3D Simulation of the Effects of Surface Defects on Field Emitted Electrons

The ev­er-grow­ing de­mand for high­er beam en­er­gies has dra­mat­i­cal­ly in­creased the risk of RF break­down, lim­it­ing the max­i­mum achiev­able ac­cel­er­at­ing gra­di­ent. Field emis­sion is the most fre­quent­ly en­coun­tered RF break­down where it oc­curs at re­gions of lo­cal­ly en­hanced elec­tric field. Elec­trons ac­cel­er­at­ed across the cav­i­ty as they tun­nel through the sur­face in the pres­ence of mi­cro­scop­ic de­fects. Upon Im­pact, most of the ki­net­ic en­er­gy is con­vert­ed into heat and stress. This can in­flict ir­re­versible dam­age to the sur­face, cre­at­ing ad­di­tion­al field emis­sion sites. This work aims to in­ves­ti­gate, through sim­u­la­tion, the physics in­volved dur­ing both emis­sion and im­pact of elec­trons. A newly de­vel­oped 3D field model of an 805 MHz cav­i­ty is gen­er­at­ed by COM­SOL Mul­ti­physics. Elec­tron track­ing is per­formed using a Mat­lab based code, cal­cu­lat­ing the rel­e­vant pa­ram­e­ters need­ed by em­ploy­ing fourth Order Runge Kutta in­te­gra­tion. By study­ing such be­haviours in 3D, it is pos­si­ble to iden­ti­fy how the cav­i­ty sur­face can alter the local RF field and lead to break­down and sub­se­quent dam­ages. The ul­ti­mate aim is to in­tro­duce new sur­face stan­dards to en­sure bet­ter cav­i­ty per­for­mance

A modeling study on lateral compressive behavior of structured needle - punched nonwovens

The aim of this work is to study the compression behavior of cord-structured needled fabrics. In order to achieve the objectives of this research, melt-spun polypropylene fibres of various linear densities have been produced by varying the spinning pump speed. The effect of linear density of melt-spun polypropylene fibres on compressive behavior of the fabrics has been investigated. A Zwick tester set in compressive mode is used to obtain variation of compressive force versus compression thickness. Van Wyk law is adapted to model the lateral compressive behavior of cord-structured needled fabrics. The experimental results are compared with the theoretical curves calculated using Van Wyk law. Results show that the increase in linear density of fibres tends to reduce the compressive force required to compress the samples to a pre-defined thickness. An acceptable conformity between Van Wyk’s equation and compressive behavior of cord-structured needled fabrics is also observed

THE EFFECTS OF FIELD EMITTED ELECTRONS ON RF SURFACE

The ever-growing demand for higher RF gradients has considerably increased the risk of breakdown in accelerating structures. Field emission is the most common form of RF breakdown that generates free electrons capable of inflicting irreversible damages on the RF surface. This paper presents a systematic experimental and simulation programme to understand possible sources and their influence on RF cavity operatio

An experimental investigation into the ‘fate’ of entrained peritectic minerals in I-type granite magmas intruded at below 2kbar.

Thesis (MSc)--Stellenbosch University, 2016.ENGLISH ABSTRACT: Entrainment of a peritectic mineral assemblage, formed through the incongruent melting of biotite and hornblende in a source of intermediate composition, has been proposed to account for the fact that I-type granites are commonly more mafic in composition than the melts from which they are derived. Magma consisting of variable proportions of melt and a distinct peritectic assemblage (Pl + Cpx + Opx + Ilm ± Grt) has been proposed to explain the substantial compositional range of I-type granitic rocks. Geochemical evidence to support the entrainment of a peritectic assemblage includes the strong, positive correlation between Ti vs maficity (atomic Fe + Mg) (with correlation coefficients typically higher than 0.9 for suites from individual plutons), as well as the very weak correlation between K and maficity in all granitic rocks. Despite this compelling geochemical evidence for the entrainment of a peritectic mineral assemblage, there is distinct lack of both mineral textural and mineral compositional evidence of these peritectic phases within I-type granites. In order to ascertain the ‘fate’ of the proposed entrained peritectic phases; the mechanisms and reactions by which these phases equilibrate with the surrounding magma, as well as the corresponding rates of these reactions needs to be established. This research used experimental techniques to determine the kinetic processes involved in the digestion of a high-pressure peritectic assemblage within a granodioritic magma under plutonic conditions within the upper crust (~2kbar and 763 to 720°C). A synthetic silicate gel (representative of the melt fraction) was mixed with a hypothetical, natural peritectic mineral assemblage (13.1Pl + 9.3Cpx + 1.1Opx + 0.8Ilm ± 5.7Grt) in a 7:3 ratio to form the experimental starting material. The peritectic mineral assemblage comprised of natural minerals of suitable compositions that were crushed and sieved to produce crystals with an average 200μm - 600μm size range. The 1.60 wt. % water within the starting composition was accommodated as kaolinite, which was added to the completed silicate gel in the correct stoichiometric proportions. The overall mineral-melt mixture was intended to represent the bulk composition of an I-type granodioritic magma produced by the fluid-absent melting of biotite and hornblende within an intermediate source composition at 10kbar and 850 - 900°C. The starting material (melt + mineral mixture) was loaded into gold capsules, which were then welded shut. The material was then reacted at pressures of 1.90kbar and 1.40kbar and at temperatures of 763 – 723°C within a cold-seal pressure vessel, with each experiment lasting 10 days. The findings of this research identified two principal reaction processes by which the high-pressure peritectic minerals react out within the experimental run products, namely; dissolution-precipitation and mineral-melt reactions. A coupled dissolution-precipitation mechanism is proposed to account for the rapid re-equilibration of ‘peritectic’ phases that are predicted (by phase equilibrium modelling) to be stable, but out of compositional equilibrium with the surrounding magma at the investigated PT conditions. ‘Peritectic’ plagioclase was seen to dissolve completely and reprecipitate a more albitic composition across all experimental pressure and temperature conditions, and within the experimental run-time (10 days). Consequently, a coupled dissolution-precipitation mechanism is inferred to be a sufficiently rapid and efficient process to account for the predominantly magmatic origin of feldspathic crystals within natural granitic rocks. At corresponding PT conditions, unstable mineral phases were seen to change both compositionally and texturally according to a separate reaction process, i.e. reaction with the surrounding melt portion. High pressure and temperature ferromagnesian minerals, such as garnet and orthopyroxene, which were not stable phases in the magma at the condition of the experiments were seen to react with the melt to produce reaction rims of biotite, which was a stable phase in the magma. The rapid growth rate of biotite resulting from this process suggests phaneritic textural features typically associated with granite bodies do not require long residency times to form within upper crustal magma chambers. The persistence of both garnet and orthopyroxene at the low pressure (1.40 – 1.90kbar) and temperature (763 - 720°C) conditions of experimentation indicates that at these conditions the mineral-melt reactions were too ‘sluggish’ to completely digest these phases. The rapid dissolution-precipitation rates of plagioclase within the experiments suggests that where garnet and orthopyroxene crystals are evident in natural granitic rocks these minerals bear no compositional or textural resemblance to the originally entrained ‘peritectic’ phases. The findings of this research corresponds well with the theory that I-type granites’ compositional trends form as a consequence of a peritectic mineral assemblage (Pl + Cpx + Opx + Ilm ± Grt) being entrained within a leucocratic melt. However, this research has far-reaching implications in terms of other formational theories, such as; fractional crystallization, magma mixing at depth and restite entrainment, that similarly involve the production of crystal-rich magmas at depth. Regardless of which process is responsible for generating the vast compositional heterogeneity of I-type granitic rocks, if the temperature conditions of the granitic magmas remain sufficiently high (>700°C) the magma will have the propensity to either partially or fully equilibrate any crystal component that is not in equilibrium with the surrounding magma.AFRIKAANSE OPSOMMING: Optel-en-meevoer van ‘n peritektiese mineraal versameling gevorm deur die ongelykvormige smelting van biotiet en hornblende in ‘n bron van intermediêre samestelling, was voorgestel om die I-tiepe graniet in ag te neem. Die I-tiepe graniet is oor die algemeen meer mafies in samestelling as wat smelt is. Magma bestaan uit verskillende hoeveelhede van smelt en ‘n spesifieke peritektiese versameling (Pl + Cpx + Opx + Ilm ± Grt) was voorgestel om die aansienlike komposisionele reeks van I-tiepe graniet smelt te verduidelik. Geochemiese bewyse om die optel-en-meevoer van die peritektiese versameling te ondersteun sluit in ‘n sterk, positiewe korrelasie tussen die Ti vs mafiese (atomies Fe + Mg) (met ‘n korrelasiekoëffisiënt wat gewoontlik hoër as 0.9 is vir rock groepe van individuele plutone) asook die baie swak korrelasie tussen K en mafiese in alle graniet klippe. Ten spyte van hierdie geochemiese bewyse van die optel-en-meevoer van die peritektiese mineraal versameling, is daar ‘n duidelike gebrek van bewyse vir beide mineraal tekstuur en mineraal komposisie van hierdie peritektiese fases in die I-tiepe graniet. Om die ‘lot’ van die voorgestelde optel-en-meevoer peritetiese fases te bepaal, moet die meganismes en reaksies van hierdie ewewigsfases, sowel as die magma in die omliggende en hul ooreenstemmende tempos, bepaal word. Hierdie navorsing het van eksperimentele tegnieke gebruik gemaak om die kinetiese prosesse te bepaal. Die prosesse is betrokke in die vertering van ‘n hoë druk perifiese mineraal versameling in ‘n granodioritiese magma onder plutoniese omstandighede in die boonste kors (~2kbar en 763 tot 720°C). ‘n Sintetiese silikahouende gel (verteenwoordiger van die smelt fraksie) was gemeng met ‘n hipotetiese, natuurlike peritektiese mineraal versameling (13.1Pl + 9.3Cpx + 1.1Opx + 0.8Ilm ± 5.7Grt) in ‘n 7:3 verhouding om die eksperimentele begin materiaal te vorm. Die peritektiese mineraal versameling bestaan uit natuurlike minerale van geskikte komposisies wat vergruis en gesif was sodat die kristalle ‘n gemiddeld grootte van 200μm - 600μm het. Die 1.6 wt. % water in die begin komposisie was geakkommodeer as kaoliniet, wat bygelas was om die silikahouende gel in die regte stoïgiometriese proporsies volledig te maak. Die hele mineraal mengsel was bedoel om die mederheid komposisies van ‘n I-tiepe granodiorite magma voor te stel en was geproduseer by die vloeistof-afwesige smelting van biotiet en hornblende in ‘n intermediêre bron komposisie by 10 kbar en 850 - 900°C. Die begin materiaal (smelt + mineraal mengsel) was in ‘n goue capsule gelaai, wat dan toe gesweis was. Die materiaal het gereageer by ‘n druk van 1.9 kbar en 1.4 kbar en by die temperatuur van 763 - 720°C wat in ‘n koue geseëlde drukvat geplaas was met elke eksperiment wat 10 dae lank geduur het. Die resultate van hierdie navorsing identifiseer twee beginsel reaksie prosesse, ontbinding-neerslag en minerale-smelt hidrasie reaksies, waar die hoë druk peritektiese minerale reageer in die eksperimentele resultante. ‘n Gekoppelde oplossings-presipitasie meganisme is voorgestel om die vinnige herewewigsproses van peritektiese fases in ag te neem wat voorspel is (deur fase ewewig modellering) om stabiel te wees, maar uit komposisionele ewewig saam met die magma in die omliggende gebied by die ondersoekte PT omstandighede. Peritektiese plagioklaas was gesien en het heeltemal opgelos en neerslag ‘n meer albeitiese komposisie oor al die eksperimentele druk en temperatuur omstandighede, asook in die eksperimentele looptyd (10 dae). Gevolglik is ‘n gekoppelde oplossings-presipitasie meganisme afgelei as ‘n effektiewe proses teen voldoende tempo om die oorwegende magmatiese oorsprong van feldspatiese kristalle in natuurlike graniet klip waar te neem. By ooreenkomstige PT omstandighede was daar onstabiele mineraal fases gesien wat volgens die afsonderlike reaksie fase in beide komposisionele en teksturele minerale verander het, bv. reaksies met die omgewind smelt proporsies. Hoë druk en temperatuur minerale met hoë Fe en Mg konsentrasies, soos granaat en ortopirokseen, was nie in stabiele fases in die magma by die omstandighede van die eksperiment nie en waargeneem dat dit reageer met die smelt om reaksie rande van ‘n stabiele biotiet fases te vorm. Dit was waargeneem dat ortopirokseen in laer druk eksperimente (1.40 kbar) reageer met die omliggende smelt proporsie om biotiet vervang ortopirokseen te vorm teen ‘n spoed van omtrent vier keer stadiger as die herewewig van peritektiese plagioklaas by die oplossings-presipitasie proses. Die vinnige groeitempo van biotiete, as gevolg van hieride proses, stel voor dat ‘phaneritic’ teksturele funksies wat tipies geassosieer word met graniet liggame, vereis nie lang verblyf tye om te vorm in die boonste kristal magma kamers nie. Die volharding van beide granaat en ortopirokseen by die lae druk (1.40 – 1.90kbar) en temperatuur (763 - 720°C) omstandighede van eksperimentasie toon dat by hieride omstandighede van eksperimentasie die mineraal-smelt hidrasie reaksies was te trae om volledig te verteer in hierdie fases. Die vinnige oplossings-presipitasie tempos van plagioklaas in die eksperimente dui duidelik aan dat granaat en ortopirokseen kristalle in natuurlike graniet klippe is. Hierdie minerale hou geen komposisionele of teksturele ooreenkoms met die oorspronklike opgetel en weggedra peritektiese fases nie. Die resultate van die navorsing stem goed ooreen met die teorie dat I-tiepe graniete komposisionele tendense vorm as ‘n gevolg van ‘n peritektiese mineraal versameling (Pl + Cpx + Opx + Ilm ± Grt) wat opgetel en weggedra is in ‘n leukokratiese smelt. Maar hierdie navorsing het verreikende gevolge in terme van ander magmatiese teorieë, bv. fraksionele kristalisering, magma vermenging in diepte en die optel-en-meevoer van die oorblywende minerale uit die bron, wat soortgelyk die produksie van kristalryk magmas in diepte insluit. Ongeag van watter van hierdie prosesse verantwoordelik is vir die generering van die oorgrote komposisionele heterogeniteit van I-tiepe graniet klippe, as die temperatuur omstandighede van die graniet magmas by voldoende hoë hoeveelhede bly (>700°C), dan sal die magma die geneigdheid hê om gedeeltlik of heeltemal ewewig te bereik met enige kristal komponent wat nie in komposisionele ewewig met die omliggende magma is nie

Prediction of heat transfer and air permeability properties of light weight nonwovens using artificial intelligence

Effects of pore sizes and distribution of pore sizes of light weight spunlace nonwovens on the heat transfer and air permeability of these fabrics have been studied. Image analysis has been applied to extract the geometrical features of the cross-section of spunlace samples (pore sizes and distribution of pore sizes) at the different layers in the thickness direction. A neural network model is also developed for the prediction of heat transfer and air permeability with respects to structural properties of light weight nonwovens. Results show that the increase in pore sizes and distribution factor of pore sizes increases the air flow rate and heat transfer properties of the nonwoven fabrics respectively. The neural network model also predicts the air permeability and heat transfer of nonwovens in terms of the measured geometrical properties

High pressure injection technique for hypochlorite treatment of polysulfone hollow fibre membranes

High pressure injection technique for hypochlorite treatment of polysulfone hollow fibre membranes has been developed. This technique allows injection of the hypochlorite solution into the channel of the fibres at a high pressure. The effect of this treatment on water flux of the membranes is studied. The results are compared with the water flux of identical membranes subjected to traditional hypochlorite treatment. Concentrated polymer solution containing polysulfone (PSf) /poly-vinyl pyrrolidone (PVP-K90)/N-Methyl-2-pyrrolidone (NMP) in weight ratio of 15/5/80 together with two types of bore fluids have been used for the production of two types of hollow fibre membranes via dry-wet-spinning process. Distilled water and mixture of NMP/ distilled water are used as bore fluids. Atomic force microscopic analysis and image processing technique (SEM microphotographs) have been employed to investigate performance of PSf hollow fibres treated with the traditional and high pressure injection techniques in relation to the composition of bore fluid. It is observed that in general both treatment methods result in the increase in water flux of the hollow fibres due to elimination of PVP (poly-vinyl pyrrolidone) swelling and alteration in pore size and pore distribution. The rate of increase in water flux in the membranes treated by high pressure injection technique is found to be higher in comparison to traditionally treated membranes. It is also found that the membranes produced using a mixture of NMP/ distilled water as bore fluid exhibit a higher rate of flux increase than those produced using distilled water. High pressure injection technique yields to production of highly permeable membranes. In addition, it is found that the composition of bore fluid controls the performance of the membranes subjected to hypochlorite treatment

Prediction of Permeability of Realistic and Virtual Layered Nonwovens using Combined Application of X-ray μCT and Computer Simulation

Fundamental understanding of transport properties of fibrous porous media is contingent upon in depth knowledge of their internal structure at the micro-scale. In this work computer simulations are explicitly coupled with X-ray micro-computed tomography (μCT) to investigate the effect of micro-structure on permeability of fibrous media. In order to reach this aim, samples of layered nonwoven fabrics were produced and realistic 3D images of their structure were prepared using X-ray μCT. A series of algorithms was developed to extract micro-structural parameters of fibrous media, including fibers population, orientation and diameter of each fiber as well as the local porosity of structure from high-resolution realistic 3D images. A Matlab-based program capable of producing fibrous structures with various fiber diameters, porosities, thicknesses and 3D fiber orientations was developed. The obtained parameters from μCT images were then implemented into the simulation code to generate virtual fibrous structures. Prediction of permeability in realistic and virtual structures was done by fluid flow simulation through the micro-structure of porous media. The results indicated that both through- and in-plane permeabilities are strongly dependent on the porosity of structure. It was established that the anisotropic nature of the geometry creates anisotropic permeability, with a ratio of 1.8. The anisotropy effect was found to be more profound at higher porosity values. Comparison of numerical results with experimentally obtained data and those of empirical, analytical, numerical, and experimental models were made. Considering the porosity of structures, acceptable agreement between the results and previously published findings was observed

Experimental analysis of surface finish in normal conducting cavities

A normal conducting 805 MHz test cavity with an in built button shaped sample is used to conduct a series of surface treatment experiments. The button enhances the local fields and influences the likelihood of an RF breakdown event. Because of their smaller sizes, compared to the whole cavity surface, they allow practical investigations of the effects of cavity surface preparation in relation to RF breakdown. Manufacturing techniques and steps for preparing the buttons to improve the surface quality are described in detail. It was observed that even after the final stage of the surface treatment, defects on the surface of the cavities still could be found

Evaluation of a new dispersion technique for assessing triboelectric charging of powders

In a number of applications, especially in pharmaceutical drug development, there is often a very small powder quantity available for evaluating the manufacturability of new drugs. However, it is highly desirable to be able to quickly evaluate processing issues, and where possible using the smallest powder quantity. In the present work, a proprietary commercial powder dispersion device (the disperser of Malvern© Morphologi G3) is adapted to evaluate the triboelectric charging tendency. A very small powder quantity (as small as 0.1 mg) is dispersed by a pressure pulse of compressed gas such as air or nitrogen. This causes the particles to become air borne and collide with the containing walls, resulting in dispersion and leading to triboelectric charge transfer between the particles and the walls. In this work, the charging propensity of a number of materials is evaluated and the effect of particle surface functional groups on the tribo-electric charge transfer is analysed. Model materials with a well-defined shape (glass ballotini) but with different silane groups deposited on their surfaces as well as a number of organic crystalline particles (such as aspirin, α-lactose monohydrate and paracetamol) are tested. Following dispersion the particles move immediately to a Faraday cup placed directly underneath the disperser. Therefore, particle charge is measured with no decay. The method can differentiate charging of different polymorphs of the same material, different silane groups on the surfaces of glass ballotini and different crystal morphologies obtained from crystallisation from various solvents

Neutrino factory

The properties of the neutrino provide a unique window on physics beyond that described by the standard model. The study of subleading effects in neutrino oscillations, and the race to discover CP-invariance violation in the lepton sector, has begun with the recent discovery that θ13>0. The measured value of θ13 is large, emphasizing the need for a facility at which the systematic uncertainties can be reduced to the percent level. The neutrino factory, in which intense neutrino beams are produced from the decay of muons, has been shown to outperform all realistic alternatives and to be capable of making measurements of the requisite precision. Its unique discovery potential arises from the fact that only at the neutrino factory is it practical to produce high-energy electron (anti)neutrino beams of the required intensity. This paper presents the conceptual design of the neutrino factory accelerator facility developed by the European Commission Framework Programme 7 EUROν Design Study consortium. EUROν coordinated the European contributions to the International Design Study for the Neutrino Factory (the IDS-NF) collaboration. The EUROν baseline accelerator facility will provide 10^21 muon decays per year from 12.6 GeV stored muon beams serving a single neutrino detector situated at a source-detector distance of between 1 500 km and 2 500 km. A suite of near detectors will allow definitive neutrino-scattering experiments to be performed