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1
Asselin, Daniel Joseph. "Characterization of the Near-Plume Region of a Low-Current Hollow Cathode". Digital WPI, 2011. https://digitalcommons.wpi.edu/etd-theses/438.
Testo completoAbstract (sommario):
Electric propulsion for spacecraft has become increasingly commonplace in recent decades as designers take advantage of the significant propellant savings it can provide over traditional chemical propulsion. As electric propulsion systems are designed for very low thrust, the operational time required over the course of an entire mission is often quite long. The two most common types of electric thrusters both use hollow cathodes as electron emitters in the process of ionizing the propellant gas. These cathodes are one of the main life-limiting components of both ion and Hall thrusters designed to operate for tens of thousands of hours. Failure often occurs as a result of erosion by sputtering from high-energy ions generated in the plasma. The mechanism that is responsible for creating these high-energy ions is not well understood, and significant efforts have gone into characterizing the plasma produced by hollow cathodes. This work uses both a Langmuir probe and an emissive probe to characterize the variation of the plasma potential and density, the electron temperature, and the electron energy distribution function in the near plume region of a hollow cathode. The cathode used in this experiment is typical of one used in a 200-W class Hall thruster. Measurements were made to determine the variation of these parameters with radial position from the cathode orifice. Changes associated with varying the propellant and flow rate were also investigated. Results obtained from the cathode while running on both argon and xenon are shown. Two different methods for calculating the plasma density and electron temperature were used and are compared. The density and temperature were not strongly affected by reductions in the propellant flow rate. The electron energy distribution functions showed distinct shifts toward higher energies when the cathode was operated at lower flow rates. The plasma potential also displayed an abrupt change in magnitude near the cathode centerline. Significant increases in the magnitude of plasma potential oscillations at lower propellant flow rates were observed. Ions formed at the highest instantaneous plasma potentials may be responsible for the life-limiting erosion that is observed during long-duration operation of hollow cathodes.
2
Sary, Gaétan. "Modélisation d'une cathode creuse pour propulseur à plasma". Thesis, Toulouse 3, 2016. http://www.theses.fr/2016TOU30182/document.
Testo completoAbstract (sommario):
La cathode creuse est un élément clef des propulseurs à plasma. Dans un propulseur à plasma, un gaz propulsif est ionisé dans un canal de décharge puis accéléré hors de celui-ci afin de créer la poussée. Dans le propulseur de Hall en particulier, l'ionisation du gaz est provoquée par l'injection dans le canal de décharge d'un intense courant électronique (de quelques ampères à plus d'une centaine d'ampères). L'élément chargé de fournir le courant électronique de la décharge, la cathode creuse, est crucial dans le fonctionnement du propulseur. Or, celle-ci est souvent idéalisée dans les modèles de propulseur et n'est que rarement étudiée pour sa physique propre. Pourtant, le développement de propulseurs de Hall de haute puissance, destinés à terme à équiper l'ensemble des missions spatiales, requiert la mise au point de cathodes capable de délivrer un fort courant (jusqu'à plus de 100 A) sur des durées de l'ordre de la dizaine de milliers d'heures. Or, la mise au point de nouvelles cathodes s'est révélée difficile en raison de l'absence de modèle susceptible de prédire a priori les performances d'une cathode en fonction de sa conception. On se propose ici de mettre en place un modèle prédictif de cathode creuse capable de retranscrire la physique du fonctionnement de la cathode. L'objectif in fine est bien sûr d'utiliser ce modèle afin de faire le lien entre la conception de la cathode et son fonctionnement dans le but de guider le développement de futures cathodes. On présentera tout d'abord brièvement le contexte d'application des cathodes creuses, et on donnera un rapide aperçu du principe de fonctionnement global de la cathode. Ensuite, après avoir effectué un tour d'horizon des différents modèles numériques de cathode creuse préexistants dans la littérature, on détaillera le modèle de la cathode développé ici, qui incorpore une description fluide du plasma, ainsi que des transferts thermiques aux parois, qui conditionnent en grande partie le bon fonctionnement de la cathode. Un soin particulier sera apporté à la validation des résultats de simulation vis-à-vis des mesures expérimentales disponibles dans la littérature, ce qui nous permettra de perfectionner certains points du modèle afin de mieux traduire la réalité physique. En particulier, une modélisation spécifique de la région de transition entre la décharge interne de la cathode et la plume du propulseur sera réalisée. Ce modèle permettra de mettre en évidence certains phénomènes d'instabilité du plasma spécifiques de cette décharge, qui ont été jusqu'ici observés expérimentalement mais jamais pleinement intégrés aux modèles de cathode creuse. A l'aide du modèle validé, on procèdera à l'analyse physique de l'ensemble des phénomènes qui gouvernent le fonctionnement d'une cathode particulière, la cathode NSTAR développée par la NASA au Jet Propulsion Laboratory. Ensuite, on s'appuiera sur le modèle numérique pour comprendre l'impact sur le fonctionnement de la cathode des choix de conception au travers d'une étude paramétrique autour de la cathode NSTAR. Les tendances dégagées nous permettront de formuler des recommandations quant au développement de cathodes de haute puissance. Enfin, dans le but d'illustrer la versatilité du modèle développé, le comportement d'une cathode creuse employant une géométrie alternative à la cathode NSTAR sera également présentéA hollow cathode is a critical component of plasma thrusters. In a plasma thruster, a propellant gas is ionized in a discharge chamber and accelerated out of it so as to generate thrust. In Hall thrusters in particular, the ionization of the gas is caused by an intense electron current (from a few to hundred amps) which flows through the discharge chamber. The hollow cathode is the device which is responsible for providing the discharge current. This key element is often idealized in thruster numerical models and its physical behavior is rarely studied for its own sake. Yet, developing high power Hall thrusters, designed to propel in the long run every type of space mission, requires new hollow cathodes able to supply an intense electron current (over 100 A) over a duration on the order of ten thousand hours. So far, designing new cathodes proved difficult because of the lack of model capable of predicting the performance of a cathode based on its design. In this work, we build up a predictive model of a hollow cathode capable of simulating the physics relevant to the operation of the cathode. In the end, we aim at using this model to associate design characteristics of the cathode to key aspects of the cathode performance during operation. Our goal with this model is to guide the development of future high power hollow cathodes. We will first briefly describe the range of application of hollow cathodes related to space propulsion. Then we will give a brief account of the working principles of the cathode and we will set the numerical models available in the literature prior to this one out. The numerical model developed in this work will then be described. It includes a fluid treatment of the plasma as well as an account of the heat fluxes to the walls which largely control the performance of the cathode. Simulation results will be thoroughly compared to experimental measurements available in the literature and specific aspects of the model will be refined to match up simulation results with the physical reality. For instance, a model that specifically represents the transition region between the internal plasma of the cathode and the plume of the cathode will be described. This model will enable us to highlight plasma instability phenomena which were so far observed experimentally, yet never properly included in hollow cathode models. Using the model just developed, we will analyze the physics of a particular hollow cathode which has been developed by NASA at the Jet Propulsion Laboratory, the NSTAR hollow cathode. Then, thanks to the numerical model, we will be able to carry out a parametric study revolving around the design of the NSTAR cathode. This will allow us to bring out the influence of the design on the cathode performance and we will eventually express recommendations regarding the design of future high power cathodes. To conclude, the versatility of the numerical model built up here will also be displayed through simulations of the behavior of a hollow cathode based on an alternate geometry
3
Pagaud, Francis. "Control and stability of magnetised plasma columns : plasma-cathode interactions and helicon plasma operation". Electronic Thesis or Diss., Lyon, École normale supérieure, 2024. http://www.theses.fr/2024ENSL0016.
Testo completoAbstract (sommario):
Le transport, les ondes et les instabilités sont des problématiques courantes des plasmas magnétisés, à l’origine de problèmes fondamentaux et de limites opératoires pour les plasmas de fusion, les accélérateurs de particules à plasma ou la propulsion plasma. Le contrôle des propriétés du plasma est souhaitable mais complexe. Ce doctorat vise à utiliser une cathode émissive comme nouveau paramètre de contrôle et à comprendre la stabilité du plasma hélicon. Le dispositif expérimental est une enceinte à vide cylindrique de 80 cm de long et 20 cm de diamètre, connecté à un tube source en verre de 11 cm de diamètre. L’argon est injecté en continu à 0,13 Pa et ionisé par une antenne radio-fréquence inductive de 1 kW enroulée autour du tube source. Un champ magnétique de 170 G à 340 G garantit une faible magnétisation. Une cathode chaude injecte un fort courant thermionique à l’autre extrémité de la colonne de plasma. Les mesures optiques de température de la cathode ont révélé un profil fortement inhomogène dû aux interactions plasma-cathode, et fut reproduit numériquement avec succès par un modèle thermique détaillé. Le régime opératoire prédit est en excellent accord avec les expériences. L’influence expérimentale de l’injection d’électrons sur les propriétés plasmas, et plus particulièrement le potentiel plasma, a été couplée à une approche analytique basée sur un modèle de plasma magnétisé. Un accord qualitatif entre les prédictions théoriques et le contrôle effectif du potentiel plasma a été trouvé. La perspective de l’utilisation d’une cathode émissive comme nouveau paramètre de contrôle ouvre la possibilité d’un réglage fin de la dynamique globale du plasma, ainsi que la mitigation du transport et des instabilités au sein du plasma. Des améliorations sont discutées en vue d’une prédiction quantitative accrue. Enfin, une source de plasma hélicon haute densité a été implémentée afin d’atteindre des taux d’ionisation importants et un couplage antenne-plasma optimal. Ce nouveau système a été caractérisé à l’aide de sondes et d’imagerie rapide. Des signatures typiques des plasmas hélicons ont été retrouvées telles que des transitions de mode E-H-W, une rupture de symétrie liée au champ magnétique et la propagation d’ondes whistler m = +1. En outre, des oscillations basse-fréquence telles que des oscillations H-W et W-W, et des instabilités coexistantes de Rayleigh-Taylor et Kelvin-Helmholtz ont été identifiées. Une forte instabilité multi-échelles à 1080 G a été également brièvement explorée. L’identification des mécanismes d’instabilité via le calcul des taux de croissance, la décomposition 2D-FT et POD ont permis de comprendre les mécanismes physiques à l’oeuvreRadial transport, azimuthal waves and instabilities are common features in magnetised plasmas, causing major challenges for plasma propulsion, plasma wakefield particle acceleration or fusion devices. Plasma properties control is desirable yet complex. This PhD thesis follows two goals, one being the use of an emissive cathode as a new parameter control and the other being the fundamental understanding of the helicon plasma operational stability. Firstly, the role of the injection of electrons inside a magnetised plasma column has been studied experimentally and numerically. The experimental set-up is a 80 cm long and 20 cm diameter vacuum vessel connected to a 11 cm wide glass tube. The argon gas at a base pressure of 0.13 Pa is ionised by a 3-turns inductive radio-frequency antenna supplied at 1 kW. Magnetic field ranging from 170 G to 340 G, ensures a weak magnetisation of the plasma. A large tungsten hot cathode was placed at the end of the plasma column to inject an important thermionic current. Electrical and optical measurements of the cathode temperature revealed a highly inhomogeneous cathode temperature profile due to plasma–cathode interactions. A detailed thermal modelling solved numerically accurately reproduces the heterogeneous rise in temperature witnessed experimentally. The operating regime was predicted in excellent agreement with experimental results.The fine understanding of the emissive cathode behaviour in presence of a surrounding magnetised plasma permitted to explore its influence on the plasma properties, and especially the plasma potential. An analytical approach based on a two-fluids plasma model and anisotropic electrical conductivities, predicting plasma potential control and plasma rotation regulation as a function of thermionic emission, has been applied and compared to a wide experimental dataset of plasma properties. The works presented confront the role of cross-field ion transport to experimental radial plasma potential scans with a semi-quantitative agreement, highlighting a new major application of emissive cathodes.Finally, a state-of-the-art helicon plasma source has been implemented to produce higher ionization rates. This new system required a complete characterisation of plasma properties through electrostatic probes and high-speed camera imaging. It reproduced well-known helicon plasma features such as E-H-W mode transitions, bistability and hysteresis, chirality emerging from the external magnetic field direction and the propagation of m = +1 whistler waves. Besides, it displayed complex behaviours such as H-W and W-W oscillations, or coexisting low-frequency Kelvin-Helmholtz and Rayleigh-Taylor instabilities. A strong multiscale core instability at 1080 G was also briefly investigated. Wave-mode identification based on theoretical growth rates, 2DFT modal decomposition and POD has been conducted, unravelling the physical mechanisms at stake
4
Taillefer, Zachary R. "Characterization of the Near Plume Region of Hexaboride and Barium Oxide Hollow Cathodes operating on Xenon and Iodine". Digital WPI, 2018. https://digitalcommons.wpi.edu/etd-dissertations/44.
Testo completoAbstract (sommario):
The use of electric propulsion for spacecraft primary propulsion, attitude control and station-keeping is ever-increasing as the technology matures and is qualified for flight. In addition, alternative propellants are under investigation, which have the potential to offer systems-level benefits that can enable particular classes of missions. Condensable propellants, particularly iodine, have the potential to significantly reduce the propellant storage system volume and mass. Some of the most widely used electric thrusters are electrostatic thrusters, which require a thermionic hollow cathode electron source to ionize the propellant for the main discharge and for beam neutralization. Failure of the hollow cathode, which often needs to operate for thousands of hours, is one of the main life-limiting factors of an electrostatic propulsion system. Common failure modes for hollow cathodes include poisoning or evaporation of the thermionic emitter material and erosion of electrodes due to sputtering. The mechanism responsible for the high energy ion production resulting in sputtering is not well understood, nor is the compatibility of traditional thermionic hollow cathodes with alternative propellants such as iodine. This work uses both an emissive probe and Langmuir probe to characterize the near-plume of several hollow cathodes operating on both xenon and iodine by measuring the plasma potential, plasma density, electron temperature and electron energy distribution function (EEDF). Using the EEDF the reaction rate coefficients for relevant collisional processes are calculated. A low current (< 5 A discharge current) hollow cathode with two different hexaboride emitters, lanthanum hexaboride (LaB6) and cerium hexaboride (CeB6), was operated on xenon propellant. The plasma potential, plasma density, electron temperature, EEDF and reaction rate coefficients were measured for both hexaboride emitter materials at a single cathode orifice diameter. The time-resolved plasma potential measurements showed low frequency oscillations (<100 kHz) of the plasma potential at low cathode flow rates (<4 SCCM) and spot mode operation between approximately 5 SCCM and 7 SCCM. The CeB6 and LaB6 emitters behave similarly in terms of discharge power (keeper and anode voltage) and plasma potential, based on results from a cathode with a 0.020�-diameter. Both emitters show almost identical operating conditions corresponding to the spot mode regime, reaction rates, as well as mean and RMS plasma potentials for the 0.020� orifice diameter at a flow rate of 6 SCCM and the same discharge current. The near-keeper region plasma was also characterized for several cathode orifice diameters using the CeB6 emitter over a range of propellant flow rates. The spot-plume mode transition appears to occur at lower flow rates as orifice size is increased, but has a minimum flow rate for stable operation. For two orifice diameters, the EEDF was measured in the near-plume region and reaction rate coefficients calculated for several electron- driven collisional processes. For the cathode with the larger orifice diameter (0.040�), the EEDFs show higher electron temperatures and drift velocities. The data for these cathodes also show lower reaction rate coefficients for specific electron transitions and ionization. To investigate the compatibility of a traditional thermionic emitter with iodine propellant, a low-power barium oxide (BaO) cathode was operated on xenon and iodine propellants. This required the construction and demonstration of a low flow rate iodine feed system. The cathode operating conditions are reported for both propellants. The emitter surface was inspected using a scanning electron microscope after various exposures to xenon and iodine propellants. The results of the inspection of the emitter surface are presented. Another low current (< 5 A), BaO hollow cathode was operated on xenon and iodine propellants. Its discharge current and voltage, and plume properties are reported for xenon and iodine with the cathode at similar operating conditions for each. The overall performance of the BaO cathode on iodine was comparable to xenon. The cathode operating on iodine required slightly higher power for ignition and discharge maintenance compared to xenon, as evident by the higher keeper and anode potentials. Plasma properties in the near- plume region were measured using an emissive probe and single Langmuir probe. For both propellants, the plasma density, electron energy distribution function (EEDF), electron temperature, select reaction rate coefficients and time-resolved plasma potentials are reported. For both propellants the cathode operated the same keeper (0.25 A) and discharge current (3.1 A), but the keeper and anode potentials were higher with iodine; 27 V and 51 V for xenon, and 30 V and 65 V for iodine, respectively. For xenon, the mean electron energy and electron temperature were 7.5 eV and 0.7 eV, with bulk drift energy of 6.6 eV. For iodine, the mean electron energy and electron temperature were 6.3 eV and 1.3 eV, with a bulk drift energy of 4.2 eV. A literature review of relevant collisional processes and associated cross sections for an iodine plasma is also presented.
5
Popov, M. Yu, A. P. Volkov, S. G. Buga, V. S. Bormashov, K. V. Kondrashov, R. L. Lomakin, N. V. Lyparev, V. V. Medvedev, S. A. Tarelkin e S. A. Perfilov. "Nanostructured metal-fullerene field emission cathode". Thesis, Sumy State University, 2011. http://essuir.sumdu.edu.ua/handle/123456789/20585.
Testo completoAbstract (sommario):
One of the important properties of carbon nanostructures is their cold electron
emission ability. Carbon nanotubes and other nanostructures are capable of
emitting high currents at relatively low electrical fields. They are already
used in functional devices such as field emitters. The conventional method of
carbon nanostructured cathodes manufacturing is thin film nanocarbon deposition
using CVD process on electrically conducting substrate like metal or doped
silicon plates. The alternative way of manufacturing of carbon field emission
cathodes is based on a special processing of carbon microfibers or composite materials in metal holders. We used the similar approach to produce composite
metal-nanocarbon material which may be easily processed and shaped to
produce an effective field emission cathode which can be easily fixed an any
environment.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/20585
6
Vaughn, Joel M. "Thermionic Electron Emission Microscopy Studies of Barium and Scandium Oxides on Tungsten". Ohio University / OhioLINK, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1279814656.
Testo completo
7
Lee, Kon Jiun. "Current limiting of field emitter array cathodes". Diss., Georgia Institute of Technology, 1986. http://hdl.handle.net/1853/19629.
Testo completo
8
Shen, Xiangqian. "Novel processing routes for oxide cathode emission materials". Thesis, Loughborough University, 2000. https://dspace.lboro.ac.uk/2134/10822.
Testo completoAbstract (sommario):
An investigation has been carried out into the production of the alkaline earth carbonate and oxide powders and coatings suitable as cathode emission materials by the ethylenediaminetetraacetic acid (EDT A) gel method for potential application as cathode emission materials. The emission performance of thermionic cathodes coated with these materials has been measured, and found to give encouraging results, comparable with conventionally prepared oxide emission materials, despite the former having not been optimised. Amorphous gels of composition Ca-EDTA, Sr-EDTA, Ba-EDTA, [SrO.5 Bao.5J-EDTA and [SrO.5 Bao.5 Cao.05J-EDTA were successfully prepared from aqueous solutions of alkaline earth nitrates and EDT A. Subsequently, the thermal decomposition of the gels and the effects of temperature and atmospheres on the decomposition have been studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The gels and the powders derived from calcination of these gels at different temperatures have been characterised by Fourier transform infrared spectroscopy (FTIR) , X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) with X-ray energy dispersive (EDX) spectroscopy. Coatings prepared by dip-coating, spraying and electrophoretic deposition (EPD) from the EDT A-sols have been characterised by FTIR, XRI> and SEM techniques. In addition, an attempt was made to study the coating of the colloidal particles in the suspension of methanolethanediol- EDTA sol by electrochemical impedance spectroscopy.
9
Mollart, T. P. "Electron emission processes in cold cathode thermal arcs". Thesis, Durham University, 1993. http://etheses.dur.ac.uk/5546/.
Testo completoAbstract (sommario):
In this Thesis the processes of electron emission from cathode electrodes are studied theoretically, and the applicability of these mechanisms to the non refractory cathodes that can be used to sustain thermal arcs was examined. Apparatus that was used to generate and manipulate thermal arcs along rail electrodes is described in this thesis. Techniques for driving arcs over polished sample electrodes with magnetic or aerodynamic forces are outlined. Scanning electron microscopy was used to study emission site formation on highly polished electrodes with a natural 2.5 nm oxide layer. Theoretical maximum electron current densities that can be extracted by the arc were calculated and these were used, in conjunction with information from the experimental work, to make estimates of the lifetime of emission spots that are seen on the cathode electrodes of thermal arc devices. The lifetime was found to be dependent on the arc velocity over a range of velocity values from 3 to 80 ms(^-1). The lifetime measured ranged from 2.4 µs to 0.024 µs. Experiments on arcs driven at a constant velocity using a combination of aerodynamic and magnetic forces showed that the formation of emission spots was independent of die applied external magnetic field. The presence of artificially grown copper (11) oxide layers, 50 nm and 100 nm thick, were found to influence the lifetime. The effect of the oxide layer was predicted using a simple model accounting for the change of resistance that such an oxide layer would be expected to cause. Additional experiments showed that the resistance of the arc was independent of the oxide layer thickness, as predicted by the model.
10
Jones, Randolph D. "Circuit model of a low-voltage field emission cathode". Diss., Georgia Institute of Technology, 1985. http://hdl.handle.net/1853/15631.
Testo completo
11
Lysenkov, Dmitry. "Optimization of nanostructures for field emission cathodes". [S.l.] : [s.n.], 2006. http://deposit.ddb.de/cgi-bin/dokserv?idn=979952913.
Testo completo
12
Sanborn, Graham Patrick. "A thin film triode type carbon nanotube field electron emission cathode". Diss., Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/50302.
Testo completoAbstract (sommario):
The current technological age is embodied by a constant push for increased performance and efficiency of electronic devices. This push is particularly observable for technologies that comprise free electron sources, which are used in various technologies including electronic displays, x-ray sources, telecommunication equipment, and spacecraft propulsion. Performance of these systems can be increased by reducing weight and power consumption, but is often limited by a bulky electron source with a high energy demand.
Carbon nanotubes (CNTs) show favorable properties for field electron emission (FE) and performance as electron sources. This dissertation details the developments of a uniquely designed Spindt type CNT field emission array (CFEA), from initial concept to working prototype, to specifically prevent electrical shorting of the gate. The CFEA is patent pending in the United States. Process development enabled fabrication of a CFEA with a yield of up to 82%. Furthermore, a novel oxygen plasma etch process was developed to reverse shorting after CNT synthesis. CFEA testing demonstrates FE with a current density of up to 293 μA/cm² at the anode and 1.68 mA/cm² at the gate, with lifetimes in excess of 100 hours. A detailed analysis of eighty tested CFEAs revealed three distinct types of damage. Surprisingly, about half of the damaged chips are not electrically shorted, indicating that the CFEAs are very robust.
Potential applications of this technology as cathodes for spacecraft electric propulsion were explored. Exposure to an operating electric propulsion thruster showed no significant effect or damage to the CFEAs, marking the first experimental study of CNT field emitters in an electric propulsion environment. A second effort in spacecraft propulsion is a collaboration with the Air Force Institute of Technology (AFIT). CFEAs are the payload on an AFIT developed Cube Satellite, called ALICE, to test electron emission in the space environment. ALICE has passed flight tests and is awaiting launch scheduled for 5 December 2013.
13
Schlise, Charles A. "Explosive emission cathodes for high power microwave devices /". Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 2004. http://library.nps.navy.mil/uhtbin/hyperion/04Jun%5FSchlise.pdf.
Testo completo
14
Cooper, Joseph Andrew. "Investigation of the effects of process variables on the properties of europium-doped yttrium oxide phosphor". Thesis, Georgia Institute of Technology, 1997. http://hdl.handle.net/1853/20503.
Testo completo
15
Kandah, Munther Issa. "Particles emission control at graphite cathode in arc ion plating deposition". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp03/NQ55420.pdf.
Testo completo
16
Kandah, Munther. "Particles emission control at graphite cathode in arc ion plating deposition". Thesis, McGill University, 1997. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=35434.
Testo completoAbstract (sommario):
In this work, the dependence of the vacuum arc spot velocity on physical and electrical properties of different graphite cathode materials is investigated in the presence of a variable magnetic field. A pulsed arc system is used to perform preliminary experiments on the arc mobility for the different types of graphite for the selection of proper material morphology and the design of a continuous vacuum arc system. The characteristics of arc mobility, erosion rate, and carbon ion flux emitted from the continuous carbon source are then evaluated in view of particle-free diamond-like protective coatings. Results show that the arc spot velocity on graphite cathodes is larger on cathodes having larger grain size, lower electrical resistivity and higher apparent density. The spot velocity is also lower for cathodes having larger pore sizes and total porosity. The arc spot velocity is also found to be increased by increasing the magnetic field intensity over the surface of any graphite type. Reduced residence time of the spot on a given site of the cathode resulting from arc velocity increase should lead to a reduction in the heat load input in the cathode spot. This correlates with results on the number of emitted particles, the film thickness and roughness, and the erosion rate that are found to decrease, while the ion flux emission is increased. Diamond-like carbon (DLC) films free of particles are produced in a continuous arc ion plating (AIP) system. The ion energy in the continuous AIP system is found to vary with the graphite surface properties and the intensity of a plasma confining magnetic field in front of the cathode. The ion energies measured vary between 39.8 eV to 62.6 eV.
17
Bajic, Stevan. "Non-metallic cold-cathode electron emission from composite metal-insulator microstructures". Thesis, Aston University, 1989. http://publications.aston.ac.uk/8063/.
Testo completoAbstract (sommario):
A detailed investigation has been undertaken into a field-induced electron emission (FIEE) mechanism that occurs at microscopically localised `sites' on uncoated, dielectric-coated and composite-coated metallic cathodes. An optical imaging technique has been used to observe and characterize the spatial and temporal behaviour of the populations of emission sites on these cathodes under various experimental conditions, e.g. pulsed-fields, gas environment etc. This study has shown that, for applied fields of 20MVm^-1, thin dielectric (750AA) and composite metal-insulator (MI) overlayers result in a dramatic increase in the total number of emission sites (typically 30cm^-2), and hence emission current. The emission process has been further investigated by a complementary electron spectroscopy technique which has revealed that the localised emission sites on these cathodes display field-dependent spectral shifts and half-widths, i.e. indicative of a `non-metallic' emission mechanism. Details are also given of a comprehensive investigation into the effects of the residual gas environment on the FIEE process from uncoated Cu-cathodes. This latter study has revealed that the well-known Gas Conditioning process can be performed with a wide range of gas species (e.g. O_2, N_2 etc), and furthermore, the degree of conditioning is influenced by both a `Voltage' and `Temperature' effect. These experimental findings have been shown to be particularly important to the technology of high-voltage vacuum-insulation and cold-cathode electron sources. The FIEE mechanism has been interpreted in terms of a hot-electron process that is associated with `electroformed' conducting channels in MI, MIM and MIMI surface microstructures.
18
Hirt, Benjamin David. "Impact of Additives on Thermionic Cathodes". Ohio University Honors Tutorial College / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=ouhonors1524832507214002.
Testo completo
19
Swartzentruber, Phillip D. "MICROSTRUCTURE AND WORK FUNCTION OF DISPENSER CATHODE COATINGS: EFFECTS ON THERMIONIC EMISSION". UKnowledge, 2014. http://uknowledge.uky.edu/cme_etds/41.
Testo completoAbstract (sommario):
Dispenser cathodes emit electrons through thermionic emission and are a critical component of space-based and telecommunication devices. The emission of electrons is enhanced when coated with a refractory metal such as osmium (Os), osmium-ruthenium (Os-Ru), or iridium (Ir). In this work the microstructure, thermionic emission, and work function of thin film Os-Ru coatings were studied in order to relate microstructural properties and thermionic emission.
Os-Ru thin film coatings were prepared through magnetron sputtering and substrate biasing to produce films with an array of preferred orientations, or texture. The effect of texture on thermionic emission was studied in detail through closely-spaced diode testing, SEM imaging, and x-ray diffraction. Results indicated that there was a strong correlation with emission behavior and specific preferred orientations.
An ultra-high vacuum compatible Kelvin Probe was used to measure the work function of W-Os-Ru ternary alloy films to determine the effect W interdiffusion has on work function. The results indicated that a high work function alloy coating corresponded to low work function cathodes, as expected. It was inferred that a high work function alloy coating results in a low work function cathode because it aligns more closely with ionization energy of Ba. The results also proved that this method of evaluating dispenser cathode coatings can distinguish small variations in microstructure and composition and may be a beneficial tool in the development of improved dispenser cathode coatings.
A novel experimental apparatus was constructed to measure the work function of dispenser cathode coatings in-vacuo using the ultra-high vacuum Kelvin Probe. The apparatus is capable of activating cathodes at high temperature and measuring the work function at elevated temperature. The design of this apparatus allows for more rapid evaluation of dispenser cathode coatings.
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Wu, Qiong. "Measurements and studies of secondary electron emission of diamond amplified photo cathode". [Bloomington, Ind.] : Indiana University, 2008. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3337275.
Testo completoAbstract (sommario):
Thesis (Ph.D.)--Indiana University, Dept. of Physics, 2008.Title from PDF t.p. (viewed on Jul 29, 2009). Source: Dissertation Abstracts International, Volume: 69-12, Section: B, page: 7588. Adviser: Shyh-Yuan Lee.
21
Pozdnyakov, D. V., A. V. Borzdov e V. M. Borzdov. "Calculation of Current-Voltage Characteristics of the Optimized Triode with a Cold Cathode Based on the Ordered Array of Single-Wall Metallic Carbon Nanotubes". Thesis, Sumy State University, 2013. http://essuir.sumdu.edu.ua/handle/123456789/35361.
Testo completoAbstract (sommario):
The current-voltage characteristics of a triode with the plane-parallel electrodes and a cold cathode
based on the ordered array of single-wall metallic carbon nanotubes with open ends have been calculated
by means of a numerical solution of both the Poisson equation and the quantum-wave equation. The topological
parameters of the triode have been optimized.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35361
22
Ayzatskiy, N., A. Dovbnya, V. Zakutin, N. Reshetnyak, V. Romas'ko, I. Chertishchev, V. N. Boriskin, V. Mitrochenko, A. B. Galat e I. Khodak. "Experimental investigation on the time characteristics of an electron beam formed in the magnetron gun with a secondari-emission cathode". Thesis, Национальный научный центр "Харьковский физико-технический институт" (ННЦ ХФТИ), 2007. http://openarchive.nure.ua/handle/document/9244.
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Patel, Jay. "GROWTH AND CHARACTERIZATION OF CARBON NANOMATERIALS". Wright State University / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=wright1310148312.
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Magnus, Sandra Hall. "An Investigation of the relationship between the thermochemistry and emission behavior of thermionic cathodes based on the BaO-Sc₂O₃-WO₃ ternary system". Diss., Georgia Institute of Technology, 1996. http://hdl.handle.net/1853/19625.
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Sabaut, Lucie. "Cathode commutable à nanotubes de carbone pour tube à rayons X". Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLX078/document.
Testo completoAbstract (sommario):
Les systèmes d'imagerie à rayons X (RX) sont des appareils volumineux et contraignants en termes de contrôle du faisceau. L'industrie des tubes électroniques est donc à la recherche de solutions pour assurer la stabilité du courant tout en permettant la miniaturisation du système.Ce travail opte pour l'amélioration de la source d'électrons, en remplaçant l'émission thermoïonique historique par l'émission de champ. En particulier, les cathodes froides à base de nanotubes de carbone possèdent l'avantage de pouvoir délivrer de forts courants (>1A/cm^2), tout en ayant un faible temps de réponse.A travers le développement d'une structure innovante de cathodes à nanotubes de carbone à grille intégrée, l'objectif de cette étude est de réaliser des sources commutables et régulées, pour des sources de rayons X miniatures, portables ou polyvalentes.La modélisation électrostatique de la nouvelle structure a conduit à la fabrication de cathodes à grille optimisées, sur lesquelles est cru un réseau vertical de nanotubes de carbone. L'analyse de défaillance permet finalement d'obtenir des dispositifs isolés fiables. Leur caractérisation en émission de champ indique des performances de modulation de courant inégalées, de l'ordre de 10^6 pour +/-40V de polarisation de grille. La régulation du courant a également été démontrée avec l'obtention d'une stabilité à 0,02% sur 100 h.Pour pallier les limitations rencontrées (courant de fuite et croissance parasite), une structure de grille enterrée a été proposée avec succès, ainsi qu'une nouvelle méthode de fabrication d'émetteurs courts et fins. Ces cathodes fonctionnelles ont finalement été intégrées en tube à rayons X et ont montré pour la première fois une modulation de courant de 2000 à une haute tension fixe de 60 kVThis work chooses to improve the electron source by replacing thermionic emission with field emission. More especially, carbon nanotubes based cold cathodes stand out by their ability to supply high currents (>1A/cm^2) while responding fast.Through the development of an innovative structure of in-plane gated carbon nanotube based cathode, this study aims at making switchable and regulated sources for miniature, portable or polyvalent X-rays sources.The electrostatic modelling of the new structure led to the fabrication of optimized gated cathodes, where a vertically aligned array of carbon nanotubes is grown. Default analysis allows to get reliable insulated devices.Field emission characterization shows unprecedented current modulation of 10^6 at +/-40V bias voltage. Current regulation is also achieved with a stability of 0.02% over 100 h.Another structure with a burried gate electrode was designed to successfully cope with leak current and parasitic growth. A new way of growing short and thin nanotubes was tackled.Finally, gated cathodes were integrated in a compact X-ray tube and showed a current modulation of 2000 at a high voltage of 60 kV
26
Yang, Sen. "Characterization of low voltage cathodo-luminescent phosphors for field emission displays". Diss., Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/30859.
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O'Neill, Kevin. "Field Emission from Excimer Laser Processed a-Si:H Thin Film Cathodes". Thesis, University of Dundee, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.521675.
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Pottinger, Sabrina. "Investigation of steady state characteristics of hollow cathode internal plasmas using optical emission spectroscopy". Thesis, University of Southampton, 2005. https://eprints.soton.ac.uk/345074/.
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Knápek, Alexandr. "Metody přípravy a charakterizace experimentálních autoemisních katod". Doctoral thesis, Vysoké učení technické v Brně. Fakulta elektrotechniky a komunikačních technologií, 2013. http://www.nusl.cz/ntk/nusl-233613.
Testo completoAbstract (sommario):
Téma doktorské práce se zabývá přípravou a popisem katod na bázi autoemise, jenž představují kvalitní a levný elektronový zdroj pro zařízení pracující s fokusovaným elektronovým svazkem. Pro přípravu kompozitní autoemisní katody byla využita elektrochemická metoda výroby. Kompozitní struktura katody zlepšuje proudovou stabilitu ve srovnání s čistě autoemisními katodami na bázi wolframu. Na základě charakterizace katody, jenž byla nově provedena metodou šumové spektroskopie, byla implementována technologická zlepšení stávající výroby. Metoda šumové spektroskopie je založena na analýze emisního proudu v časové a kmitočtové rovině, ale především poskytuje informace o nosiči náboje, o jeho pohyblivosti a dále o životnosti katody. Výsledky experimentální části byly rozšířeny teoretickými simulacemi, vedoucími k návrhu metodiky charakterizace vylepšené autoemisní katody.
30
Munné, Vicente. "Investigation of barium titanate PTCR films for current limiting of field emitter arrays". Diss., Georgia Institute of Technology, 1999. http://hdl.handle.net/1853/19612.
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Neuerman, Robert A. "Simulation and design methods for free-electron laser systems". Thesis, Monterey, California : Naval Postgraduate School, 2009. http://edocs.nps.edu/npspubs/scholarly/theses/2009/Dec/09Dec%5FNeuerman.pdf.
Testo completoAbstract (sommario):
Thesis (M.S. in Applied Physics)--Naval Postgraduate School, December 2009.Thesis Advisor(s): Colson, William B. ; Blau, Joseph. "December 2009." Description based on title screen as viewed on January 28, 2010. Author(s) subject terms: Free-electron lasers, FEL simulations, undulators, FEL oscillators, FEL amplifiers, diamond field-emitter arrays, field emission cathodes, cathode test cell. Includes bibliographical references (p. 45). Also available in print.
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Forrest, Roy Duncan. "Electron field emission from amorphous semiconductor thin films". Thesis, University of Surrey, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.484237.
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Little, Scott A. "Experimental and numerical studies of a new thermionic emitter structure based on oxide coated carbon nanotubes operating at large emission currents". Virtual Press, 2007. http://liblink.bsu.edu/uhtbin/catkey/1380104.
Testo completoAbstract (sommario):
We have developed a thermionic cathode capable of high emission currents. The structure of this cathode is oxide coated carbon nanotubes (CNTs) on a tungsten (W) substrate. This cathode was superior in emission due to the combination of the field enhancement effect from the CNTs and the lowered work function from the semiconducting oxide surface. Such oxide coated CNTs were excellent electron emitters. Conventional electron emission theories, such as Richardson's and Fowler-Nordheim's, did not accurately describe the field enhanced thermionic emission from such emitters. A unified electron emission theory was adopted and numerical simulations were performed to explain the deviation of electron emission from conventional field and thermionic emission theories. Also, the thermionic measurement system and measurement methods were improved in order to measure and characterize the strong electron emission from this new cathode. Large electron emission current from such structures also made a new thermionic cooling device a possibility. Cooling due to the electron emission was measured in terms of temperature drop, and a large temperature drop was observed from this cathode structure. Finally, applications of this cathode in plasma discharge devices were explored. This new cathode was tested in a plasma environment and initial results were obtained.Department of Physics and Astronomy
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Liu, Xiaotao. "CHARACTERIZATION OF NANOSTRUCTURE, MATERIALS, AND ELECTRON EMISSION PERFORMANCE OF NEXT-GENERATION THERMIONIC SCANDATE CATHODES". UKnowledge, 2019. https://uknowledge.uky.edu/cme_etds/96.
Testo completoAbstract (sommario):
Scandate cathodes, where scandia is added to the tungsten cathode pellets, have recently received substantial and renewed research interest owing to significantly improved electron emission capabilities at lower temperatures, as compared with conventional dispenser cathodes. However, there are several persistent issues including non-uniform electron emission, lack of understanding regarding scandium’s role in the emission mechanism, and unreliable reproducibility in terms of scandate cathode fabrication. As a result, scandate cathodes have not yet been widely implemented in actual vacuum electron devices (VEDs).
The surface structure and chemical composition of multiple scandate cathodes – prepared with the powder using the liquid-solid (L-S) technique – and exhibiting excellent emission behavior were characterized to give insight into the fundamental mechanism(s) of operation. This was achieved with high-resolution electron microscopy techniques that include high-precision specimen lift-out. These studies showed that the micron-sized tungsten particles that compose the largest fraction of the cathode body are highly faceted and decorated with nanoscale Ba/BaO (~10 nm), as well as larger (~150 nm) Sc2O3 and BaAl2O4 particles. The experimentally identified facets were confirmed through Wulff analysis of the tungsten crystal shape and were determined to consist of {110}, {100}, and {112} facets, in increasing order of surface area prevalence. Furthermore, it is estimated that Ba atoms decorating the tungsten crystal surfaces are present in quantities such that monolayer coverage is possible at elevated temperatures.
The high-resolution electron microscopy techniques used to investigate the cross section (near-surface) of the L-S scandate cathodes also revealed that the BaAl2O4 particles (100-500 nm) that attach to the larger tungsten particles are either adjacent to the smaller Sc2O3 nanoparticles or encompass them. Furthermore, high-resolution chemical analysis and 3D elemental tomography show that the two oxides always appear to be physically distinct from each other, despite their close proximity. 3D elemental tomography also showed that the Sc2O3 particles can sometimes appear inside the larger tungsten particles, but are inhomogeneously distributed. Nanobeam electron diffraction confirmed that the crystal structure of the tungsten particles are body-centered cubic, and imply that the structure remains unchanged despite the numerous complex chemical reactions that take place throughout the impregnation and activation procedures.
The role of Sc and the emission mechanism for scandate cathodes are discussed. Based on characterization results and materials computation, the role of Sc in scandate cathodes is possibly related to tuning the partial pressure of oxygen in order to establish an oxygen-poor atmosphere around the cathode surface, which is a necessary condition for the formation of the (near) equilibrium tungsten shape. A thin Ba-Sc-O surface layer (~8 nm) was detected near the surface of tungsten particles, using electron energy loss spectroscopy in the scanning transmission electron microscope. This stands in stark contrast to models invoking a ~100 nm Ba-Sc-O semiconducting surface layer, which are broadly discussed in the literature. These results provide new insights into understanding the emission mechanism of scandate cathodes.
35
Del, Pozo Rodriguez Sofia. "Investigation and optimisation of a plasma cathode electron beam gun for material processing applications". Thesis, Brunel University, 2016. http://bura.brunel.ac.uk/handle/2438/13565.
Testo completoAbstract (sommario):
This thesis describes design, development and testing work on a plasma cathode electron beam gun as well as plasma diagnosis experiments and Electron Beam (EB) current measurements carried out with the aim of maximising the power of the EB extracted and optimising the electron beam gun system for material processing applications. The elements which influence EB gun design are described and put into practice in a thermionic EB gun case study. The relevant principles of plasma EB gun systems, such low-temperature, low-pressure, RF excitation, are described along with the test rigs developed to investigate different plasma cathode configurations. The first experimental setup was for optical spectroscopy measurements of the light emitted from the plasma and the second included current measurements from EBs generated at –30 and –60 kV as well as the spectroscopic measurements. Comparison of EB current measurements with different plasma cathode configurations and correlation with spectroscopic measurements are presented. The maximum current extracted from the Radiofrequency (RF) gun was 38 mA at –60 kV using a hollow cathode geometry and permanent magnets for electron confinement. The RF gun was compared to a Direct Current (DC) gun which generated higher currents. This was reflected in the spectra which indicated a higher ionisation level than in the RF plasma. Simulation work carried out using Opera-2d to model beam trajectories indicated that the beam shape is largely influenced by the plasma boundary. Particle In Cell (PIC) simulations of a parallel plate RF plasma cathode demonstrated that higher excitation frequencies produced higher ionisation, however the RF sheaths were larger and thus the current extracted may be limited in practice due to fewer electrons being available near the aperture. The sheath thickness decreased in the simulations as the discharge gap was increased. RF plasma also produced larger currents from larger plasma chambers.
36
Serbun, Pavel [Verfasser]. "A systematic investigation of carbon, metallic and semiconductor nanostructures for field-emission cathode applications / Pavel Serbun". Wuppertal : Universitätsbibliothek Wuppertal, 2014. http://d-nb.info/1048757064/34.
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Mroz, Michael V. "Characterization of Morphological and Chemical Properties of Scandium Containing Cathode Materials". Ohio University / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou158470102410177.
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Calderón-Colón, Xiomara Zhou Otto. "Fabrication, characterization and integration of carbon nanotube cathodes for field emission x-ray source". Chapel Hill, N.C. : University of North Carolina at Chapel Hill, 2009. http://dc.lib.unc.edu/u?/etd,2721.
Testo completoAbstract (sommario):
Thesis (Ph. D.)--University of North Carolina at Chapel Hill, 2009.Title from electronic title page (viewed Mar. 10, 2010). "... in partial fulfillment of the requirement for the degree of Doctor of Philosophy in the Curriculum in Applied Science and Engineering." Discipline: Applied and Materials Sciences; Department/School: Applied and Materials Sciences.
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Chalamala, Babu Reddy. "The Physics of Gaseous Exposures on Active Field Emission Microcathode Arrays". Thesis, University of North Texas, 1996. https://digital.library.unt.edu/ark:/67531/metadc278588/.
Testo completoAbstract (sommario):
The interaction of active molybdenum field emission microcathode arrays with oxygen, water, carbon dioxide, methane, hydrogen and helium gases was studied. Experiments were setup to measure the emission characteristics as a function of gas exposures. The resulting changes in the surface work function of the tips were determined from the Fowler-Nordheim plots. The kinetics of the FEA-gas interaction were studied by observing the ion species originating from the array during and after gas exposures with a high resolution quadrupole mass spectrometer. With the work function data and the mass spectrometry information, the mechanisms responsible for emission degradation and subsequent device recovery after exposures have been determined.
The data obtained was used in estimating the device lifetimes under various vacuum environments. Also it was found that the gas exposure effects are similar in dc and pulsed modes of operation of the arrays, thus permitting the use of dc mode testing as an effective acceleration method in establishing the device lifetimes under various vacuum conditions. The vacuum conditions required for the long term emission current stability and reliability of vacuum microelectronic devices employing FEAs are established.
Exposure of Mo field emitter arrays to oxygen bearing species like oxygen, water and carbon dioxide resulted in serious emission current degradation. Whereas, exposure to methane and hydrogen caused a significant increase in emission current. The control of residual gases like 02, C02 and H20 in the vacuum envelope is essential for the emission current stability and long term reliability of vacuum microelectronic devices employing field emission microcathode technology.
40
Navitski, Aliaksandr [Verfasser]. "Scanning field emission investigations of structured CNT and MNW cathodes, niobium surfaces and photocathodes / Aliaksandr Navitski". Wuppertal : Universitätsbibliothek Wuppertal, 2010. http://d-nb.info/1009494678/34.
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Dong, Chen Dan. "Scaling of ultrafast photon-triggered field emission cathodes composed of arrays of sharpened single-crystal Si pillars". Thesis, Massachusetts Institute of Technology, 2014. http://hdl.handle.net/1721.1/100329.
Testo completoAbstract (sommario):
Thesis: M. Eng., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2014.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 53-55).
Ultrafast (<1 ps-long pulses) cathodes with spatially structured emission are an enabling technology for exciting applications such as free-electron lasers, tabletop coherent x-ray sources, and ultrafast imaging. In this thesis, we explore the scaling down and multiplexing limits of ultrafast photon-triggered field emission cathodes composed of arrays of nano-sharp high-aspect-ratio silicon pillars. We are interested in exploring how the geometry of the array and the morphology of the pillars affect the emission of the electrons. Both the multi-photon regime and the strong-field tunneling regime of the emission process were studied using near-IR pulses at various pulse energies. We model the structure using 2D and 3D COMSOL Multiphysics, collect the charge-energy characteristics of the actual devices, compare and interpret the results from both simulations and experiments. We find that the field factor is a strictly increasing function of the pitch at the fixed height, and the field factor saturates when the pitch is very large. The field factor is also strictly increasing with respect to the height with a diminishing return. In addition, due to the trade-off between the field factor and the pitch scaling, there exists an optimum pitch at 2.5 [mu]m, which yields the highest emission of electrons. This work also confirms earlier work by the group on the emission characteristics and polarization dependence: at low pulse energy, the emission is in the multiphoton regime and has an sin² ([theta]) dependence on the polarization; at high pulse energy, the emission is in the strong-field tunneling regime and has an sin²([theta]) dependence on the polarization.
by Chen Dan Dong.
M. Eng.
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Christy, Larry A. "Field Emission Properties of Carbon Nanotube Fibers and Sheets for a High Current Electron Source". University of Cincinnati / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1406819279.
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Cheng, An-jen Tzeng Y. "Cold cathodes for application in poor vacuum and low pressure gas environments carbon nanotubes versus zinc oxide nanoneedles /". Auburn, Ala., 2006. http://repo.lib.auburn.edu/2006%20Spring/master's/CHENG_AN-JEN_7.pdf.
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Hudanski, Ludovic. "Modulation d'un faisceau électronique issu d'une cathode à base de nanotubes de carbone - Applications aux tubes hyperfréquences". Phd thesis, Ecole Polytechnique X, 2008. http://pastel.archives-ouvertes.fr/pastel-00004268.
Testo completoAbstract (sommario):
Les tubes amplificateurs ont été quelque peu oubliés du grand public depuis leur disparition des récepteurs de radio et d'autres équipements à la fin des années 50 et leur remplacement par les composants état solide. Toutefois, il est des régimes de fonctionnement où ils demeurent incontournables, surpassant tous les dispositifs état solide en termes de fréquence de fonctionnement, de puissance, de rendement et de fiabilité. C'est la raison pour laquelle les satellites de télécommunications embarquent à leur bord une centaine de tubes à ondes progressives pourtant lourds et volumineux. Afin de réduire leur poids et leur taille et d'atteindre des rendements encore plus élevés, différents laboratoires étudient des cathodes capables d'émettre un faisceau électronique modulable en hyperfréquence. Les! laboratoires de TRT, de l'Ecole Polytechnique (Nanocarb) et de TED, dans lesquels cette thèse a été préparée, étudient de nouvelles cathodes à émission de champ basées sur des réseaux de nanotubes de carbone. Ce travail de thèse a porté sur l'étude et le développement de méthodes pour moduler directement le faisceau électronique issu de ces cathodes. La croissance de nanotubes de carbone est ici réalisée par dépôt chimique en phase vapeur assisté par plasma. Des travaux précédents ont permis de développer et d'optimiser une technologie de croissance de nanotubes individuels orientés verticalement, et d'étudier leurs caractéristiques d'émission en continu. La première partie de cette thèse a consisté à démontrer la modulation de l'émission en utilisant des cavités résonantes qui permettent l'obtention de forts champs électriques hyperfréquence. Nous avons ainsi démontré à une fréquence de 1.5 GHz la modulation d'un courant crête de 30 mA correspondant à une densité de courant de 12 A.cm-2, puis, à 32 GHz, la modulation d'un courant crête de 3.43 mA. Ce sont les premières et les seules modulations hyperfréquence de cathodes à base de nanotubes de carbone publiées à ce jour. Cependant l'utilisation de cavités résonantes limite l'emploi de ces cathodes à des applications bande étroite. La deuxième partie de la thèse a été consacrée au développement d'un nouveau concept de photocathodes basées sur l'association de photodiodes p-i-n et de nanotubes de carbone. L'émission électronique de ces dispositifs est alors commandée par la puissance lumineuse reçue et ne requiert plus de cavité. Le contrôle de la modulation est ainsi rendu compatible large bande. Une première génération de photocathodes à nanotubes de carbone sur p-i-n silicium a permis de montrer la modulation d'un courant de 500 µA avec un taux de modulation de 97% pour une puissance optique absorbée de 12 mW.
45
Dionne, Martin. "Optimized carbon nanotube array cathodes for thermo-field emission in plasmas: a theoretical model and an experimental verification". Thesis, McGill University, 2011. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=97045.
Testo completoAbstract (sommario):
In this work we developed a 3-D theoretical model for plasma-enhanced thermo-field emission from nanostructured cathodes in the absence of significant erosion. Our first studies indicated that very dense arrays of vertically-aligned carbon nanotubes (CNT) acting as electron emitters in vacuum could sustain the temperatures resulting from very high surface-averaged current densities such as those found in the cathode spots of arc discharges on non-refractory cathodes. A comparative study of the electron emission models for cold surfaces subjected to strong electric field revealed the existence of a simple relation between the inaccurate Fowler-Nordheim (F-N) equation and the accepted result provided by the theory of Murphy and Good (M-G). We therefore proposed a parametric equation for the emitted current density which was as convenient as the F-N equation but more accurate. The use of M-G theory has also provided an explanation for the tip cooling effect described in a previous study on the destruction of field emitting CNT. We identified the source of the tip cooling effect as the Nottingham effect. For very short emitters, this particular effect heats the surface and accelerates the destruction process and for long emitters, it creates a small isothermal zone at the emitter's tip which is destroyed when it reaches a critical temperature (approximately 1850 K according to our calculations). When combined to existing data on the emitter's length, its diameter, the applied voltage and the measured current, our model can provide the emitter's work function, its room temperature resistivity and the value of the thermal contact resistance between the emitter and its substrate. Another version of this model includes a calculation of the surface electric field in the presence of a non-thermal plasma. To this end we modified the model developed by Mackeown and obtained a general result for 3-D surfaces. This general expression requires the calculation of the ion flux enhancement factor which can be obtained by solving Laplace equation above the surface of interest. This simple approach allows us to describe how the ions are redistributed within the sheath towards the tip of the CNT where the surface field increases. These theoretical predictions were tested by developing simultaneously a fabrication process for a composite electrode matching the optimized design we suggested. Anodic aluminum oxide templates were used as substrates to grow CNT arrays. In order to facilitate their large scale use we modified a standard CNT production process to allow the direct use of as anodized commercial aluminum. Our resulting electrodes were then used as cathodes in low pressure gas discharges. The operating parameters of these discharges are different from the typical voltages and current densities found in glow discharges using as electrodes bare aluminum surfaces. In fact, due to the very low work function of the sharp and relatively ordered emission sites and the simultaneous presence of a ceramic template around them, our electrodes produced very diffuse attachment points for the plasma in a similar fashion as thermionic cathodes do for high pressure arcs. They also required lower (38-140 V) sustaining voltages than what is necessary to sustain a conventional glow discharge. Our electrodes also showed the ability to sustain these low voltage discharges for as much as 500 hours if their bulk temperature was maintained below 60 Celsius and if water vapour was added to the feed gas. Our experiments in nitrogen-water mixtures demonstrated the feasibility of producing large amounts of UV photons at an operating voltage (anode, grounded cathode) of 90-100 V. These results are very promising for future applications in lighting.Dans le cadre de ce projet nous avons développé un modèle en 3-D pour l'émission électronique par effet thermo-champ stimulée par un plasma. Nos premiers résultats ont indiqué que des réseaux denses de nanotubes de carbone (NTC) agissant en tant qu'émetteurs dans le vide pouvaient supporter les températures résultant de densité de courant moyennes très élevées de l'ordre de celles présentes dans les taches cathodiques d'arcs opérant sur des cathodes non-réfractaires. Une étude comparative des modèles pour l'émission électronique pour les surfaces froides soumises à de forts champs électriques a révélé l'existence d'une relation simple entre l'équation imprécise de Fowler-Nordheim (F-N) et le résultat accepté fourni par la théorie de Murphy et Good (M-G). Nous avons donc proposé une équation paramétrique précise et simple pour la densité de courant émise. L'usage de la théorie de M-G a aussi fourni une explication pour l'effet de refroidissement à la pointe décrit dans une étude précédente sur la destruction de NTC émettant par effet de champ: l'effet Nottingham. Pour des émetteurs très courts, cet effet particulier chauffe la surface et accélère le processus de destruction et pour de longs émetteurs, il crée une petite zone isotherme à la pointe de l'émetteur qui est détruite lorsqu'elle atteint une température critique (approximativement 1850 K). Lorsque combiné à des données sur la longueur de l'émetteur, son diamètre, la tension appliquée et le courant mesuré, notre modèle peut fournir le travail de sortie de l'émetteur, sa résistivité à la température ambiante et la valeur de la résistance de contact thermique entre l'émetteur et son substrat. Une autre version de ce modèle inclut un calcul du champ électrique de surface en présence d'un plasma froid. À cette fin, nous avons modifié le modèle développé par Mackeown et obtenu un résultat général pour des surfaces en 3-D. Cette expression générale requiert le calcul du facteur d'amplification du flux ionique, lequel peut être obtenu en résolvant l'équation de Laplace au dessus de la structure d'intérêt. Cette approche simple nous permet de décrire comment les ions sont redistribués à l'intérieur de la gaine vers la pointe des NTC où le champ électrique augmente. Ces prédictions ont été testées en développant simultanément un procédé de fabrication pour une électrode composite correspondant au schéma optimisé que nous suggérions. Des patrons d'oxyde anodique d'aluminium furent utilisés en tant que substrats pour faire croître nos réseaux de NTC mais afin de faciliter leur usage à grande échelle nous avons modifié le procédé de production des NTC pour permettre l'usage directe d'aluminium commercial anodisé. Nos électrodes furent ensuite utilisées comme cathodes dans des décharges à basse pression. Les tensions et densités de courant mesurées sont différentes des valeurs typiques rencontrées dans les décharges électroluminescentes utilisant comme électrodes, des surfaces d'aluminium. En fait, en raison du très faible travail de sortie des sites d'émission pointus et relativement ordonnés et de la présence simultanée d'un patron de céramique autour d'eux, nos électrodes ont produit des points d'attachement très diffus pour le plasma d'une façon similaire à ce qui est observé pour des cathodes réfractaires chaudes telles que le tungstène. Les tensions d'opération, dans la plage 38-140 V, sont de beaucoup inférieures aux tensions observées avec des électrode d'aluminium (>200 V). Nos électrodes ont aussi démontré leur capacité de maintenir ces décharges à basse tension pour au moins 500 heures si leur température moyenne était maintenue sous 60 Celsius et si de la vapeur d'eau était ajoutée au gaz injecté. Nos expériences dans des mélanges d'azote et d'eau ont démontré la faisabilité de produire de larges quantités de photons UV pour un potentiel anodique (cathode mise à la terre) de 90-100 V. Ces résultats sont très prometteurs pour de futures applications en éclairage.
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Orozco, Nieto Pedro Francisco. "Manufacturing strategy for high current cold field emission cathodes : floating catalyst chemical vapour deposition grown carbon nanotube fibres and films enhanced by laser patterning and laser purification process". Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/278104.
Testo completoAbstract (sommario):
The aim of this work is to produce a manufacturing strategy for high current (>10 mA) field emission (FE) devices for military (microwave generation) and civilian (particle accelerator electron beam) applications using carbon nanotubes (CNT) as base material. With a particular focus on the relationship of the laser time pulse duration used for cutting CNTs and how this affects the field emission performance. Material selection for this work was narrowed to CNT as they possess unique characteristics such as: high aspect ratio; high thermal conductivity; high chemical stability and high current carrying capacities up to a theoretical limit of 1,200 MA∙cm-1 making them an ideal material for FE. The CNT material studied in this work is produced in two distinct forms, fibres (∅~7-10 μm) and films (h~30 μm), using a floating catalyst chemical vapour deposition process which produces high quantities of CNT material with mixed mechanical and electrical properties. The material is difficult to handle because of its dimensions and is susceptible to environmental changes i.e. electrostatic forces. In order to reduce the variability in electrical properties, a laser purification process was developed. The process consists of locally irradiating an infra-red (IR) laser several microseconds directly at the material. A percentage is vaporised (mainly non-conductive or defective material) and the remaining CNT material shows very high crystallinity with an increase of up to ten times (G/D ratio > 100) compared to the original material and electron mean free path is increased by an order of magnitude. The production strategy is based on directly coating the CNT material with copper using an electroplating process. This allowed for CNT fibre and film to be easily handled and improved the overall electrical contact. Emitter geometry was customised by a laser cutting process to achieve increased enhancement factor geometries, in this case, triangles with 29 tips whilst reducing FE variability. FE performance was quantified by testing the devices in a continuous DC mode with a sweep up to 1,000 V until the material suffered catastrophic failure. The gap distance between the tip of the triangles and the anode was varied to increase the electric field until failure. FE results using the production strategy improved more than 400% compared to untreated material. Applications for these devices are intended to be in the creation of high energy electron beam lines and generation of high powered directed microwaves.
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Schmitt, Thorsten. "Resonant Soft X-Ray Emission Spectroscopy of Vanadium Oxides and Related Compounds". Doctoral thesis, Uppsala University, Atomic and molecular physics, 2004. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-4290.
Testo completoAbstract (sommario):
This thesis addresses the electronic structure of vanadium and copper oxides using soft X-ray absorption (SXA) spectroscopy and resonant inelastic X-ray scattering (RIXS) at high brightness synchrotron radiation sources. In RIXS incident photons, tuned to the energy of specific absorption resonances, are inelastically scattered leaving behind a low energy valence excitation in the system studied. Effects of electron localization are reflected by the occurrence of low-energy excitations in form of dd- and charge-transfer excitations that are modelled by cluster calculations. Band-like states are dominating when the intermediate core excited state is delocalized.
RIXS at V 2p and O 1s resonances has been used to study the electronic structure of the monovalent vanadium oxides VO2 and V2O3, and of the mixed valence compounds, NaV2O5 and V6O13. For NaV2O5 and V6O13 significant contributions from localized low-energy excitations reflect the partly localized character of their valence band electronic structure, whereas VO2 and V2O3 appear mostly as band-like. Effects of carrier doping are addressed for the case of Mo doping into VO2 and reveal a quasi-rigid band behavior. In the cases of VO2 and V6O13 the temperature dependent metal-insulator transition could be monitored by following the spectral evolution of bands originating from V 3d and V 3d - O2p hybridized states. For Na2V3O7 nanotubes it was possible to selectively probe states from the apical and the basal oxygen sites of VO5 pyramids that constitute these nanotubes. Furthermore, the RIXS technique has been demonstrated to be highly valuable in characterizing the charge transfer processes that accompany lithium insertion into vanadium oxide battery cathodes. Finally, for insulating cuprates RIXS at O 1s, Cu 3p and Cu 3s resonances has been recorded at high-resolution for the detailed investigation of crystal field excitations.
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Vašíček, Martin. "Charakterizace autoemisních zdrojů pro elektronovou mikroskopii". Master's thesis, Vysoké učení technické v Brně. Fakulta elektrotechniky a komunikačních technologií, 2013. http://www.nusl.cz/ntk/nusl-220188.
Testo completoAbstract (sommario):
This work deals with the theoretical foundations of electron emission into vacuum, various types of emissions, focused on the cold-emission and Schottky emission and the principle of quantum tunneling. The next part deals with the technical implementation of electron sources with a detailed study of the methodology of laboratory production of cathodes by electrochemical etching and construction of electron microscopes, using field-emission sources. This work also contains methods for measuring, processing and evaluation of electrical characteristics of emission sources.
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ORLIKOWSKI, JEAN-PIERRE. "Contribution a l'etude des cathodes a emission de champ a effet schottky : utilisation a basse tension, dans un canon a lentille magnetique incorporee". Reims, 1990. http://www.theses.fr/1990REIMS013.
Testo completoAbstract (sommario):
L'objectif du travail qui est presente ici a ete d'etudier les performances a basse tension (10 kv) d'une colonne de microscope electronique a balayage, utilisant une source d'emission intense et parfaitement stable, pour essayer d'obtenir un courant de faisceau electronique important dans des tailles de sonde de l'ordre de 100 nm. Les qualites du faisqceau electronique emis sont en effet primordiales dans plusieurs domaines comme la spectroscopie des electrons auger, la microanalyse x, la microlithographie et, en particulier, le test des circuits integres par stroboscopie du faisceau d'electrons. Les deux principaux chapitres de la these sont conscares a l'etude et au test de cathodes a oxyde permettant d'obtenir des courants de faisceau intenses et stables puis, ensuite, a l'evaluation des performances a basse tension (de 1 a 10 kv) d'une colonne de microscope a balayage equipe d'un canon a emission de champ a lentille magnetique fonctionnant avec ce type de cathode. Le troisieme volet de la these concerne le test d'un nouveau type de canon a emission de champ qui permet le filtrage en energie des electrons. Grace a un dispositif imagine par troyon, on montre qu'il est possible de diminuer assez fortement la dispersion energetique des electrons du faisceau. Typiquement, cette dispersion peut devenir 0,30 ev au lieu de 1,50 ev. Ceci est au detriment de l'intensite globale transportee par le faisceau qui malgre tout reste appreciable (on obtient encore 20 na avec une ouverture angulaire de 1,2 mrad). Il est aussi demontre que l'utilisation des cathodes a oxyde en association a un canon a emission de champ a lentille magnetique incorporee permet d'obtenir des courants de faisceau intenses et tres stables. L'ensemble de ce travail sur les cathodes a oxyde permet de mieux dominer les conditions optimales de fonctionnement et permet d'envisager rapidement un developpement de leur utilisation dans les domai
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Cousin, Richard. "Compréhension des mécanismes régissant le fonctionnement d'un tube hyperfréquence de type MILO (Magnetically Insulated Line Oscillator)". Phd thesis, Ecole Polytechnique X, 2005. http://pastel.archives-ouvertes.fr/pastel-00001651.
Testo completoAbstract (sommario):
Le MILO (Magnetically Insulated Line Oscillator) est une source micro-onde de forte puissance capable de produire des puissances crêtes supérieures au gigawatt à des fréquences de quelques gigahertz. Ce tube hyperfréquence est un oscillateur à champs électrique et magnétique croisés qui ne requiert pas de structure externe pour produire le champ magnétique nécessaire au guidage du faisceau d'électrons. Ceux-ci sont produits par émission explosive à partir d'une cathode de type velours et sont accélérés par la différence de potentiel dans l'espace anode – cathode. L'anode constitue une cavité hyperfréquence capable d'emmagasiner de l'énergie électromagnétique, transférée depuis le faisceau d'électrons. Ainsi, la géométrie du tube fixe les conditions de propagation des électrons dans le vide (isolement magnétique) et les conditions de synchronisme permettant les échanges d'énergie dans la structure hyperfréquence. Cette thèse vise à détailler les caractéristiques de fonctionnement d'un MILO compact, c'est-à-dire à l'échelle ½ en comparaison des dispositifs existants, en vue d'un développement expérimental. Pour cela nous avons démontré la faisabilité d'un modèle réduit en redimensionnant tous les paramètres géométriques. Des simulations en géométrie 2D et 3D sous code PIC – Electromagnétique MAGIC ont permis de caractériser, d'une part, la structure hyperfréquence afin d'optimiser les couplages cavité – guide d'onde de sortie pour l'extraction du rayonnement et de démontrer, d'autre part, les contraintes physiques limitant la puissance micro-onde de sortie. Le prototype ainsi optimisé a conduit à la conception d'un dispositif expérimental où les tests préliminaires à froid (sans faisceau) ont montré une bonne concordance avec les prévisions théoriques. Cette thèse ouvre la voie à l'expérimentation et au développement d'une source micro-onde de puissance compacte.