Dissertations / Theses on the topic 'Plasmon damping'

The aim of this thesis was to study the cyclotron damping in magnetized plasmas using a different approach to the dielectric tensor that is the stardard way to study this case. In this approach given we deduce a set of coupled differential equations that give us the evolution of the electric field and the distribution function. The system of coupled equations can not be solved analitically, that is why we have found numerical solutions. The algorithm we used to obtain the numerical solutions is the staggered leap-frog method that common used in problems involving electromagnetic fields. We have studied two cases where we consider two different initial conditions for our distribution function in the velocity space. In the first example we used ˜g(t = 0, v_n) = 0. In this case we found that the electric field decays exponentially and there is phase mixing in the evolution of the distribution function. As second example we used as initial condition the expression ˜g(t = 0, v_n) = E_n/(iv_n −\gamma). In this case the phase mixing is less pronounced compared to the first example, and the electric field start growing until the oscillations of the distribution function start to become important, then the electric field start to decay slowly.

Two rich and vibrant fields of investigation - graphene and plasmonics - strongly overlap in this work, giving rise to a novel hybrid photodetection device. The intrinsic photoresponse of graphene is significantly enhanced by placing the gold nanorods exhibiting unique anisotropic localized surface plasmon resonances on the graphene surface. The reported enhanced photoresponse of graphene is caused by the redistribution of localized surface plasmons in the nanoparticles into graphene. The exact underlying energy redistribution mechanism is thoroughly studied by a single particle scattering spectroscopy monitoring the particle plasmon linewidth as a function of the number of underlaying graphene layers. The obtained extraordinary plasmon broadening for nanoparticles placed on graphene suggests the contribution of a novel energy redistribution channel attributed to the injection of hot electrons from gold nanorods into graphene.

A theoretical model for the radio frequency (RF) heating system of a two current strap antenna in a tokamak is presented, considering a finite reflection of plasma waves and taking passive conducting components, e.g. conducting limiters, close to the antenna into account. Specifically, scenarios resulting in undesirable effects of the coupling, such as a lowering of the coupling resistance or the current drive, and variational structures of quantities in continuous parameter intervals are being investigated. A plane slab geometry is used, neglecting poloidal variations and using an equidistant discretization of toroidal coordinates. It is shown that most notable effects of passive components are found in the regime of low single pass damping. The quality factor (fraction of resistive power to apparent power transmitted to the plasma) and the plasma directivity averaged with respect to frequency is lowered when taking passive components into account. The presence of passive components also affects the spectrum of coupling resistance, but with no frequency averaged effects of the coupling resistance being observed. There are heavy oscillations of the coupling resistance, quality factor and plasma directivity for small variations of antenna frequency in the case of low single pass damping, which in turn relates to small variations of plasma density.

Inici de l’estudi de l’efecte de les col•lisions entre ions i neutres en l’esmorteïment d’ones magnetohidrodinàmiques. Es considera un plasma parcialment ionitzat, infinit i homogeni, i s’analitza l’esmorteïment temporal i espacial de les ones magnetoacústiques i les ones d’Alfvén tan en el cas de plasmes adiabàtics com en el de no adiabàtics. Mentre l’esmorteïment temporal de les ones MHD en plasmes adiabàtics parcialment ionitzats és degut a les col•lisions entre ions i neutres, en el cas no adiabàtic és possible estudiar la importància de cada mecanisme d’esmorteïment involucrat. Per altre banda, en el cas de l’esmorteïment espacial s‘han estudiat també ones MHD adiabàtiques i no adiabàtiques en plasmes resistius totalment ionitzats així com en plasmes parcialment ionitzats, i hem inclòs la presència de fluxes. S’inicia l’estudi amb el desenvolupament de les equacions magnetohidrodinàmiques per un fluid considerant ionització parcial i s’aplica aquest conjunt d’equacions a diferents configuracions de plasmes.
The study of the effect of ion-neutral collisions on the damping of magnetohydrodynamic waves is started. We develop a set of one-fluid equations for a partially ionised plasma and use it in different plasma configurations. As a first step, the simplest plasma configuration is considered, an unbounded homogeneous partially ionised plasma. We study the temporal and spatial damping of magnetoacoustic and Alfvén waves in the case of adiabatic and non-adiabatic plasmas. While the time damping of MHD waves in adiabatic partially ionized plasmas is due to ion-neutral collisions, in the non-adiabatic case it is possible to study the importance of each of the different damping mechanisms involved. In the case of spatial damping we have considered adiabatic and non-adiabatic MHD waves in fully ionized resistive and partially ionised plasmas, and we have also included flows.

Devido à sua importância em turbulência causada por ondas de deriva e à aplicação com propósitos em diagnósticos de plasma, a investigação de fluxos zonais (ZF) e modos acústicos geodésicos (GAM) tem atraído bastante atenção na literatura em física de plasmas. Nesta tese, primeiramente consideramos efeitos de equilíbrio com rotação poloidal e toroidal nestes modos, posteriormente investigamos efeitos diamagnéticos em GAM a partir de um modelo de dois fluido, no qual incluímos viscosidade paralela de íons e, na parte final, consideramos amortecimento de Landau e efeitos diamagnéticos simultaneamente no estudo de GAM, porém, a partir do modelo girocinético. Efeitos diamagnéticos são causados por termos que envolvem gradientes de densidade e de temperatura provenientes da função Maxwelliana de equilíbrio. O acoplamento entre os harmônicos poloidais, $m = \\pm1$, e as derivadas radiais de quantidades macroscópicas do plasma é responsável pelo aumento no valor da frequência no GAM de alta frequência e pela instabilidade no GAM de baixa frequência. Este tipo de instabilidade, que é proporcional à frequência diamagnética de elétrons e à razão entre os gradientes de temperatura e de densidade, é mais propenso a ocorrer em posições radiais em que o fator segurança é alto. Modos geodésicos são fracamente amortecidos devido a um mecânismo não colisional conhecido por amortecimento de Landau, o qual é causado pela interação entre a onda eletrostática e partículas carregadas, íons no caso, e a taxa de amortecimento é maior próximo ao centro da coluna de plasma, onde o fator de segurança assume valores mais baixos. O equilíbrio MHD com rotação foi investigado em três regimes com relação às superfícies magnéticas: isotérmico, adiabático e isométrico. Foi observado que o gradiente de temperatura possui sentido oposto em relação à velocidade de rotação poloidal apenas no regime isométrico. Ao considerar equilíbrio com rotação e superfícies magnéticas isotérmicas e incluir fluxo de calor na equação da energia, observamos que ZF apresentam frequência não-nula, a qual é proporcional à velocidade de rotação poloidal e inversamente proporcional ao fator de segurança. Como direções futuras ressaltamos que é importante considerar efeitos eletromagnéticos, estudar automodos geodésicos e incluir o efeito de partículas aprisionadas para o desenvolvimento da física de ZF e GAM. Tal desenvolvimento beneficiará tanto a área de transporte em tokamaks como a área de diagnósticos, na qual a obtenção do perfil radial da temperatura de íons e do fator de segurança é um dos objetivos. Nesta área, um novo tipo de diagnóstico conhecido como espectroscopia em modos acústicos geodésicos está sendo desenvolvido baseado no estudo de automodos.
Due to the important role in drift wave turbulence and applications for plasma diagnostic purposes, the investigation of zonal flows (ZF) and associated geodesic acoustic modes (GAM) has arisen much attention in the plasma physics literature. In this thesis, first we consider equilibrium poloidal and toroidal rotation effects on these modes using the ideal MHD model, then we investigate diamagnetic effects on GAM using a two fluid model that includes parallel ion viscosity, and, in the final step, we include both Landau damping and diamagnetic effects on the study of GAM within the framework of the gyrokinetic model. By diamagnetic effects we mean the density and temperature radial gradients terms coming from the equilibrium Maxwellian distribution function. The effects caused by the coupling between the $m = \\pm1$ poloidal harmonics and the radial derivatives of equilibrium macroscopic quantities are responsible for an increase in the frequency value of the high frequency GAM and for an instability in the low frequency GAM. This instability, which is proportional to the electron drift frequency and the ratio between ion temperature and density gradients, are more likely to occur in radial positions where the safety factor is high. We observe that geodesic modes are slowly damped by a collisionlees mechanism known as Landau damping which is caused by the wave particle interaction between the eletrostatic potential and the íons. This damping is enhanced near the center of the plasma column, where the safety factor has lower values. Equilibrium MHD with plasma rotation were investigated in three regimes regarding the magnetic surfaces: isotherm, adiabatic and isometric. It is found that the temperature gradient has opposite directions compared to the poloidal rotation only for the isometric regime. By considering equilibrium rotation with isotherm magnetic surfaces and including heat flux we observed that ZF has a non-zero frequency which is proportional to the poloidal velocity and the inverse of the safety factor. For future directions we point out that electromagnetic effects, geodesic eigenmodes and trapped particles physics should be important for the development of the ZF and GAM physics, either in the area of anomalous transport caused by drift wave turbulence or for diagnostic purposes for obtaining the radial profile of the ion temperature and the safety factor. In this area, a new kind of diagnostic known as geodesic acoustic mode spectroscopy is being developing based on the study of eigenmodes.

Cette thèse porte sur le comportement en temps long de solutions d’équations de type Vlasov, principalement le modèle Vlasov-HMF. On s’intéresse en particulier au phénomène d’amortissement Landau, prouvé mathématiquement dans divers cadres, pour plusieurs équations de type Vlasov, comme l’équation de Vlasov-Poisson ou le modèle Vlasov-HMF, et présentant certaines analogies avec le phénomène d’amortissement non visqueux pour l’équation d’Euler 2D. Les résultats qui y sont décrits sont les suivants. Le premier est un théorème d’amortissement Landau pour des solutions numériques du modèle Vlasov-HMF, obtenues par discrétisation en temps de ce dernier via des méthodes de splitting. Nous prouvons en outre la convergence des schémas numériques. Le second est un théorème d’amortissment Landau pour des solutions du modéle Vlasov-HMF linéarisé autour d’états stationnaires inhomogènes. Ce théorème est accompagné de nombreuses simulations numériques destinées à étudier numériquement le cas non-linéaire, et semblant mettre en lumière de nouveaux phénomènes. Enfin, le dernier résultat porte sur la discrétisation en temps de l’équation d’Euler 2D par un intégrateur de Crouch-Grossman symplectique. Nous prouvons la convergence du schéma
This thesis concerns the long time behavior of certain Vlasov equations, mainly the Vlasov- HMF model. We are in particular interested in the celebrated phenomenon of Landau damp- ing, proved mathematically in various frameworks, foar several Vlasov equations, such as the Vlasov-Poisson equation or the Vlasov-HMF model, and exhibiting certain analogies with the inviscid damping phenomenon for the 2D Euler equation. The results described in the document are the following.The first one is a Landau damping theorem for numerical solutions of the Vlasov-HMF model, constructed by means of time-discretizations by splitting methods. We prove more- over the convergence of the schemes. The second result is a Landau damping theorem for solutions of the Vlasov-HMF model linearized around inhomogeneous stationary states. We provide moreover a quite large amount of numerical simulations, which are designed to study numerically the nonlinear case, and which seem to show new phenomenons. The last result is the convergence of a scheme that discretizes in time the 2D Euler equation by means of a symplectic Crouch-Grossmann integrator

Plasmons, the collective electronic oscillations of metallic nanoparticles and nanostructures, are at the forefront of the development of nanoscale optics. Metallic nanostructures with their geometry-dependent optical resonances are a topic of intense current interest due to their ability to manipulate light in ways not possible with conventional optical materials. As optical frequency nanoantennas, reduced-symmetry plasmonic nanoparticles have light-scattering properties that depend strongly on geometry, orientation, and variations in dielectric environment. Particularly fascinating aspect of these systems is the recently realized possibility of creating optical frequency “magnetic plasmon” responses of comparable magnitude to the “electric plasmon” response. It is of our central interest to understand better the plasmonic system so as to manipulate the energy transport mechanism. With the much more advanced numerical calculations, and based on the Finite Element Method (FEM) and Finite-Difference Time-Domain (FDTD) method, we are now able to study various kinds of nanostructures for different interesting optical properties. With the help of FDTD, we show the geometry dependent dissipation rate in different nanosystems. We brought up a new damped harmonic oscillator model to account for the observed difference. We show that our new model better completes the full map of the energy dissipation mechanism, and the predicted outcome agreed very well with the FDTD calculations. Elliptical nanorings were investigated by applying both FEM and FDTD methods. The mulitiple plasmonic resonaces exhibited by elliptical nanorings and the well tunability of the nanosystem make elliptical nanorings very interesting. Different features can be realized by controlling the aspect ratios of the elliptical nanorings. We show another interesting nanostructures, light bending nanocup as well. Due to its unique light scattering properties, nanocup is a very promising candidate in solar cell applications. We studied more about its light redirection properties with the presence of a dielectric substrate and its sensitivity to the subtle geometry differences. Plasmonic heptamer has been shown to possess an intriguing Fano resonance due to the interference of its hybridized subradiant and super-radiant modes. Neighboring fused heptamers can support magnetic plasmons due to the generation of antiphase ring currents in the metallic nanoclusters. We use such artificial plasmonic molecules as basic elements to construct low-loss plasmonic waveguides and devices. The manipulation of magnetic plasmons in heptamer interconnects can further be expanded to more complex systems, for example, by integrating more optical paths to achieve multiple input and output plasmonic networks. With their compact dimensions, outstanding low-loss propagation characteristics, and range of functionalities, magnetic plasmon-based devices based on these structures should be key to the further development of high- performance energy transport components in informa- tion processing and data storage applications.

Plasmonic structures permit the focusing of light into volumes far below the diffraction limit. In particular Metal-Insulator-Metal (MIM) gap plasmonic structures can reach nanoscale energy confinement if the gap is sufficiently miniaturized. Under classical models, gap plasmonics can achieve indefinite confinement, down to the single atom level. However, these classical models fail to consider quantum effects that occur as the confinement approaches the single nanometer level. Recently, it has been demonstrated that Landau Damping, the absorption of highly confined plasmonic energy, is the dominant effect in highly confined MIM devices until the tunneling regime is reached. However, the effects of Landau Damping on MIM gap devices are poorly understood. In this work, we analyze the effects of Landau Damping on MIM gap devices, specifically MIM waveguides and cavities. It is found that in waveguides, Landau Damping does not limit the confinement but does limit the maximum propagation length achievable. Moreover, in cavity structures, Landau Damping causes the Quality Factor to drop significantly as the gap is further miniaturized. In terms of quantum optics applications, this causes the radiative spontaneous emission enhancement to actually decrease as the gap is miniaturized sufficiently and a saturation of the coupling-loss ratio limiting the achievement of strong coupling. These effects will limit the possibilities for high performance nanogap plasmonic devices.

碩士
國立中央大學
物理學系
105
The discrete-particle effects in particle-in-cell (PIC) simulation can numerically enhance the thermalization of collisionless plasmas, such that they can potentially change the dynamic properties of the simulated plasma system. The simulation results show that the numerical fluctuation induced by discrete-particle effects can be remedied by taking ensemble average over many computer runs to obtain the Landau damping rate, which is consistent with the theoretical estimation. But the nonlinear phase trapping can only be recovered from the numerical noise by using a reasonable number of macro-particle number in a Debye region. Moreover, both Krook-type and head-on collision models are implemented in the PIC simulation for studying the Landau damping in collisional plasmas. The convergence of numerical results due to discrete particle effects in PIC simulations will be discussed in the paper.