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Numerical evidence for the existence of spatially chaotic magnetic field lines about the collapse phase of tokamak sawteeth with incomplete reconnection is presented. This uses the results ofextensive test particle simulations in different sets of electromagnetic perturbations tested against experimental JET measurements. In tokamak sawteeth, that form a laboratory prototype of magnetic reconnection, the relative magnetic perturbation dB B may reach a few percents. This does not apply to tokamak operating regimes dominated by turbulence where dB B is usually not larger than 10^- 4 . However, this small magnetic perturbation being sustained by a large spectrum of modes is shown to be sufficient to ensure the existence of stochastic magnetic field lines. This has important consequences for magnetic confinement fusion where electrons are dominantly governed by the magnetic force. Indeed some overlap between magnetic resonances can locally induce chaotic magnetic field lines enabling the spatial redistribution of the electron population and of its thermal content. As they are the swiftest plasma particles, electrons feed back the most rapid perturbations of the magnetic field. Fil: Firpo, Marie Christine. Centre National de la Recherche Scientifique; Francia Fil: Lifschitz, Agustin Federico. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Centre National de la Recherche Scientifique; Francia Fil: Ettoumi, Wahb. Centre National de la Recherche Scientifique; Francia. Universidad de Ginebra; Suiza Fil: Farengo, Ricardo. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina Fil: Ferrari, Hugo Emilio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina Fil: Garcia Martinez, Pablo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina

Using X-ray absorption spectroscopy (XAS), we studied the local structure in LaMnO3 under applied pressure across and well above the insulator to metal (IM) transition. A hysteretic behavior points to the coexistence of two phases within a large pressure range (7 to 25 GPa). The ambient phase with highly Jahn-Teller (JT) distorted MnO6 octahedra is progressively substituted by a new phase with less-distorted JT MnO6 units. The electronic delocalization leading to the IM transition is finger-printed from the pre-edge XAS structure around 30 GPa. We observed that the phase transition takes place without any significant reduction of the JT distortion. This entails band-overlap as the driving mechanism of the IM transition., Comment: 7 pages, 4 figures