Guido Van Oost

Measurements with an emissive probe in the CASTOR tokamak

An emissive probe has been used in the edge region of the CASTOR tokamak in order to test the possibility of direct measurements of the plasma potential. The difference between the floating potential of a cold probe and that of an emissive probe has been found to be approximately 1.3 times the electron temperature, which is less than predicted by the probe theory. Several possible reasons to explain this discrepancy are offered, such as secondary electron emission, uncertainties in the ion temperature, different collecting areas for electrons and ions, etc. The possible impact of a space charge formed by the emitted electrons is also discussed.

Plasma based waste treatment and energy production

During the past centuries, industrial processes and energy conversion plants have shown no or little care for environmental quality. The result is a huge accumulation of pollution and hazardous by-products, left as a heritage for the present and future generations. Recuperation of by-products or thermal energy is not only motivated by cost saving, but also by resource saving considerations. Environmental awareness is more than staying within the lines of the existing regulations.By the application of a plasma based system to a wide range of possible feedstocks which are CO2 neutral, a clean syngas of high caloric value is produced from the organic substances simultaneously with a non-leachable vitrified lava from the inorganic substances. The results will provide the advanced technology for the environmentally friendly treatment of hazardous wastes, biomass and low grade fuel. The driving force behind the task is to give priority to environmental quality at affordable costs. Thus, the investigation of ways to increase the efficiency of the process is very important. A plasma based remediation system is the only technology that prevents undesired pollution in the by-products and end product (such as syngas or other gases). The problem to be solved is twofold: recuperate clean energy from waste and renewables without pollution at affordable costs. Such a technique fulfils the objectives of sustainable development.Today, one of the main reasons that restricts the use of plasma based methods is the cost of electrical energy. The crucial element is the plasma torch performance. Hence, the physics of modern plasma torches is addressed in detail. The optimistic scenario holds the promise to provide 10?15% of the energy needs for the European Union (EU). Thus, the investigation of ways to increase the efficiency of the process is very important.

Potential of a Bayesian Integrated Determination of the Ion Effective Charge via Bremsstrahlung and Charge Exchange Spectroscopy in Tokamak Plasmas

Reliable and accurate estimates of the ion effective charge Zeff in tokamak plasmas are of key importance with respect to the impurity transport studies and the establishment of thermonuclear burn criteria. These issues are of fundamental interest to ITER and reactor operational scenarios in general. However, Zeff estimates derived from bremsstrahlung spectroscopy on the one hand and from the weighted summation of individual impurity concentrations obtained via charge exchange spectroscopy (CXS) on the other hand often are not compatible. This is a longstanding problem in fusion plasma diagnosis. A rigorous analysis of uncertainty sources and their propagation in the experimental determination of Zeff can contribute significantly to the derivation of a Zeff value with reduced uncertainty that is consistent with both the bremsstrahlung and CXS data sets. In this paper, Bayesian probability theory is used in an integrated approach as a powerful tool for an advanced error analysis in the derivation of Zeff, even in the presence of systematic errors on the data. A simple probabilistic model is proposed for the estimation of Zeff, first assuming only statistical uncertainty and then taking into account also the systematic deviations. The obtained Zeff estimates have smaller error bars than the Zeff values derived from the individual bremsstrahlung and CXS measurements, approaching ITER requirements. The estimates are shown to be consistent with all available information. In addition, the systematic errors on the data are quantized through the requirement of data consistency between different time slices in the acquired measurements.

Modeling Fusion Data in Probabilistic Metric Spaces: Applications to the Identification of Confinement Regimes and Plasma Disruptions

Pattern recognition is becoming an increasingly important tool for making inferences from the massive amounts of data produced in fusion experiments. In this work, we present an integrated framework for (real-time) pattern recognition for fusion data. The main starting point is the inherent probabilistic nature of measurements of plasma quantities. Since pattern recognition is essentially based on geometric concepts such as the notion of distance, this necessitates a geometric formalism for probability distributions. To this end, we apply information geometry for calculating geodesic distances on probabilistic manifolds. This provides a natural and theoretically motivated similarity measure between data points for use in pattern recognition techniques. We apply this formalism to classification for the automated identification of plasma confinement regimes in an international database and the prediction of plasma disruptions at JET. We show the distinct advantage in terms of classification performance that is obtained by considering the measurement uncertainty and its geometry. We hence advocate the essential role played by measurement uncertainty for data interpretation in fusion experiments. Finally, we discuss future applications such as the establishment of scaling laws.

3-D Discrete Dispersion Relation, Numerical Stability, and Accuracy of the Hybrid FDTD Model for Cold Magnetized Toroidal Plasma

The finite-difference time-domain (FDTD) method in cylindrical coordinates is used to describe electromagnetic wave propagation in a cold magnetized plasma. This enables us to study curvature effects in toroidal plasma. We derive the discrete dispersion relation of this FDTD scheme and compare it with the exact solution. The accuracy analysis of the proposed method is presented. We also provide a stability proof for nonmagnetized uniform plasma, in which case the stability condition is the vacuum Courant condition. For magnetized cold plasma we investigate the stability condition numerically using the von Neumann method. We present some numerical examples which reproduce the dispersion relation, wave field structure and steady state condition for typical plasma modes.

Thermonuclear burn criteria for D-T plasmas

Abstract After about 50 years of fusion research the time has arrived when fusion processes in experimental plasmas are increasingly getting important. In JET the genuine fuel of a fusion reactor was used for the first time in late 1991, in TFTR the same happened in 1993, and in JET an extended period of experiments of this kind was performed in 1997. Therefore, it is getting more and more rewarding to deal with the problems related to the ignition and burning of plasmas.

Thermonuclear burn criteria

Abstract After more than 50 years of fusion research the time has arrived when fusion processes in experimental plasmas are increasingly getting important. In JET the genuine fuel (deuterium-tritium) of a fusion reactor was used for the first time in late 1991, in TFTR the same happened in 1993, and in JET an extended period of experiments of this kind was performed in 1997. Therefore, it is getting more and more rewarding to deal with the problems related to the ignition and burning of plasmas.

Charge shielding in magnetized plasmas

The shielding of a charge sheet in a magnetized plasma is investigated by taking account of the diamagnetic drift start-up current in addition to the polarization current. For a charge sheet with an infinitesimal width, the shielding is the same as the conventional Debye shielding if the charge sheet is perpendicular to the magnetic field; the shielding length is 2 times larger than the conventional one if the charge sheet is parallel to the magnetic field. When the scale length of the charge sheet is comparable or smaller than the ion Larmor radius, the electric field is significantly enhanced within the charge sheet, while far away from the charge sheet, the electric field is shielded to the usual 1/er level (where er is the diamagnetic coefficient of the magnetized plasma).

Advanced probes for boundary plasma diagnosis in fusion devices

Abstract Since the 1990s it became increasingly clear that boundary plasmas play a major role in magnetic fusion devices (MCD), and strongly relate to and even dominate central plasma processes. On the one hand, the conditions of the boundary plasma are crucial to obtain high fusion triple products; on the other hand, plasma-surface interactions, a sufficiently low impurity concentration in the fusion volume, heat removal and helium exhaust which directly relate to the boundary plasma, have emerged as equally important goals, and even more difficult to reach in the state of self-sustained thermonuclear burn. Successful resolution of these issues is critical to establish the viability of the MCD concept for a fusion power reactor.

Identification of confinement regimes in tokamak plasmas by conformal prediction on a probabilistic manifold

Pattern recognition is becoming an increasingly important tool for making inferences from the massive amounts of data produced in magnetic confinement fusion experiments. However, the measurements obtained from the various plasma diagnostics are typically affected by a considerable statistical uncertainty. In this work, we consider the inherent stochastic nature of the data by modeling the measurements by probability distributions in a metric space. Information geometry permits the calculation of the geodesic distances on such manifolds, which we apply to the important problem of the classification of plasma confinement regimes. We use a distance-based conformal predictor, which we first apply to a synthetic data set. Next, the method yields an excellent classification performance with measurements from an international database. The conformal predictor also returns confidence and credibility measures, which are particularly important for interpretation of pattern recognition results in stochastic fusion data.