Neutral particles play an important role on the performance of tokamak plasmas. In this dissertation, the original TEP methodology has been extended to take into account linearly (DP_1) and quadratically (DP_2) anisotropic distributions of angular fluxes for calculations of transmission probabilities. Three approaches, subdivision of optically thick regions, expansion of collision sources and the diffusion approximation, have been developed and implemented to correct effects of the preferential probability of collided neutrals escaping back across the incident surface. Solving the diffusion equation via the finite element method has been shown to be the most computationally efficient and accurate for a broader range of D/l by comparisons with Monte Carlo simulations. The average neutral energy (ANE) approximation has been developed and implemented into the GTNEUT code. The average neutral energy approximation has been demonstrated to be more accurate than the original local ion temperature (LIT) approximation for optically thin regions. The simulations of the upgraded GTNEUT code excellently agree with the DEGAS predictions in DIII-D L-mode and H-mode discharges, and the results of both the codes are in a good agreement with the experimental measurements.

A detailed analysis of neutral atom recycling and pedestal fueling in a DIII-D [J. Luxon, Nucl. Fusion, 42, 614, 2002] high-confinement mode discharge is presented. Experimental data and 2D edge plasma fluid code calculations are employed to provide ion wall recycling and recombination neutral sources and background edge plasma parameters for a 2D edge neutral code calculation of detailed neutral density, ionization and charge-exchange distributions throughout the edge pedestal, scrape-off layer and surrounding halo region, divertor, and private flux regions. The effectiveness of the different neutral sources for fueling the confined plasma is evaluated.

The presence of a large pinch velocity in the edge pedestal of high confinement (H-mode) tokamak plasmas implies that particle transport in the plasma edge must be treated by a pinch-diffusion theory, rather than a pure diffusion theory. Momentum balance also requires the inclusion of a pinch term in descriptions of edge particle transport. A numerical investigation of solving generalized pinch-diffusion theory using methods extended from the numerical solution methodology of pure diffusion theory has been carried out. The generalized diffusion equation has been numerically integrated using the central finite-difference approximation for the diffusion term and three finite difference approximations of the pinch term, and then solved using Gauss reduction. The pinch-diffusion relation for the radial particle flux was solved directly and used as a benchmark for the finite-difference algorithm solutions to the generalized diffusion equation. Both equations are solved using several mesh spacings, and it is found that a finer mesh spacing will be required in the edge pedestal, where the inward pinch velocity is large in H-mode plasmas, than is necessary for similar accuracy further inward where the pinch velocity diminishes. An expression for the numerical error of various finite-differencing algorithms is presented.

Thèse doctorat : Physique des Plasmas : Nancy 1 : 2005. / Titre provenant de l'écran-titre.

Thèse de doctorat : Physique des Plasmas : Nancy 1 : 2006. / Titre provenant de l'écran-titre.

In this thesis a model for saturated tearing mode islands is developed. The equations for the mode amplitudes are essentially those of R B White et al,after a pertubation expansion has been made. It is well known that these equations are not then analytic at the mode rational surface. In our model this problem is overcome when a suitable choice of the axisymmetric current density perturbation is added to the unperturbed equilibrium current density profile. The modelled axisymmetric current density perturbation flattens the unperturbed profile locally at the rational surface and is sufficient to induce an island. No modelling in the interior of the island is necessary. The axisymmetric perturbation has a free variable which adjusts the amount of local flattening. However, when the boundary conditions are taken into account, this free parameter is determined, and the problem becomes an eigenvalue problem. The boundary condition thus determines the amount of local flattening at the rational surface. The saturated island widths are determined using D.' (W) criterion. The model allows for non axsymmetric plasma surface in a simple way, requiring careful choice of D (W). The different criteria are compared to establish the validity of the use of such criteria for perturbed boundaries. In the cylindrical approximation, one or two modes may be included in the model. In the case of two modes, non-linear coupling via the current density profile is introduced. Toroidal coupling between modes can also be simply introduced. Two modes that are toroidally coupled are considered, but mode-mode coupling is ignored. The emphasis falls in large part on the boundary conditions. Various boundary conditions can be considered because distortion of the plasma surface can be fixed by wall effects, plasma rotation, external DC coil currents, plasma rotation with external coil currents, etc. Of particular interest is the case of toroidally coupled modes, coupled in turn to these external conditions as this is the first study of such a nature. Results flowing from the study include among others that: for the special case of circular boundaries the model agrees reasonably with the results of R B White et al. No significant difference was found between the D. I (W) criterion of P H Rutherford, which is valid for circular boundaries, and that of A H Reiman, which is also valid for perturbed boundaries, when the boundary is perturbed significantly. Toroidally coupled islands do not increase in size if the boundary condition of that particular mode is not changed. If a coil current of particular helicity is switched on, it will only affect the mode of that particular helicity. Toroidally induced sideband islands have approximately the same width as natural tearing islands when the size of the natural island is large. / Thesis (Ph.D.)-University of Natal, Durban, 1992.

The turbulence and fluctuation induced transport in the edge plasma of the Tokoloshe tokamak was studied using a Langmuir probe array. In this thesis three separate experiments are presented, each of which examines a particular aspect of the edge plasma. In the first experiment measurements of edge plasma parameters are presented. These include standard parameters (such as Ne, Op , Te, etc.) as well as features such as the velocity shear, T(t) during periods of both high and low Mirnov activity, Te/Te and Q. These are compared with results from other machines as well as predictions of several turbulence theories. It was found that many of the results are very similar to those obtained on other machines and that, since the operating parameter space on Tokoloshe is well within the parameter space described by drift wave theories, resistivity-driven gradient driven turbulence theories do not describe the edge turbulence. In the second experiment external windings are used to produce fields which can slow and lock magnetic islands in the toroidally rotating plasma. Edge parameters are again presented and these results compared with those from the so-called 'reference' plasmas, i.e. ones in which no locking occurred. During locking some parameters are dramatically altered, e.g.Te/Te Standard transport theory ignores the effect of Te/Te since they are usually small in reference discharges. During the locked phase, however, certain measurements used to deduce T and Q are greatly affected by increases in Te/Te. As a result, certain assumptions regarding these measurements are no longer valid. Comparison of results for different island positions (produced by different coils) indicates that the assumption of poloidal and toroidal symmetry of edge conditions is invalid. The third experiment investigates the high frequency (~60 kHz), low amplitude, magnetic oscillation which characterises the locked phase and which exhibits some small degree of correlation with the fluctuations observed on (e.g.) Of'. Since over 80% of the spectral power of Te/Te lies below 70 kHz and since Of /Te depends strongly on Te/Te , it is suggested that the magnetic mode and these large variations in Te, may be due to a similar physical process. / Thesis (Ph.D.)-University of Natal, 1993.

Este trabalho constitui-se numa monografia para ser apresentada como requisito para o título de Doutor em Ciências dentro do Programa de Pós-Graduação em Física da Universidade Federal do Rio Grande do Sul. Faz parte da pesquisa teórica desenvolvida pelo Grupo de Física de Plasmas do IF-UFRGS, e tem por objetivo participar do esforço de tornar comercialmente viável a tecnologia de fusão termonuclear como fonte geradora de energia elátrica. Conhecimento científico e tecnológico que é imprescindível para um país em desenvolvimento como o Brasil. Dentro deste contexto, o trabalho desenvolvido segue a linha de pesquisa do estudo de processos de geraçãao de correntes em tokamaks por meio de interações onda-partícula. Assim, estudamos os efeitos devido a ocorrência de transporte radial de partículas em um tokamak na eficiência de geração de corrente devido a ondas de radio freqüência (RF), na presença de barreiras internas de transporte (ITB - Internal Transport Barriers). Para isso, considerou-se dois casos: no primeiro, a geração de corrente ocorre devido somente a ondas do tipo híbrida inferior (LH - Lower Hybrid), e no segundo, a geração com ação combinada das ondas LH e do tipo eletrônica de cíclotron (EC - Electron Cyclotron). Os resultados foram obtidos por solução numérica da equação de Fokker-Planck que rege a evolução da função distribuição de elétrons. Nós consideramos que o transporte radial de partículas pode ser devido a utuações magnéticas ou eletrostáticas. Considerando ambos os tipos de utuações, a eficiência da geração de corrente mostrou um crescimento com o crescimento das utuações que originam o transporte. A dependência da eficiência da geração de corrente na profundidade e posição da barreira também foi investigada. / This work constitutes a monograph to be presented as requirement to the title of PhD at the Physics Graduate Program at Federal University of Rio Grande do Sul. It is part of a theoretical research developed by the Plasma Physics Group of the IF-UFRGS, and it have the purpose to participe on the e ort to make feasible the thermonuclear fusion technology as a source of electric energy. Scienti c and technological knowledge that are indispensable to a developing country as the Brazil. In that context, the work under development follows the research line of the study of the process of current drive in tokamaks by means of wave-particle interaction. Therefore, we studied the e ects due to the occurrence of radial transport of particles in a tokamak on the e ciency of current drive due the radio frequency waves (RF), in the presence of an Internal Transport Barrier (ITB). For this, we consider two cases: at rst, the current drive occur due to Lower Hybrid waves (LH) only, and at second case, the current drive with combined action of LH and Electron Cyclotron (EC) waves. The present results are obtained by numerical solution of the Fokker-Planck equation which rules the evolution of the electron distribution function. We assume that the radial transport of particles can be due to magnetic or to eletrostatic utuations. It considering both the utuations types, the e ciency of current drive is shown to increase with the increase of the utuations which originate the transport. The dependence of the current drive e ciency on the depth and position of the barrier is also investigated.

Usamos a teoria quase-linear para estudar os efeitos do transporte radial de partículas na eficiência da geração de corrente por ondas do tipo híbrida inferior (lower hybrid ou LH), em um tokamak modelado como uma lâmina. Nossos resultados numéricos foram obtidos com cinco diferentes modelos do termo de transporte e indicaram que embora a potência absorvida e a corrente gerada possam ser modificadas por efeito do transporte, a proporção de variação dessas quantidades não é muito sensível a uma forma particular do termo de transporte. Na formulação quase-linear utilizada, a evolução no tempo da função distribuição de elétrons, em um dado ponto da geometria de lâmina proposta, ocorre sob a ação de ondas do tipo híbrida inferior, colisões e transporte, e é descrita pela seguinte equação: 8rfe = (8rfehH + (8rfe)COL + (8rfeh . Oterceiro termo pretende demonstrar a natureza e a magnitude dos efeitos de transporte, e é dado pela seguinte forma: (8rfeh = 8s [DT(S) 8sie] , com um coeficiente para difusão espacial dependente de posição. Utilizamos cinco formas totalmente arbitrárias para a dependência de posição, com as quais pretendemos verificar a sensibilidade do processo de geração de corrente a aspectos do termo de difusão.

To understand the physical mechanism underlying the cross-field transport of mass and energy in magnetoplasmas is a long-standing problem in fusion research. Woods (1987) has recently developed a second-order transport theory which has been used to explain a number of transport-related phenomena observed in tokamaks. Here, we apply second-order transport theory to the reverse field pinch (RFP) and a phenomenon observed in tokamaks known as 'snakes'. Expressions for the mass and energy confinement times in the RFP, τ_p and τ_e, are deduced and agreement with experimental results from HBTX is found. For typical operating conditions the times τ_p ~ 0.1ms and τ_e ~ 0.2ms are observed in HBTX. Second-order transport theory predicts τ_p ~ 0.4ms and τ_e ~ 0.4ms for this machine. Scaling laws for β_p versus η_e,β_p versus I_φ and τ_e versus I_φ are compared with measurements from HBTX and agree well with observation. Snakes are large density perturbations observed in JET after fuel pellet injection. Typical snakes in JET are remarkably stable and are found to have density decay times longer than predictions based on neoclassical theory (Stringer 1987). After their formation, snakes have even been observed to grow (Weller <i>et al.</i> 1987), which suggests the presence of an inward diffusion mechanism. There is also some evidence for a temperature depression in the snakes region. An explanation of the stability and energy balance in snakes based on second-order transport theory is proposed.