Minimal domain size necessary to simulate the field enhancement factor numerically with specified precision

In the literature about field emission, finite elements and finite differences techniques are being increasingly employed to understand the local field enhancement factor (FEF) via numerical simulations. In theoretical analyses, it is usual to consider the emitter as isolated, i.e, a single tip field emitter infinitely far from any physical boundary, except the substrate. However, simulation domains must be finite and the simulation boundaries influences the electrostatic potential distribution. In either finite elements or finite differences techniques, there is a systematic error ($\epsilon$) in the FEF caused by the finite size of the simulation domain. It is attempting to oversize the domain to avoid any influence from the boundaries, however, the computation might become memory and time consuming, especially in full three dimensional analyses. In this work, we provide the minimum width and height of the simulation domain necessary to evaluate the FEF with $\epsilon$ at the desired tolerance. The minimum width ($A$) and height ($B$) are given relative to the height of the emitter ($h$), that is, $(A/h)_{min} \times (B/h)_{min}$ necessary to simulate isolated emitters on a substrate. We also provide the $(B/h)_{min}$ to simulate arrays and the $(A/h)_{min}$ to simulate an emitter between an anode-cathode planar capacitor. At last, we present the formulae to obtain the minimal domain size to simulate clusters of emitters with precision $\epsilon_{tol}$. Our formulae account for ellipsoidal emitters and hemisphere on cylindrical posts. In the latter case, where an analytical solution is not known at present, our results are expected to produce an unprecedented numerical accuracy in the corresponding local FEF

The diffusion of military power : a neoclassical realist analysis

This doctoral dissertation explains why some states are able to take advantage of the diffusion of major military innovations while others are not. It further develops Neoclassical Realism by answering questions neglected by Structural Neorealism and Adoption Capacity Theory. While the former consciously ignores the causes that constrain military diffusion and whether states are successful—or not—in benefiting from diffusion processes, the latter does so by overly emphasizing organizational aspects of diffusion to the detriment of political elites’ preferences and international alliances. Studies of military diffusion have concentrated on single case studies or small-n comparisons of specific military technological innovations. This research is explicit about necessary and sufficient conditions, processes, and causal mechanisms. It also develops and tests hypotheses with intermediate-n observations, covering 34 countries and 32 technologies from 1991 to 2014. Such a framework binds two main methods into a Neoclassical Realism analysis: Multiple Correspondence Analysis (MCA) and Fuzzy-Set Qualitative Comparative Analysis (fsQCA). By explaining military diffusion more strategically and comparatively, one may assess whether the diffusion of military innovations leads to power centralization or decentralization in the international system. In short, backwardness advantage, logistical capacity, and the absence of alliance are necessary but trivial conditions for successful military diffusion. On the other hand, threat and the absence of threat are the only two conditions that turned out to be a condition that could be considered “almost necessary” for successful and unsuccessful military diffusion, respectively.Esta tese de doutorado explica por que alguns Estados conseguem tirar proveito da difusão das principais inovações militares, enquanto outros não conseguem. Nela, aprofunda-se o desenvolvimento do Realismo Neoclássico, respondendo a questões negligenciadas pelo Neorrealismo Estrutural e pela Teoria da Capacidade de Adoção. Enquanto o primeiro ignora conscientemente as causas que limitam a difusão militar e se os Estados são bem-sucedidos ou não em se beneficiar dos processos de difusão, a segunda faz o mesmo enfatizando demasiadamente os aspectos organizacionais da difusão em detrimento das preferências das elites políticas e das alianças internacionais. Em geral, estudos sobre difusão militar concentraram-se em estudos de caso ou em comparações de pequeno-n em termos de inovações tecnológicas militares específicas. Este trabalho é explícito sobre as condições necessárias e suficientes, os processos e os mecanismos causais envolvidos. Também desenvolve e testa hipóteses com observações intermediárias, abrangendo 34 países e 32 tecnologias, de 1991 a 2014. Esse modelo analítico combina dois principais métodos em uma análise realista neoclássica: a Análise de Correspondência Múltipla (MCA) e a Análise Qualitativa Comparativa de Conjuntos Difusos (fsQCA). Ao explicar a difusão militar de forma mais estratégica e comparativa, pode-se avaliar se a difusão de inovações militares leva à centralização ou descentralização do poder no sistema internacional. Em resumo, a vantagem do atraso, a capacidade logística e a ausência de alianças são condições necessárias, mas triviais, para uma difusão militar bem-sucedida. Por outro lado, a presença ou ausência de ameaça são as únicas duas condições que se mostraram "quase necessárias" para a difusão militar bemsucedida ou mal-sucedida, respectivamente

Physics-based derivation of a formula for the mutual depolarization of two post-like field emitters

Recent analyses of the field enhancement factor (FEF) from multiple emitters have revealed that the depolarization effect is more persistent with respect to the separation between the emitters than originally assumed. It has been shown that, at sufficiently large separations, the fractional reduction of the FEF decays with the inverse cube power of separation, rather than exponentially. The behavior of the fractional reduction of the FEF encompassing both the range of technological interest $0<c/h\lesssim5$ ($c$ being the separation and $h$ is the height of the emitters) and $c\rightarrow\infty$, has not been predicted by the existing formulas in field emission literature, for post-like emitters of any shape. In this letter, we use first principles to derive a simple two-parameter formula for fractional reduction that can be of interest for experimentalists to modeling and interpret the FEF from small clusters of emitters or arrays in small and large separations. For the structures tested, the agreement between numerical and analytical data is $\sim1\%$

Balancing in unipolarity: who is afraid of balance of power

This article aims to analyze, through a critical bias, the implications of unipolarity to balancing behavior. In order to do so, it discusses the dynamics of balance of power theory, assumed to be inoperative in the post-Cold War period by the main academic debates over unipolarity: i) unipolar stability; ii) balance of threats; iii) soft balancing; iv) liberal institutionalism. What is argued is that, including the unipolar illusion view, tied to the balance of power theory, these approaches overestimated the effects of the unipolarity to the balancing behavior of other states. In this sense, it is assumed here that the issues related to the unipolar moment are directly connected to the hegemonic interregnum discussions. By concluding that the dynamics of balance of power, especially those of hard balancing, are still observed in the post-Cold War era, the two main ponderations of the literature become inverted: i) that balancing became inoperative and; ii) that the only available strategies to other states would be soft balancing and bandwagoning. In sum, this conclusion has directly implication to the available strategies both to the United States and its main peer competitors.&nbsp; &nbsp;&nbsp; &nbsp; &nbsp; Recebido em: Agosto/2018. Aprovado: Setembro/2018.Through a critical bias, this article aims to analyze the implications of unipolarity for balancing behavior. In order to do so, it discusses the dynamics of balance of power theory, assumed to be inoperative in the post-Cold War period by main academic debates over unipolarity: i) unipolar stability; ii) balance of threats; iii) soft balancing; iv) liberal institutionalism. We argue that these approaches, including the unipolar illusion view, tied to the balance of power theory, overestimate the effects of unipolarity on balancing behavior of other states. In this sense, we assume here that issues related to the unipolar moment are directly connected to discussions on hegemonic interregnum. Concluding that balance of power dynamics, especially those of hard balancing, are still observed in the post-Cold War era, we criticize two main conclusions from the literature: i) that balancing became inoperative and; ii) that the only available strategies to other states are soft balancing and bandwagoning. In sum, this conclusion has directly implication on strategies available both to the United States and to its main competitors.&nbsp;&nbsp;&nbsp;&nbsp

The rapid soybean growth in Brazil.

Título em francês: La rapide croissance du soja au Brésil