Abstract
Heat transport is studied in strongly heated fusion plasmas, far from thermodynamic equilibrium. The radial propagation of perturbations is studied using a technique based on the transfer entropy. Three different magnetic confinement devices are studied, and similar results are obtained. “Minor transport barriers” are detected that tend to form near rational magnetic surfaces, thought to be associated with zonal flows. Occasionally, heat transport “jumps” over these barriers, and this “jumping” behavior seems to increase in intensity when the heating power is raised, suggesting an explanation for the ubiquitous phenomenon of “power degradation” observed in magnetically confined plasmas. Reinterpreting the analysis results in terms of a continuous time random walk, “fast” and “slow” transport channels can be discerned. The cited results can partially be understood in the framework of a resistive Magneto-HydroDynamic model. The picture that emerges shows that plasma self-organization and competing transport mechanisms are essential ingredients for a fuller understanding of heat transport in fusion plasmas.
Highlights
The initial goal of fusion research is to design a system that sustains fusion reactions in a safe manner on Earth, which is a necessary first step towards the development of a fusion reactor, potentially a nearly inexhaustible power source for humankind, free from the pernicious greenhouse effect.Currently, one of the most promising approaches is magnetic confinement, in which the ionized gas or plasma is bound to a strong magnetic field
Using a relatively novel analysis method, the transfer entropy, we have shown that heat transport in magnetic fusion devices exhibits qualitatively similar properties in two stellarators and one tokamak
Analysis based on the use of the transfer entropy demonstrates the existence of radially localized zones that can be described as “minor barriers” and associated “trapping regions”
Summary
The initial goal of fusion research is to design a system that sustains fusion reactions in a safe manner on Earth, which is a necessary first step towards the development of a fusion reactor, potentially a nearly inexhaustible power source for humankind, free from the pernicious greenhouse effect.Currently, one of the most promising approaches is magnetic confinement, in which the ionized gas or plasma is bound to a strong magnetic field. To achieve sustained fusion reactions, the parameters of the plasma must fulfill the Lawson criterion: nTτ > θ, where n is the particle density, T the temperature, τ the confinement time, and θ a threshold value [1]. To comply with this requirement in the core region of the plasma, the plasma is heated and fueled by various methods. We note that temperatures achieved in the core of present-day fusion devices range from about 1000 to several times 10,000 eV, corresponding to equivalent temperatures of 107 –108 K. The system as a whole is necessarily very far from thermodynamic equilibrium, and standard approaches to study the transport of particles and heat in the plasma must be used with great caution
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