. To that end, a controlled porosity dispenser (CPD) photocathode is currently being explored and developed to replace the cesium during operation and increase photocathode lifetime. A theoretical model of cesium resupply, diffusion, and evaporation on the surface of a sintered wire CPD photocathode is developed to understand and optimize the performance of future controlled porosity photocathodes. For typical activation temperatures within the range of 500K--750K, simulation found differences of less than 5 % between the quantum efficiency (QE) maximum and minimum over ideal homogenous surfaces. Simulations suggest more variation for real cases to include real surface non uniformity. The evaporation of cesium from a tungsten surface is modeled using an effective one-dimensional potential well representation of the binding energy. The model accounts for both local and global interactions of cesium with the surface metal as well as with other cesium atoms. The theory is compared with the data of Taylor and Langmuir comparing evaporation rates to sub-monolayer surface coverage of cesium, gives good agreement, and reproduces the nonlinear behavior of evaporation with varying coverage and temperature