Scielo RSS <![CDATA[Brazilian Journal of Physics]]> http://www.scielo.br/rss.php?pid=0103-973319980003&lang=en vol. 28 num. 3 lang. en <![CDATA[SciELO Logo]]> http://www.scielo.br/img/en/fbpelogp.gif http://www.scielo.br <![CDATA[<b>Testing a new strategy to treat divergent amplitudes in QED</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97331998000300001&lng=en&nrm=iso&tlng=en We analyse a new strategy to manipulate and calculate divergent amplitudes in the context of Quantum Electrodynamics. We compare our results with results of Dimensional Regularization and (one parameter) Pauli-Villars regularization prescriptions and conclude that the present technique allows for an unambiguous determination of the physical content of the divergent amplitude. The results obtained for QED can be easily extended to nonrenormalizable theories where they should be most especially useful. <![CDATA[<b>On the Markovian limit in a kinetic theory for dissipative systems</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97331998000300002&lng=en&nrm=iso&tlng=en We reconsider a nonlinear quantum kinetic theory which is built within the context of a nonequilibrium statistical ensemble formalism. This is the Nonequilibrium Statistical Operator Method based on a variational principle, namely, the Maximization of the Informational-Statistical Entropy, and referred to as MaxEnt-NESOM. It may be considered as encompassed within the framework of E. T. Jaynes' Predictive Statistical Mechanics. These theory has an ample domain of application covering a large class of experimental conditions. We consider a particular <FONT FACE="Symbol">¾</font> and quite important <FONT FACE="Symbol">¾</font> limiting case, consisting in the Markovian approximation. For illustration we applied it to the study of a spin system in interaction with the lattice. The presentation is an extended and detailed version of a Brief Report published in Phys. Rev. E 57, 3637-3640 (1998). <![CDATA[<b>A kinetic model for the charged triple layer in low pressure arc discharges</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97331998000300003&lng=en&nrm=iso&tlng=en A one dimensional model of a peculiar configuration of charged layers in equilibrium composed by one electron rich layer surrounded by two ion rich layers adjacent to plasmas at distinct potentials and which is formed in a low pressure arc discharge (usually know as a triple layer) has been constructed using the BGK method [1] viz., with the help of the Poisson-Vlasov's system of equations applied to the free and reflected populations of electrons and ions in a supposedly existing electrostatic potential with the free populations assumed to be monoenergetic beams and the reflected ones obeying the Maxwell-Boltzmann distribution. Sagdeev potentials derived for the charged region and matched by appropriate plasma boundary conditions are numerically integrated to obtain the electrostatic potential for some set of free input parameters, compatible with those of a specific group of experiments. Limitations of the model are addressed to. <![CDATA[<b>Plasma processes in pulsar magnetospheres and eclipsing binary pulsar systems</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97331998000300004&lng=en&nrm=iso&tlng=en Plasma processes that may be responsible for pulsar radio emission and for eclipses observed for binary pulsars are discussed. High brightness temperature of pulsar radio emission implies that the radiation mechanism must be coherent. Several emission mechanisms are discussed. The high brightness temperature of radio emission also implies that nonlinear effect on wave propagation through pulsar magnetospheric plasmas is important and may result in radio pulse microstructure or cause fluctuation in dispersion measure. The discovery of eclipsing binary pulsars provides us with an opportunity to study nonlinear wave-wave interactions in electron-ion plasmas in the winds (or magnetospheres) of companion stars. <![CDATA[<b>Transitions between periodic orbits and control of chaos assisted by an external force in Hamiltonian systems</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97331998000300005&lng=en&nrm=iso&tlng=en We investigate the possibility of inducing transitions between periodic orbits in two-dimensional Hamiltonian systems by means of a time-localized external perturbation. We show that the amplitude of the perturbation can be approximately calculated in the limit of a delta-type force in terms of the initial and final periodic orbits. For a specific Hamiltonian, we show several numerical examples where the external perturbation, varied from delta-type to gaussian, allows transitions between specifically chosen members of families of periodic orbits. The same mechanism is then applied to move aperiodic chaotic orbits into periodic ones, presenting a new way to control chaotic behavior in Hamiltonian systems. <![CDATA[<b>Stochastic diffusion of energetic ions due to lower hybrid waves</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97331998000300006&lng=en&nrm=iso&tlng=en This paper discusses the interaction between energetic ions and lower hybrid waves in tokamak plasmas, presenting a numerical analysis which employs a quasilinear formalism appropriated for the situation in which ion stochastic diffusion occurs. The results show that significant wave-particle interaction may take place when a population of energetic ions in present in the plasma, in partial agreement with evidence from experimental results available in the literature. <![CDATA[<b>The threshold condition for stochastic diffusion of energetic ions due to lower hybrid waves</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97331998000300007&lng=en&nrm=iso&tlng=en In the present paper we consider the interaction between energetic ions and lower hybrid waves in tokamak plasmas and study the parametric dependence of the threshold condition for stochastic ion diffusion. It is shown that the threshold condition as obtained in the work by Karney [C. F. F. Karney, Phys. Fluids 22, 2188 (1979)] may be not easily satisfied in present day large tokamaks, but can attained in small tokamaks with relatively modest levels of wave power. <![CDATA[<b>Using beryllium and polypropylene-aluminium absorbers to measure the TBR-1 tokamak electron temperature</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97331998000300008&lng=en&nrm=iso&tlng=en 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. <![CDATA[<b>Eigenmode frequencies calculations in the charge-density-wave states of 2H-TaSe<sub>2</sub> </b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97331998000300009&lng=en&nrm=iso&tlng=en We have performed some calculations involving two eigenmode frequencies of 2H - TaSe2. We have obtained a good comparison with some well established Raman scattering data for the commensurate and incommensurate charge-density-wave phases in this compound. We outline an approximated method to compute the determinantal equation. <![CDATA[<b>The Lorentz-Dirac equation and the structures of spacetime</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97331998000300010&lng=en&nrm=iso&tlng=en An interpretation of the causality implementation of the Lienard-Wiechert solution raises doubts against the validity of the Lorentz-Dirac equation and the limits of validity of the Minkowski structure of spacetime.