Este artigo descreve em detalhe a construção e operação de um sistema mecânico que levita quando submetido a tensões maiores ou iguais a 20 kV. Este dispositivo, que corresponde basicamente a um capacitor assimétrico, é conhecido na literatura pelo nome de ‘lifter’. Também é mostrado neste trabalho, através de cálculos razoavelmente simplificados, que o mecanismo físico responsável pelo efeito de levitação está intrinsecamente relacionado com a transferência de momento linear de íons (positivos ou negativos) para o ar circundante. Estes íons são produzidos ao redor do condutor de menor curvatura (portanto com maior potencial elétrico) devido ao chamado efeito corona e, uma vez criados, eles são imediatamente acelerados pela configuração do campo elétrico existente, produzindo múltiplas colisões com as moléculas de ar, causando a transferência de momento e, consequentemente, provocando no lifter um movimento ascendente.

This paper shows in detail the construction and operation of an apparatus that levitates when submitted to voltages greater or of the order of 20 kV. This device, which basically corresponds to an asymmetric capacitor, is commonly known in the literature by the name ‘lifter’. It also will be shown in this article, through rather simplified calculations, that the physics grounds of the observed levitation effect are intrinsically related to the transference of linear momentum of the ions (positive or negative) to the surrounding atmosphere. These ions are produced within the conductor of very small curvature (hence, great electrical potential) due to the so called corona effect and, once created, they are immediately accelerated by the intrinsic electric field lines configuration, producing multiple collisions with the air molecules, causing the transference of momentum and, therefore, provoking the observed lifter ascendant impulsion.

Este artigo foi escrito para mostrar aos alunos de graduação em física e engenharia como estimar as correntes de Foucault. Inicialmente fazemos uma breve análise das condições de contorno entre dois meios com diferentes parâmetros ε, µ e σ, que devem ser obedecidas tanto por campos eletromagnéticos estáticos quanto dependentes do tempo. Em seguida, usando as equações de Maxwell calculamos as "correntes de Foucault", ou eddy currents, que surgem em um condutor plano metálico (paramagnético ou diamagnético) quando sobre ele é aplicado um campo magnético B (t) variável no tempo, gerado por um solenoide longo de seção reta circular.

This paper was written to show to undergraduate students of Physics and Engineering how to estimate the Foucault or eddy currents, in conductors. We begin with a brief discussion about the boundary conditions at the interface of two different medias with respective parameters ε, µ and σ, which must be satisfied by both static and time dependent electromagnetic fields. Afterwards, using the Maxwell equations, we calculate the eddy currents which are induced in a plane metallic conductor (either paramagnetic or diamagnetic) when a time variable magnetic field B (t) produced by a long solenoid of circular cross section is applied upon it.

Chaotic magnetic field lines play an important role in plasma confinement by tokamaks. They can either be generated in the plasma as a result of natural instabilities or artifficially produced by external conductors, like resonant helical windings and ergodic magnetic limiters. This is a review of works carried out at the Universidade de São Paulo and Universidade Federal do Paraná on theoretical and experimental aspects of generation and control of chaotic magnetic field lines in tokamaks.

In this work, we discuss the preliminary analysis of some disruptive plasma discharges in the TCABR tokamak, operating in the high density limit. The Fourier analysis of the MHD activity was greatly facilitated because the magnetic coils inside the TCABR were installed as to take into consideration the toroidal geometry of the system, in a straight forward manner. What we have observed is that the m = 3, 4 and 7 MHD components dominate during almost the whole discharge duration, whereas the m = 2 MHD mode increased substantially just before the occurrence of a major disruption. Also, we could estimate the angular velocity of the magnetic islands, which was observed to increase up to three times just before the major disruption.

The influence of the toroidal geometry, and the plasma column displacement, over the usual tokamak poloidal magnetic field configuration was investigated for circular plasmas through a series of simulations. The results obtained clearly showed the existence of a change in phase modulation of the Mirnov signals, which are not usually considered when the magnetic signals are Fourier analyzed in the frame of cylindrical approximation, that is, by neglecting any possible toroidal effect. Furthermore, if the occasional displacements of the plasma column in tokamaks are considered, some differences on the mode composition are also observed. These discrepancies, however, do not seem to affect the analysis of the magnetic signals as strongly as in the former case, when toroidal effects are neglected.

Beryllium foils of different thickness have been employed for measuring the electron temperature in tokamaks using the absorbing method based on ratio of the bremsstrahlung intensities, emited by the plasma. However, in the case of small tokamaks with low electron temperatures, such as TBR-1, the foils must be very thin, becoming very fragile, hard to handle and with insufficient absorbing power of line radiation from impurities. Also they are very expensive. In this work, we have investigated whether other absorber materials, obtained by depositing Aluminium on polypropylene films, for example, could give similar results. These new kind of filters have good mechanical properties and are light tight, as opposed to the thin beryllium foils which have shown, very often, the presence of pinholes. Also, the attenuation of the line emission from impurities, with the use of such filters, is more effective. Silver foils have also been manufactured and used, but with unsatisfactory results due to the TBR-1 low plasma temperature. However, silver foils should be considered a good candidate for measuring Te through the absorbing method in large tokamaks with higher plasma temperatures.