133 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

    Quasi-geometric integration of guiding-center orbits in piecewise linear toroidal fields

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    A numerical integration method for guiding-center orbits of charged particles in toroidal fusion devices with three-dimensional field geometry is described. Here, high order interpolation of electromagnetic fields in space is replaced by a special linear interpolation, leading to locally linear Hamiltonian equations of motion with piecewise constant coefficients. This approach reduces computational effort and noise sensitivity while the conservation of total energy, magnetic moment and phase space volume is retained. The underlying formulation treats motion in piecewise linear fields exactly and thus preserves the non-canonical symplectic form. The algorithm itself is only quasi-geometric due to a series expansion in the orbit parameter. For practical purposes an expansion to the fourth order retains geometric properties down to computer accuracy in typical examples. When applied to collisionless guiding-center orbits in an axisymmetric tokamak and a realistic three-dimensional stellarator configuration, the method demonstrates stable long-term orbit dynamics conserving invariants. In Monte Carlo evaluation of transport coefficients, the computational efficiency of quasi-geometric integration is an order of magnitude higher than with a standard fourth order Runge-Kutta integrator.Comment: 38 pages, 11 figure

    Erziehung und sozialer Wandel - Brennpunkte sozialp√§dagogischer Forschung, Theoriebildung und Praxis. Eine Einf√ľhrung 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.

    The monoenergetic approximation in stellarator neoclassical calculations

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    In the standard "monoenergetic" approach to numerical calculation of stellarator neoclassical transport, to expedite computation, ad-hoc changes are made to the kinetic equation so speed enters only as a parameter. Here we examine the validity of this approach by considering the effective particle trajectories in a model magnetic field. We find monoenergetic codes systematically under-predict the true trapped particle fraction, with the error in the trapped ion fraction being of order unity when the electric field is large, suggesting some results of these codes may be unreliable in this regime. This inaccuracy is independent of any errors introduced by approximation of the collision operator.Comment: 8 pages, 2 figures, submitted to Plasma Phys. Controlled Fusio
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