Theoretical studies on nuclear fusion plasmas and propagation of electromagnetic waves in disordered media

Se realizarán investigaciones en varias áreas de interés para el desarrollo de la fusión nuclear controlada y para la comprensión de los efectos del desorden sobre la propagación de ondas electromagnéticas. Esto incluye:1- Efecto de las perturbaciones electromagnéticas producidas por inestabilidades sobre el confinamiento de partículas energéticas.2- Control de los perfiles cinéticos y magnéticos del plasma.3- Equilibrio, estabilidad y sostenimiento de configuraciones tipo tokamak esférico con columna central de plasma.4- Propagación de ondas electromagnéticas en medios desordenados.Research on various areas of interest for the development of controlled nuclear fusion and for understanding the effects of disorder on the propagation of electromagnetic waves will be performed. This includes:1- Effect of the electromagnetic perturbations produced by instabilities on the confinement of energetic particles.2- Control of the kinetic and magnetic plasma profiles.3- Equilibrium, stability and sustainment of spherical tokamaks with a plasma central column configurations.4- Propagation of electromagnetic waves in disordered media

Microinestabilidades producidas por gradientes de corriente en plasmas magnetizados

Se estudian inestabilidades producidas por gradientes de velocidad y temperatura electrónica en un sustrato de plasma inmerso en un campo magnético con cizalladura. Utilizando diversos modelos para describir la dinámica de iones y electrones se muestra que ambos gradientes pueden excitar modos inestables de baja frecuencia, y se determinan la tasa de crecimiento y estructura espacial de estos modos dentro de un amplio rango de valores de los parámetros de interés. Dependiendo del modelo utilizado, la condición de cuasineutralidad conduce a una ecuación diferencial lineal de segundo orden o a una ecuación integral. En el límite de plasma frío la ecuación diferencial resultante puede ser resuelta analíticamente para perfiles sencillos de velocidad y campo magnético pero cuando se utiliza un modelo cinético para los electrones (y el de fluido frío para los iones) es necesario recurrir a métodos numéricos. En el caso de temperatura iónica finita la fuerte localización de los modos considerados hace que sea necesario recurrir a un formalismo que conduce a una ecuación integral (equivalente a la condición de cuasineutralidad en el espacio k), que debe ser resuelta numéricamente. Se analiza finalmente la importancia de las inestabilidades estudiadas en el proceso de difusión de un perfil de corriente.Fil:Farengo, Ricardo. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Rotating magnetic field current drive in a hollow plasma column with a steady toroidal field

The effect of a steady azimuthal magnetic field on rotating magnetic field current drive is studied. The configuration considered consists of an infinitely long plasma column with a finite radius conductor, which carries a steady longitudinal current, running along its axis. The ions are assumed to be fixed and the electrons are described using an Ohm's law that contains the Hall term. A fully two-dimensional computer code is developed to solve the resulting time-dependent equations. For some values of the steady azimuthal field, two steady-state solutions with different efficiencies are found. (C) 2001 American Institute of Physics.841193119

Spherical tokamaks with a high current carrying plasma center column

Spherical tokamaks (STs) have many advantages from the perspective of a fusion reactor. A further improvement would be to replace the center post by a plasma center column (PCC). In this case, biased electrodes could be used to drive current along the PCC and produce the toroidal magnetic field. Moreover, the magnetic helicity injected (HI) by the PCC can be used to form and sustain the configuration, via magnetic relaxation. The magnetic structure and stability of these so-called ST-PCC configurations are studied in detail. In particular, it is shown that stable equilibria with tokamak-like safety factor (q) profiles can be obtained in the regime of high PCC current and moderate poloidal flux amplification. Using numerical simulations, the feasibility of forming and sustaining ST-PCC configurations via HI is demonstrated. The sustainment in this case involves a significant level of fluctuations and is shown to occur at a marginally stable configuration having a q = 1 surface in the ST. This behavior is in close analogy to that of spheromaks sustained by a coaxial plasma gun but presents two major differences. First, the current density in the open flux region (PCC) is significantly larger. Second, the mean current density gradient in the ST has the opposite sign, leading to q profiles with regular magnetic shear (i.e., q increases from the magnetic axis to the separatrix, as in tokamaks). When helicity injection is switched off, the fluctuations decay and nested, closed magnetic surfaces appear. This opens the possibility of using HI to form the ST-PCC and a combination of auxiliary current drive (neutral beams and/or RF) and high bootstrap current to sustain a fluctuation free configuration.Fil: Lampugnani, Leandro Gabriel. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Garcia Martinez, Pablo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Universidad Nacional de Río Negro. Sede Andina; ArgentinaFil: Farengo, Ricardo. Universidad Nacional de Río Negro. Sede Andina; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentin

Effect of the helicity injection rate and the Lundquist number on spheromak sustainment

The dynamics of the magnetic relaxation process during the sustainment of spheromak configurations at different helicity injection rates is studied. The three-dimensional activity is recovered using time-dependent resistive magnetohydrodynamic simulations. A cylindrical flux conserver with concentric electrodes is used to model configurations driven by a magnetized coaxial gun. Magnetic helicity is injected by tangential boundary flows. Different regimes of sustainment are identified and characterized in terms of the safety factor profile. The spatial and temporal behavior of fluctuations is described. The dynamo action is shown to be in close agreement with existing experimental data. These results are relevant to the design and operation of helicity injected devices, as well as to basic understanding of the plasma relaxation mechanism in quasi-steady state.Fil: Garcia Martinez, Pablo Luis. Universidad Nacional de Río Negro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Lampugnani, Leandro Gabriel. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Farengo, Ricardo. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentin

Double-helix current drive revisited

The conditions required for efficient current drive in a weakly resistive plasma column, subject to a double helix traveling magnetic field, are examined in detail by using a simple non-linear Ohm's law for the plasma. In agreement with previous numerical work on the subject by Bertram [5], it is shown that field penetration is indeed strongly limited when large external bias longitudinal magnetic fields are used. However, there is a range of small external bias fields that allow for significant penetration when reasonable driving fields are used. This should be of interest for sustaining reversed-field pinches and toroidal screw pinches.867872Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Self-Organization in a Driven Dissipative Plasma System

We perform a fully self-consistent 3-D numerical simulation for a compressible, dissipative magneto-plasma driven by large-scale perturbations, that contain a fairly broader spectrum of characteristic modes, ranging from largest scales to intermediate scales and down to the smallest scales, where the energy of the system are dissipated by collisional (Ohmic) and viscous dissipations. Additionally, our simulation includes nonlinear interactions amongst a wide range of fluctuations that are initialized with random spectral amplitudes, leading to the cascade of spectral energy in the inertial range spectrum, and takes into account large scale as well as small scale perturbation that may have been induced by the background plasma fluctuations, also the non adiabatic exchange of energy leading to the migration of energy from the energy containing modes or randomly injected energy driven by perturbations and further dissipated by the smaller scales. Besides demonstrating the comparative decays of total energy and dissipation rate of energy, our results show the existence of a perpendicular component of current, thus clearly confirming that the self-organized state is non-force free.Comment: Paper is accepted in Journal of Plasma Physics - In Pres

Development of Magnetohydrodynamic Modes During Sawteeth in Tokamak Plasmas

A dynamical analysis applied to a reduced resistive magnetohydrodynamics model is shown to explain the chronology of the nonlinear destabilization of modes observed in tokamak sawteeth. A special emphasis is put on the nonlinear self-consistent perturbation of the axisymmetric m = n = 0 mode that manifests through the q-profile evolution. For the very low fusion-relevant resistivity values, the q-profile is shown to remain almost unchanged on the early nonlinear timescale within the central tokamak region, which supports a partial reconnection scenario. Within the resistive region, indications for a local flattening or even a local reversed-shear of the q-profile are given. The impact of this ingredient in the occurrence of the sawtooth crash is discussed.Fil: Firpo, Marie C.. Ecole Polytechnique; Francia. Centre National de la Recherche Scientifique; FranciaFil: Ettoumi, W.. Ecole Polytechnique; Francia. Centre National de la Recherche Scientifique; FranciaFil: Farengo, R.. Comision Nacional de Energia Atomica. Gerencia del Area de Investigaciones y Aplicaciones no Nucleares. Gerencia de Fisica (CAB); Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Ferrari, Hugo Emilio. Comision Nacional de Energia Atomica. Gerencia del Area de Investigaciones y Aplicaciones no Nucleares. Gerencia de Fisica (CAB); Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Garcia Martinez, Pablo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Lifschitz, A. F.. Ecole Polytechnique; Francia. Centre National de la Recherche Scientifique; Franci