Repositório RCAAP

Fast acting gas valves that operate between two very different pressure environments, which may differ in several orders of magnitude, are not available commercially and, therefore, they must be specifically projected and constructed. For the TCABR impurity pellet injector, under construction, an electromagnetic valve has been built which operates for gas pressure that ranges from 20 to 40 atm at one side of the valve to 10-9atm at the other side. The working principle of the valve is based on the displacement of an aluminum disk (which controls the gas flux) from its closing position by an electromagnetic force, in result to an induced electric current on the disk caused by a magnetic flux variation. Experimental results showed that the electromagnetic impulse on the disk lasts for 200 mus and takes less than 30 ms for the valve to shut up again.

A compact Plasma Focus operated in deuterium which produces 2 10(8) neutrons per pulse, has been used as a radiation source for water detection by neutron scattering. The detecting system is composed by two silver activation sensors operated simultaneously on every shot. These detectors have quite different responses depending on whether the incoming neutrons are energetic or thermalised. Energetic neutrons come from the Plasma Focus itself, whereas those thermalized come, scattered, from the substance to be detected. The comparison between the readouts of each detector allows to reveal the presence of the blanket. The shot to shot variation of the Plasma Focus neutron yield does not preclude the detection. In practice shots with yields belonging to the 2 10(7)- 2 10(8)range, can be conveniently used. The obtained results indicate that the method is able to detect water contents of few percents in volume placed about 8.5 cm away from the Plasma Focus chamber. The presented method admits side-on as well as directional detection.

The structure of magnetic field lines in a tokamak with reversed magnetic shear is investigated by means of analytically derived area-preserving non-twist Poincaré maps. The basic configuration is the magnetic field produced by an ergodic limiter, superimposed to the tokamak equilibrium field in suitable coordinates. We consider the cases of one and two resonant modes, focusing on magnetic island dimerization and the formation of a transport barrier in the chaotic layer of field lines.

One alternative application in the decomposition and destruction of volatile organic compounds (VOCs) by a silent plasma dielectric barrier discharge (DBD) has been successfully accomplished. For this purpose, we have designed and constructed two pairs of cells, of rectangular and circular geometries, 333.96 cm³ each cell, and a similar second pair of 62.25 cm³ each one. Resonant inverters for low (3.3 kHz) and high (100 kHz) frequencies were also designed and applied to these cells. The specification of the main physical parameters of each cell contemplates: i) a first order degradation ratio of the compound, and ii) air breakdown at atmospheric pressure as a function of the carrying gas. The power consumed by the cells during the discharges was computed both theoretically and experimentally by Manley's method. The equipment was applied to the degradation of toluene, which has been degraded by an oxidation process in air-oxygen and argon-oxygen gas mixtures at atmospheric pressure within the cells. The destruction efficiency was measured as a function of the initial concentration. When air is used as an oxidant, a clear formation of solid products on the walls of dielectric glass plates has been observed, such deposits being polymeric in nature. These deposits seem to be responsible for a decline in the degradation efficiency of the treated compounds.

A millimeter/microwave detection system, in operation in the TCABR Tokamak is described. The system is used for electron cyclotron measurements. The main part of the system is a heterodyne sweeping radiometer based on a BWO oscillator that operates in the frequency range of 52 to 85GHz. The system operates in two modes : fixed frequency (maximum resolution of 10 mus) and sweeping mode (50 mus per frequency step). The radiometer is calibrated in frequency and in radiation intensity. The frequency calibration is made by means of a precision harmonic oscillator. The absolute calibration was done using a blackbody (microwave absorber) immersed in liquid nitrogen (77 K) and also put in an oven with adjustable temperature up to 1470 K. Two others components are also used for periodic intensity calibration check and sensibility measurements : a Criogenic Matched Load and a Noise Source. A Gaussian antenna is used for better space resolution measurements. Between the antenna and the radiometer, oversized waveguides are used to reduced the signal attenuation. The antenna axis is in the equatorial plane of the machine and perpendicular to the plasma column axis. The accessibility and absorption conditions are discussed. Results showing time and radial profi les of the detected ECE radiation for the TCABR are presented. For a magnetic fi eld of B TO = 1.14T it was verifi ed that the maximum permissible density to access the second harmonic in the X mode is n e0 <FONT FACE=Symbol>@</FONT> 2.3 × 10(19) m-3.

In tokamak machines with low toroidal magnetic fields and high plasma densities, the accessibility conditions impose restrictions to the detection of the Electron Cyclotron Emission (ECE). In these machines, the righthand cutoff condition can be used as an independent method to determine the local electron density from the ECE data in thermal discharges. In this paper is shown the results obatined from the detection of ECE radiation in the TCABR tokamak, in operation at the Institute of Physics of University of São Paulo. The effect of the ECE radiation cutoff was observed for different radial positions of the plasma column. To reach the ECE cutoff condition, the electron density was increased monotonically by the use of an external gas puffing system. For sufficient high densities, the emission at some frequencies is cutoff and the first and the last frequency to be cutoff depends on the shape of the density profile. These measurements do not require the plasma to be optically thick. It was observed that, for a toroidal field B O = 1:14T, the first cutoff of ECE occurs for a radial position r <FONT FACE=Symbol>@</FONT> 5 cm. From these measurements the radial electron density was determined. For a symmetric parabolic profile n e = n eo [1 - (r/a)²]alpha , values of alpha between 0.86 and 0.97 were experimentally obtained. A good agreement of these values with those obtained from the microwave interferometer measurements (<FONT FACE=Symbol>a »</FONT>0:85) was found. Therefore, the ECE right-hand cutoff constitutes an independent method to obtain information about the electron density profile.

Thermonuclear ignition condition for deuterium-tritium plasmas can be achieved in compact, high magnetic field devices such as Ignitor. The main scientific goals, the underlying physics basis, and the most relevant engineering solutions of this experiment are described. Burning plasma conditions can be reached either with ohmic heating only or with small amount of auxiliary power in the form of ICRH waves, and this condition can be sustained for a time considerably longer than all the relevant plasma time scales. In the reference operating scenario, no transport barriers are present, and the resulting thermal loads on the plasma facing component are estimated to be rather modest, thanks to the high edge density and low edge temperature that ensure an effective intrinsic radiating mantle in elongated limiter configurations. Enhanced confinement regimes can also be obtained in configurations with double X-points near the first wall.

In this work, we concentrate in the analysis of radio slowly drifting fine structures associated with solar flares recorded in the frequency range of (1000 - 2500) MHz by the Brazilian Solar Spectroscope (BSS), in regular operation at National Institute of Space Research (INPE) - Brazil. The main morphological aspects of each fine structure are narrow-band of about 5 - 10 MHz and small duration of the order of 50 milliseconds. The majority of these fine structures are observed over a time interval of a couple of minutes before the maximum of the associated flare, in the impulsive phase. However, some observations during the gradual decay phase are also reported. They drift towards lower frequencies, with slow rates typically of about of 10 - 100 MHz s-1 Estimated velocities of the exciter from the negatively drifting structures suggest that the shock-like exciter is propagating in the higher chromosphere. Than, those fine structures are interpreted as a possible signature in decimetric emissions from the chromospheric evaporation phenomenon. Details of these observations and their interpretation in terms of the plasma emission produced by accelerated particles and the chromospheric evaporation front is presented.

Electromagnetic ion cyclotron waves in the plasma depletion layer measured by Wind on three inbound passes of the magnetosheath near the stagnation streamline are modeled using theoretical results from Gnavi et al., J. Geophys. Res., 105, 20973, 2000. The kinetic dispersion relation in a plasma composed of electrons, protons, and alpha particles, is solved with each species modeled by a bi-Maxwellian distribution function with parameters taken from observations, where available, and from average values found in the literature. While one pass was under substantially high solar wind dynamic pressure (~ 6.4 nPa), the other two passes were under normal dynamic pressure at 1 AU (~ 2.2 nPa). The presence of electromagnetic ion cyclotron waves in the terrestrial plasma depletion layer under normal dynamic pressure is documented and analyzed for the first time. The power spectral density of the magnetic fluctuations transverse to the background field, using high resolution (~ 11 samples/s) data from the Magnetic Field Investigation, is obtained for the inner, middle and outer regions of the plasma depletion layer. The analysis of spectra and comparison with theory is extended to the normal dynamic pressure regime. The observations show that at the inner plasma depletion layer position the spectral power density weakens as the dynamic pressure decreases, and that the frequency range of emission shifts downward with diminishing pressure. Using bipolytropic laws for the anisotropic magnetosheath, we argue that the effect of a reduction of Pdyn is to lower Ap, thereby weakening the driver of EICWs leading to marginally bifurcated spectra and weaker EICW activity in the PDL under typical conditions. Qualitative and in some cases quantitative agreement between theory and data is very good.

Working within the domain of inviscid incompressible MHD theory, we found that a tangential discontinuity (TD) separating two uniform regions of different density, velocity and magnetic field may be Kelvin-Helmholtz (KH) stable and yet a study of a transition between the same constant regions given by a continuous velocity profile shows the presence of the instability with significant growth rates. Since the cause of the instability stems from the velocity gradient, and since a TD may be considered as the ultimate limit of such gradient, the statement comes as a surprise. In fact, a long wavelength (lambda) boundary for the KH instability does not exist in ordinary liquids being instead a consequence of the presence of magnetic shear, a possibility that has passed unnoticed in the literature. It is shown that KH modes of a magnetic field configuration with constant direction do not have the long lambda boundary. A theoretical explanation of this feature and examples of the violation of the TD stability condition are given using a model that can be solved in closed form. Stability diagrams in the (kd, MA) plane are given (where kd = 2pid/lambda, 2d is the velocity gradient length scale, and MA is the Alfvénic Mach number) that show both the well-known limit at small lambdas and the boundary for large but finite lambdas noted here. Consequences of this issue are relevant for stability studies of the dayside magnetopause as the stability condition for a TD should be used with care in data analysis work.

At present the Plasma Physics and Plasma Technology Group of the Comisión Chilena de Energía Nuclear (CCHEN) has the experimental facilities in order to study fast dense transient discharges in a wide range of energy and current, namely: I) energy from hundred of kilojoules to tens of joules, II) current from megaamperes to tens of kiloamperes. Also several diagnostics have been implemented. An overview of the work being carried out on dense pinch plasma focus discharges at the Comisión Chilena de Energía Nuclear is presented. The plasma energy density and scaling laws for the neutron yield are discussed. Possible applications of the radiation emitted are also discussed.

The concept of ExB flow velocity shear suppression is utterly fundamental in modern fusion research. It is asserted that there are models enabling to understand the physics involved in LH transitions. To improve the understanding of the mechanisms leading to the formation of Transport Barriers, especially the relation between Internal and Edge barriers it is necessary to invoke the issue of electric fields. Edge transport barriers are the feature of the H-mode, the baseline regime of ITER, whereas Internal Transport Barriers are used to develop regimes that might be employed for steady state operation of ITER , definitely beneficial for design and operation of fusion power plants in the future. Their synergy will be addressed. Plasma flows are closely connected to electric fields. Therefore, their role is crucial for understanding of tokamaks aimed at the achievement of fusion energy. This appears in the well known neoclassical theory as the most accomplished and selfconsistent basis for understanding of fusion plasmas. It pertains to the novel concept of "zonal flows" emerging from the recent development of gyro-kinetic transport codes. The equilibrium poloidal and toroidal flows are also crucial for the concept of the electric field shear suppression of plasma turbulence in tokamaks. Yet, this timely and topical issue has remained largely unaddressed experimentally because of great difficulties in measuring flows in plasmas.

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The Interacting Gluon Model (IGM) is a tool designed to study energy flow, especially stopping and leading particle spectra, in high energy hadronic collisions. In this model, valence quarks fly through and the gluon clouds of the hadrons interact strongly both in the soft and in the semihard regime. Developing this picture we arrive at a simple description of energy loss, given in terms of few parameters, which accounts for a wide variety of experimental data. This text is a survey of our main results and predictions.

A survey is given on the applications of hydrodynamic model of nucleus-nucleus collisons, focusing especially on i) the resolution of hydrodynamic equations for arbitrary configurations, by using the smoothed-particle hydrodynamic approach; ii) effects of the event-by-event fluctuation of the initial conditions on the observables; iii) decoupling criteria; iv) analytical solutions; and others.

A survey of various mechanisms for particle emission in hydrodynamics is presented. First, in the case of sudden freeze out, the problem of negative contributions in the Cooper-Frye formula and ways out are presented. Then the separate chemical and thermal freeze out scenario is described and the necessity of its inclusion in a hydrodynamical code is discussed. Finally, we show how to formulate continuous particle emission in hydrodynamics and discuss extensively its consistency with data. We point out in various cases that the interpretation of data is quite influenced by the choice of the particle emission mechanism.

I review the history of HBT interferometry, since its discovery in the mid 1950's, up to the recent developments and results from BNL/RHIC experiments. I focus the discussion on the contributions to the subject given by members of our Brazilian group.

We review the main results we have obtained in the area of high-energy elastic hadron scattering and presented in this series of Workshops on Hadron Interactions. After an introduction to some basic experimental and theoretical concepts, we survey the results reached by means of four approaches: analytic models, model-independent analyses, eikonal models and nonperturbative QCD. Some of the ongoing researches and future perspectives are also outlined.

Properties of the ground-state, zero temperature 'phase diagram' of the Bose-Hubbard hamiltonian as understood by Fisher et al. [5] are studied using simple tools in the case of nite systems. The results obtained provide a transparent picture of the thermodynamic limit and reveal features reminiscent of shell structure in many-fermion systems. Independent sites and many-site correlations appear in correspondence with independent fermions and many-fermion correlations respectively. This paper is dedicated to Nicim Zagury, on the occasion of his seventieth birthday.