A. Vannucci

Using artificial neural networks to forecast chaotic time series

Two-layer feedforward neural network was used in this work to forecast chaotic time series with very promising results, especially for the Lorenz system, as in comparison to others that had been previously published elsewhere. It was observed that the architecture m:2m:m:1, where m is the embedding dimension of the attractor of the dynamical system in consideration, is a very good initial guess for the process of finding the ideal architecture for the neural network, which is usually hard to achieve. The results we obtained with this particular type to series, and also with some others like Henon and Logistic maps, clearly indicate that there is an interplay between the architecture of a multilayer network and the embedding dimension m of the time series used. From the very good forecasting results we obtained, it can be concluded that neural networks can be considered to be an important tool for making predictions of the time evolution of nonlinear systems.

Forecast of TEXT plasma disruptions using soft X rays as input signal in a neural network

A feedforward neural network with two hidden layers is used to forecast major and minor disruptive instabilities in TEXT tokamak discharges. Using the experimental data of soft X ray signals as input data, the neural network is trained with one disruptive plasma discharge, and a different disruptive discharge is used for validation. After being properly trained, the networks, with the same set of weights, are used to forecast disruptions in two other plasma discharges. It is observed that the neural network is able to predict the occurrence of a disruption more than 3 ms in advance. This time interval is almost 3 times longer than the one already obtained previously when a magnetic signal from a Mirnov coil was used to feed the neural networks. Visually no indication of an upcoming disruption is seen from the experimental data this far back from the time of disruption. Finally, by observing the predictive behaviour of the network for the disruptive discharges analysed and comparing the soft X ray data with the corresponding magnetic experimental signal, it is conjectured about where inside the plasma column the disruption first started.

Disruptive instabilities in the discharges of the TBR-1 small Tokamak

Minor and major disruptions as well as sawteeth oscillations (internal disruptions) were identified in the discharges of the small Tokamak TBR-1, and their main characteristics were investigated. The coupling of a growing m=2 resistive mode with an m=1 perturbation seems to be the basic process for the development of a major disruption, while the minor disruption could be associated with the growth of a stochastic region of the plasma between the q=2 and q=3 islands. Measured sawteeth periods were compared with those predicted by scaling laws and good agreement was found. The time necessary for the sawteeth crashes also agrees with the values expected from Kadomtsevs model. However, there are some sawteeth oscillations, corresponding to conditions of higher plasma Zeff, which showed longer crashes and could not be explained by this model.

Magnetic field structure in the TCABR tokamak due to ergodic limiters with a non-uniform current distribution : theoretical and experimental results

The ergodic magnetic limiter is a device designed to generate a cold boundary layer of chaotic magnetic field lines at the peripheral region of a tokamak, with the main purpose of reducing the deleterious effects of the plasma–wall interaction. In the TCABR tokamak an ergodic limiter was constructed and recently installed inside the vacuum chamber. We developed a theoretical model for the action of an ergodic magnetic limiter in a large aspect-ratio tokamak taking into account the finite width of the limiter. The theoretical results are in good agreement with measurements of the vacuum magnetic field created by the limiter. Poincare maps of field line flow are computed to reveal the resulting magnetic field line structure due to the ergodic limiter and show that the operation of the ergodic limiter in the TCABR tokamak is feasible and results in a chaotic boundary layer for limiter currents of about 6% of the plasma current.

Control of chaotic magnetic fields in tokamaks

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 Sao Paulo and Universidade Federal do Parana on theoretical and experimental aspects of generation and control of chaotic magnetic field lines in tokamaks.

High current (Ipmax ≳ 5 MA) disruptive pulses in JET

High current (Ipmax 5 MA) and low beta disruptive pulses in JET have been investigated. No association was found between the disruptions and a possible triggering when the radiated power (Prad) equalled the input power (Pin), as was reported previously for disruptions in discharges with a lower plasma current. Often, Prad remained well below Pin throughout the discharge, but a disruption did occur, even though qψ 3. These pulses were well away from the limiting boundaries associated with the high density and low-q limits in the Hugill diagram. There was evidence of MHD mode interaction as the mechanism triggering a major disruption. The m/n = 1/1 perturbation played an important role in this process. Also, a limit for these high current disruptive pulses was found by plotting the internal inductance of the plasma versus qψ when this limit was reached, there was a very high probability for the plasma to disrupt through an MHD mode coupling process.

Influence of resonant helical windings on the mirnov oscillations in a small tokamak

SummaryThe influence of resonant helical perturbations on the plasma confined by the small tokamak TBR-1 was experimentally investigated. Strong attenuations of the Mirnov oscillations could be easily obtained by activating different resonant helical windings. The amplitudes of these oscillations were restored as the helical perturbation ended.RiassuntoSi è studiata in via sperimentale l’influenza delle perturbazioni elicoidali risonanti sul plasma confinato dal piccolo Tokamak TBR-1. Si sono potute facilmente ottenere forti attenuazioni delle oscillazioni di Mirnov attivando differenti avvolgimenti elicoidali risonanti. Si sono ripristinate ampiezze di queste oscillazioni nel momento in cui è terminata la perturbazione elicoidale.РезюмеЭкспериментально исследуется влияние резонанснЫх спиралевиднЫх возмущений на плазму, удерживаемую в малом токамаке TBR-I. Можно легко получить сильное затухание осцилляций Мирнова с помощью различнЫх резонанснЫх спиралевиднЫх обмоток. АмплитудЫ этих колебаний восстанавливаются, когда спиралевиднЫе возмущения вЫключаются.

Alfvén wave heating and runaway discharges maintained by the avalanche effect in TCABR

Recent results of Alfven wave heating experiments and the characteristics of a new regime of runaway discharges found in Tokamak Chauffage Alfven Bresilien (TCABR) are discussed. (1) Wave excitation was carried out with one module of the antenna system, with and without a Faraday screen. Evidence of plasma heating was obtained in both cases, for coupled wave powers up to half of the Ohmic power, approximately, without uncontrollable density rise during the RF pulse. The antenna coupling with the plasma seems to have increased when the Faraday screen was removed. (2) The new regime of runaway discharges is produced by initiating the main plasma breakdown without pre-ionization and strongly increasing the neutral gas fuelling at the end of the current ramp-up phase. Consequently, the plasma cools down substantially and switches to a runaway mode in conditions under which the primary (Dreicer) mechanism is strongly suppressed. This new regime of runaway discharges is characterized by strong enhancement of the relaxation oscillations, which are seen in the H α and ECE emissions, coupled with large spikes in the line density, loop voltage, bolometer, and other diagnostic signals.

Observation of disruptions in tokamak plasma under the influence of resonant helical magnetic fields

SummaryDisruptive instabilities were investigated in the small-tokamak TBR-1 during the application of resonant helical magnetic fields created by external helical windings. Indications were found that the main triggering mechanism of the disruption was the rapid increase of them=2/n=1 mode which, apparently after reaching a certain amplitude, interacts with other resistive modes: the internal 1/1 mode in the case of a major disruption, or with higherm components, as the 3/1 or 4/1, in the case of minor disruptions. After the coupling, the growth of the associated islands would create a chaotic field line distribution in the region between the corresponding rational magnetic surfaces which caused the gross particle transport and, finally, destroyed the confinement. In addition, investigations on higherZeff discharges in which a mixture of helium and hydrogen was used resulted in much more unstable plasmas but apparently did not alter the basic characteristics of the disruptions.

Magnetic islands and plasma rotation in the Tokamak Chauffage Alfven Bresilien tokamak

Collisional plasma rotation in the Tokamak Chauffage Alfven Bresilien (TCABR) tokamak [J. H. F. Severo, I. C. Nascimento, V. S. Tsypin, and R. M. O. Galvao, Nucl. Fusion 43, 1047 (2003)] has been experimentally studied. It was found that the measured plasma poloidal rotation velocity agrees within error limits with neoclassical theoretical predictions, and toroidal velocity with experimental results obtained in analogous tokamaks, almost everywhere along the minor radius r, except for measurements at r/a≃0.56 and r/a≃0.89 (the minor radius of TCABR tokamak a=18 cm). For the first point, the measured plasma rotation velocities are higher than the velocity of the background plasma, respectively ∼30% and ∼10% for the poloidal and toroidal rotation velocities. Using a set of 22 Mirnov coils displaced poloidally, magnetic field perturbations were measured in shots adjusted to reproduce the ones of the previous plasma rotation measurements, and the results confirm that in the region r/a≃0.89 the plasma rotates ...