Repositório RCAAP

We compare and discuss several approximations to the dispersion relation for electrostatic waves in inhomogeneous plasmas, either obtained directly from Poisson's equation, or from the dielectric constant obtained using a dielectric tensor derived using the plane wave approximation, or from the dielectric constant derived using the effective dielectric tensor.

The preservation of Onsager symmetry for the effective dielectric tensor is discussed for a homogeneous plasma immersed in a inhomogeneous magnetic field, using the unperturbed orbits correct up to order kB, which is the scalelength of the field inhomogeneity. General features of the calculation of the components of the tensor are discussed and detailed calculations are developed for the zz component, which is shown to satisfy the conditions for Onsager symmetry, in agreement with previous results obtained using less precise expressions for the unperturbed orbits.

Large scale magnetic fields in galaxies are thought to be generated, by a mean field dynamo. In order to have generated the fields observed, the dynamo would have had to have operated for a sufficiently long period of time. However, magnetic fields of similar intensities to the one in our galaxy, are observed in high redshift galaxies, where a mean field dynamo would not have had time to produce the observed fields. MHD turbulence produces small scale magnetic fields at a faster rate than it does mean fields, which can diffuse toward larger scales. If the turbulence is helical, magnetic fields generated at small scales can become correlated over large scales. We study the evolution of magnetic field correlations in the first objects formed in the universe, due to the action of a turbulent, helical, stochastic dynamo, for redshifts 5 < z < 10. Ambipolar diffusion can play a significant role in this process due to the low level of ionization of the gas in the first objects. We show that for reasonable values of the parameters that characterize the turbulent plasma in the time interval considered, fields can grow to high intensities (~ muG), with large coherence lengths (~ 2 - 6 kpc).

One of the main objectives of the research program at ETE is devoted to the study of confinement properties in a low aspect ratio tokamak plasma configuration, and to attain this purpose the determination of plasma parameters of the edge region plays an essential role. A Fast Neutral Lithium Beam (FNLB) diagnostics was developed to perform the measurement of the density profile and its fluctuations at the edge of the plasma in ETE. The first light signals were collected after the filling of ETE chamber with He gas (10-4 Torr), where the optimization of this diagnostics was done, to adjust the better point for focalization of the beam and for the optical detection system setup. An extensive study of the variation of the intensity of the light signal with the variation of the gas pressure was carried out. This calibration will be used during shots in ETE to measure the rapidly varying pressure of the pulsed discharge by FNLB technique, a requirement imposed by the use of puff valves for injection of the gas. This paper will also describe the several phases of the development of this diagnostics, discuss the problems occurred and the adopted solutions and show the first measurements of the probing of the edge plasma of ETE with FNLB.

The electrical breakdown has been investigated for low-pressure argon and nitrogen discharges under the influence of an external longitudinal magnetic field. Plane-parallel aluminum electrodes (5 cm diameter) separated by a variable distance d (4.0 cm < d < 11.0 cm) were sustained with a dc voltage (0 < V < 1 kV). A Helmholtz coil was used to produce an uniform magnetic field(B) parallel to the discharge axis. Paschen curves were obtained and the secondary electron emission coefficient (gamma), the first Townsend ionization coefficient (alpha) and the ionization efficiency(eta), were plotted with respect to the variation of the reduced field (E/P). To observe the effect of the magnetic field these curves were plotted for fixed values of B=0 and B=350 Gauss. As consequence of the longitudinal magnetic field, the free paths of the electrons in the Townsend discharge are lengthened and their lateral diffusion is reduced, thus reducing electron losses to the walls. The data presented in this paper give a quantitative description of the B-field effect on the Townsend's coefficients and overall it is concluded that the DC electrical breakdown of the gases is facilitated if a longitudinal magnetic field is applied along the discharge axis.

Nitrogen Plasma Immersion Ion Implantation (PIII) has been used to modify the surface chemical structure of Ultra High Molecular Weight Polyethylene (UHMWPE). Grinding and polishing processes based on abrasive papers and alumina pastes have been evaluated with regard to their results on the improvement of polymer surface roughness, which has shown to be of crucial importance for hardness characterization. Raman spectroscopy, XPS, and Nanoindentation tests were used to characterize the modified surfaces. Experimental results has shown that UHMWPE surface mechanical properties such as hardness and elastic modulus can be improved by induced chain cross-linking between the macromolecules on the polymer surface caused by nitrogen PIII. The new material formed on the surface is Diamond Like Carbon (DLC). As a significant improvement in hardness was obtained by DLC synthesis on the treated surface, it is expected a dramatic improvement of abrasion resistance and overall durability of prostheses made with PIII treated UHMWPE.

Doppler or Stark line broadening effects are generally used to determinate plasma temperature. These methods are difficult to apply to spectra of highly ionized atoms due to the short wavelengths involved. It is not at all easy to achieve sufficient wavelength resolution in this spectral range. In this case, a spectroscopic technique based on the relative intensities of lines must be used to measure the electron temperature in a plasma. However the relation of the measure of relative line intensity and the plasma electron temperature is complex and a number of issues must be examined for the diagnostic. In simple cases, only a two levels system need be considered. Here we introduce a semi-empirical method to determine the plasma temperature that takes into account multiple levels structure. In the theoretical model we consider a local thermodynamic equilibrium(LTE). The greatest difficult in the determination of plasma temperature using a multiple levels approach is overcome by calculating the transition probabilities in terms of the oscillator strength parameters. To check the method we calculated the oscillator strengths for the Cu I using a multi-configurational Hartree-Fock relativistic (HFR) approach. The electrostatic parameters were optimized by a least-squares procedure, in order to obtain the best fitting to the experimental energy levels. This method produces gf- values that are in better agreement with their experimental values than the produced by the ab initio calculation. The temperature obtained was 5739.3 K, what is compatible with direct measurements made for cupper DC discharge.

The rotating low frequency (RLF) field penetration and dissipation and the effect of ponderomotive forces driven by Ergodic Magnetic Limiter (EML) on the poloidal/ toroidal flow in tokamak plasmas are discussed. EML coils are represented as a sheet current expanded in Fourier series with poloidal/toroidal wave numbers M/N depending on coil shape and feeding. The Alfvén wave mode conversion effect in the RLF range is found responsible for wave dissipation at the rational magnetic surfaces qr = -M/N = 3 typical for EML coil design. Analytical and numerical calculations show maximums of LF field dissipation at the local Alfvén wave resonance omega = <FONT FACE=Symbol>½</FONT>k||cA<FONT FACE=Symbol>½</FONT> near the rational magnetic surface qr = 3 in Tokamak Chauffage Alfvén Brésilien. The poloidal rotation velocity U, taken into account in the dielectric tensor, can strongly modify the LF field and dissipated power profiles. Even stationary EML fields can dissipate at the local Alfvén wave resonance (UM/rA = k||cA). Preliminary estimations show that the stationary EML fields can decelerate the plasma rotation.

Some recent results concerning the synthesis of carbon nanostructures in a thermal plasma generating by a plasma torch are presented. Several tests were carried out in different operational conditions. The plasma was formed with argon and different gas mixtures of argon-acetylene or argon-methane to which some catalyst materials (ferrocene and cerium oxide) were added. These catalysts were introduced into the plasma in a solid (powder) or/and a gaseous state. Their feeding rate into the plasma jet was fixed along with some other operating conditions such as plasma power, gas flow rate and reactor pressure. The principal main feature observed was a short reaction time so that each test lasted for no longer than 5 minutes. The solid products obtained were collected and prepared for following analyses. The products were examined using XRD and TEM techniques in order to characterize the morphological structure of their samples. The spatial distribution of temperature in the plasma was evaluated by in-situ emission spectroscopy. The self-absorption was taken into account by simulating an integrated radiation in relation to the Swan band d³pig -> a³piu(0,0), emitted by the C2 radical. Also, the exhaust gases were characterized by gas chromatography during each test.

The biggest advantage of plasma immersion ion implantation (PIII) is the capability of treating objects with irregular geometry without complex manipulation of the target holder. The effectiveness of this approach relies on the uniformity of the incident ion dose. Unfortunately, perfect dose uniformity is usually difficult to achieve when treating samples of complex shape. The problems arise from the non-uniform plasma density and expansion of plasma sheath. A particle-in-cell computer simulation is used to study the time-dependent evolution of the plasma sheath surrounding two-dimensional objects during process of plasma immersion ion implantation. Before starting the implantation phase, steady-state nitrogen plasma is established inside the simulation volume by using ionization of gas precursor with primary electrons. The plasma self-consistently evolves to a non-uniform density distribution, which is used as initial density distribution for the implantation phase. As a result, we can obtain a more realistic description of the plasma sheath expansion and dynamics. Ion current density on the target, average impact energy, and trajectories of the implanted ions were calculated for three geometrical shapes. Large deviations from the uniform dose distribution have been observed for targets with irregular shapes. In addition, effect of secondary electron emission has been included in our simulation and no qualitative modifications to the sheath dynamics have been noticed. However, the energetic secondary electrons change drastically the plasma net balance and also pose significant X-ray hazard. Finally, an axial magnetic field has been added to the calculations and the possibility for magnetic insulation of secondary electrons has been proven.

Using high energy, pulsed X-rays, generated by a 4.7 kJ Plasma Focus operated with deuterium - argon admixtures, images of fast rotating metallic pieces were obtained. This illustrates the potential application of Plasma Focus devices for non invasive scanning of moving metallic objects. The samples for radiographic imaging were located outside the plasma focus chamber. High-sensitivity, fast-response commercial radiographic film was used as X-ray detector. The use of small doses of argon helps to enhance both the neutron production and the hard X-ray emission. By analyzing the mean attenuation of the studied radiation in different metals, an X-ray effective energy of about 100 keV was obtained.

Separatrix reconnection in the standard nontwist map is described, including exact methods for determining the reconnection threshold in parameter space. These methods are implemented numerically for the case of oddperiod orbit reconnection, where meanders (invariant tori that are not graphs) appear. Nested meander structure is numerically demonstrated, and the idea of meander transport is discussed.

Using two dimensional numerical codes, we show that the direct electron heating found with electron cyclotron emission (ECE) radiometry is related to local Alfvén wave resonances in Tokamak Chauffage Alfvén Brésilean (TCABR) plasmas. The m/N =1/2, 0/2 and 0/3 modes are dominant in the observed wave heating, due to Alfvén wave continuum absorption. In particular, the resonant absorption of the m = 0 sideband harmonic plays an important role in the central core and peripheral heating. The increase of the electron temperature during the radio frequency pulse was accompanied by a local density rise that produced a cut-off of the ECE emission. The observed heating profile is also consistent with the reflectometer measurements of the density fluctuations induced by radio frequency fields in the local Alfvén wave resonance of the m/N =-1/-2, 1/-3 modes in TCABR.

The experimental results on the Alfvén mode structure identification by the microwave reflectometry in TCABR tokamak are presented. The knowledge of the spatial spectrum of the excited waves is crucial for optimization of Alfvén wave plasma heating and noninductive current drive scenarios in tokamak plasmas. No less important is the possibility to use the Alfvén wave excitation for diagnostic purposes. A microwave reflectometer with fixed frequency was used to register plasma density oscillations driven by the excited Alfvén waves, under the condition of the spectrum scanned by a controlled plasma density rise. It is shown that when the position of the local Alfvén resonance, rA, which is defined by relation omega = k||(rA)C A(rA), is close to the plasma zone where the microwave signal is reflected, the high-frequency modulation of the output signal of the reflectometer at the RF generator frequency increases. The possibility to use the observed effect for finding the plasma current profile in tokamaks is discussed.

The preliminary design study of the Alfvén wave system for the ETE spherical tokamak is presented. The main objective of the proposed RF system is generation of travelling Alfvén waves, which can be used for the study of the non-inductive current drive and the plasma heating in a wide range of operational regimes, which are specific for tokamaks with small aspect ratio R/a <IMG SRC="/img/revistas/bjp/v34n4b/til.gif">1.2 - 1.5. The antenna system consists of four modules, which are separated by 90º in toroidal direction. Each module has two sets of RF current carrying poloidal straps that are positioned symmetrically in relation to the equatorial plane of the vacuum chamber at the poloidal coordinates ±45º. The poloidal extension of each strap is approximately 90º. The excited mode numbers can be controlled by the phasing of the feeding RF current. In the basic regime of operation, the straps in the same toroidal cross-sections are fed by the RF currents with (0, ±pi/2) phasing so that they can excite travelling modes M = ±1, N = ±1. In order to decrease the periphery RF power deposition and to improve the selectivity of the Alfvén mode excitation, the inclination of the antenna straps in toroidal direction can be adjusted in the range of angles ±30º. The dependence of the antenna efficiency on the antenna orientation is studied numerically in the frame of 1-D MHD model.

There are reasons to believe that the solar matter density fluctuates around an equilibrium profile. One of these reasons is a resonance between the Alfvén waves and the g-modes inside the Sun that creates spikes in the density profile. The neutrinos are created in the solar core and passing through these spikes feel them as a noisy perturbation, whose correlation length is given by the distance between the spikes. When we consider these perturbations on the density profile, the values of the neutrino parameters necessary to obtain a solution to the solar neutrino problem are affected. In particular, in the present work, we show that the values of the parameters of mass and mixing angle that satisfy both the Large Mixing Angle solution to the solar neutrinos and the data from KamLAND - that observes neutrinos created in earth nuclear reactors - are shifted in the direction of lower values as the amplitude of the density noise increases. This means that, depending on the new data of KamLAND and other detectors, it can be necessary to invoke random perturbations in the Sun to recover compatibility with solar neutrino observations. In this case, the neutrino observations will be used as a real probe of the solar interior, giving information of the density profile in the central part of the Sun, which can not be observed directly.

Experiments in a low pressure glow discharge on gases of environmental relevance: CO2,CO y O2 are reported. We studied the various atomic processes: ionization, excitation and molecular dissociation with typical discharge parameters of p~0.6 mbar, I< 0.1 A and V~1100 Volts. We used visible and near UV spectroscopy to observe the emitted radiation on both electrodes on pure gases and hydrogen seeded mixtures. We also set up Langmuir probes in both electrodes.

We solve the problem of a resistive toroid carrying a steady azimuthal current. We use standard toroidal coordinates, in which case Laplace's equation is R-separable. We obtain the electric potential inside and outside the toroid, in two separate cases: 1) the toroid is solid; 2) the toroid is hollow (a toroidal shell). Considering these two cases, there is a difference in the potential inside the hollow and solid toroids. We also present the electric field and the surface charge distribution in the conductor due to this steady current. These surface charges generat not only the electric field that maintains the current flowing, but generate also the electric field outside the conductor. The problem of a toroid is interesting because it is a problem with finite geometry, with the whole system (including the battery) contained within a finite region of space. The problem is solved in an exact analytical form. We compare our theoretical results with an experimental figure demonstrating the existence of the electric field outside the conductor carrying steady current.

In this work we present a comparison between the solar origins of two solar-terrestrial connection events occurring on April 1999 and February 2000, respectively. On April 13 (1999), a coronal mass ejection (CME) appeared for the first time in LASCO-C2 coronagraph field of view at 03:30 UT. The possible solar disc source of this CME was located at coordinates N25 E05, and it consisted on flaring activity together with a filament eruption as seen in EIT images. The geomagnetic storm with a Dst peak of -91 nT registered on April, 17 was attributed to this solar surface activity. On February 8, 9 and 10 (2000) three Earth-directed halo CMEs were recorded by LASCO coronagraphs. The interaction of two of them caused the intense interplanetary geomagnetic storm registered on February 12 (Dst = -110 nT). The CMEs detected on February 8 and 10 were launched from AR 8858 and the source of the activity was attributed to two solar flares of X-ray class M1.3 and C7.3 respectively. The CME detected on February 9 was originated in AR 8853 and flaring activity was also identified as the solar source for this event. In this study we discuss the solar origins of the events that gave rise to coronal and interplanetary disturbances and as a consequence produced geomagnetic activity at the Earth. This work is part of a complete study that is searching the solar origin, the interplanetary aspects and the magnetospheric effects of these events.

This work presents a study of multilayered Ti/TiN/TiC thin films obtained using a PAPVD (Plasma Assisted Physical Vapor Deposition) system by pulsed-arc discharge. For this purpose, a titanium target and a stainless steel substrate were used, placed on the cathode and on the anode, respectively, inside a vacuum chamber. To grow these films, different gases and concentrations were required. Ti is obtained with argon gas, TiN with nitrogen, and TiC with methane, at 2.5 mbar and 5 mm distance between electrodes. By means of X ray diffraction (XRD), the phases present in the film were determined, observing (111), (200), and (100) orientations for both, TiN and TiC. Also, by employing XRD techniques, Titanium Atoms Distance (DTA) was calculated at the interface of TiN and TiC, in order to study the crystallographic match. Energy Dispersive Spectroscopy (EDS) was employed in order to carry out elemental analyses in the materials. These analyses were obtained for 12 keV and 30 keV, observing the effects in the results. Taking advantage of defects generated during the growth of the multilayer, chemical composition maps were carried out, probing the combination of Ti and N in one layer and Ti and C in the other layer. Scanning Electron Microscopy (SEM) technique allowed observing the presence of the multilayer, as well as the measurement of the thicknesses of each layer, which are in the order of nanometers.