60 research outputs found

    On the magnetic nature of electron transport barriers in tokamaks

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    The formation of internal transport barriers in the vicinity of rational magnetic surfaces in tokamaks with braided magnetic fields is studied for a simplified model of the perturbed magnetic field with a broad spatial spectrum and a monotonous shear profile. The island overlap criterion is used to derive a condition for barrier formation. This condition links the amplitude and the spectral width of the perturbation with the shear parameter. Numerical experiments with the MHD Monte-Carlo code E3D, where the problem of plasma heat conductivity is solved in 3D, confirm this formation of transport barriers in the case of a monotonous shear profile. Assuming that experimentally observed electron internal transport barriers are the result of local reduction of electron heat transport due to the magnetic field braiding, the amplitude and spectral width of magnetic perturbations are estimated for the tokamak RTP

    Erziehung und sozialer Wandel - Brennpunkte sozialpädagogischer Forschung, Theoriebildung und Praxis. Eine Einführung in die Thematik des Beihefts

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    Verallgemeinernd wird im Beitrag festgestellt: "Es geht zunächst um die Vergegenwärtigung der Wandlungen und Veränderungen in den sozialen Problemlagen und Lebensverhältnissen der Adressaten der Sozialpolitik, wie sie sich im Zusammenhang der dramatischen Veränderungen auf der gesellschaftlichen Ebene in der zweiten Hälfte der neunziger Jahre darstellen; deswegen ist von den Erziehungsverhältnissen im sozialen Wandel die Rede... Es geht um die Frage, wie Erziehungs- und Bildungsverhältnisse im Zeichen und im Kontext gesellschaftlicher Wandlungsprozesse sich verändern und welche Problemlagen daraus resultieren." (DIPF/Sch.

    Publisher Correction: Demonstration of reduced neoclassical energy transport in Wendelstein 7-X

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    Demonstration of reduced neoclassical energy transport in Wendelstein 7-X

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    Experimental confirmation of efficient island divertor operation and successful neoclassical transport optimization in Wendelstein 7-X

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    Towards a new image processing system at Wendelstein 7-X: From spatial calibration to characterization of thermal events

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    Wendelstein 7-X (W7-X) is the most advanced fusion experiment in the stellarator line and is aimed at proving that the stellarator concept is suitable for a fusion reactor. One of the most important issues for fusion reactors is the monitoring of plasma facing components when exposed to very high heat loads, through the use of visible and infrared (IR) cameras. In this paper, a new image processing system for the analysis of the strike lines on the inboard limiters from the first W7-X experimental campaign is presented. This system builds a model of the IR cameras through the use of spatial calibration techniques, helping to characterize the strike lines by using the information given by real spatial coordinates of each pixel. The characterization of the strike lines is made in terms of position, size, and shape, after projecting the camera image in a 2D grid which tries to preserve the curvilinear surface distances between points. The description of the strike-line shape is made by means of the Fourier Descriptors

    Forward modeling of collective Thomson scattering for Wendelstein 7-X plasmas: Electrostatic approximation

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    In this paper, we present a method for numerical computation of collective Thomson scattering (CTS). We developed a forward model, eCTS, in the electrostatic approximation and benchmarked it against a full electromagnetic model. Differences between the electrostatic and the electromagnetic models are discussed. The sensitivity of the results to the ion temperature and the plasma composition is demonstrated. We integrated the model into the Bayesian data analysis framework Minerva and used it for the analysis of noisy synthetic data sets produced by a full electromagnetic model. It is shown that eCTS can be used for the inference of the bulk ion temperature. The model has been used to infer the bulk ion temperature from the first CTS measurements on Wendelstein 7-X

    Experimental confirmation of efficient island divertor operation and successful neoclassical transport optimization in Wendelstein 7-X

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    We present recent highlights from the most recent operation phases of Wendelstein 7-X, the most advanced stellarator in the world. Stable detachment with good particle exhaust, low impurity content, and energy confinement times exceeding 100 ms, have been maintained for tens of seconds. Pellet fueling allows for plasma phases with reduced ion-temperature-gradient turbulence, and during such phases, the overall confinement is so good (energy confinement times often exceeding 200 ms) that the attained density and temperature profiles would not have been possible in less optimized devices, since they would have had neoclassical transport losses exceeding the heating applied in W7-X. This provides proof that the reduction of neoclassical transport through magnetic field optimization is successful. W7-X plasmas generally show good impurity screening and high plasma purity, but there is evidence of longer impurity confinement times during turbulence-suppressed phases.EC/H2020/633053/EU/Implementation of activities described in the Roadmap to Fusion during Horizon 2020 through a Joint programme of the members of the EUROfusion consortium/ EUROfusio
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