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The formation of internal transport barriers (ITB) in tokamaks has been experimentally associated with the region of low magnetic shear and the presence of main rational magnetic surfaces. We study the ITB from a mathematical point of view in a Hamiltonian model (the rev-tokamap model) that describes the magnetic field lines dynamics in reversed shear tokamaks. The non-twist properties of the map that generates the system are exploited in order to explain analytically the existence of ITB surrounding the shearless curve (in the low magnetic shear zone) and the robustness of invariant circles (corresponding to closed magnetic surfaces) situated in its proximity. We describe the location of the ITB for various q-profiles and perturbations of the (ideal) integrable system. We use an analytical method to estimate the maximum radius of magnetic field lines confinement, hence the position of the barrier, and we explain the destruction of invariant circles when the amplitude of the perturbation increases. The reconnection phenomena are also related with ITB and an explanation for the (experimentally observed) enlargement of ITB when the minimal value of the q-profile becomes close but less than a main rational is obtained.

Additional information

Authors: CONSTANTINESCU D, Department of Applied Mathematics, University of Craiova, Craiova (RO);MISGUICH J H, Département de Recherches sur la Fusion Contrôlée, Association Euratom-CEA sur la Fusion, CEA Cadarache, Saint-Paul-lez-Durance (FR);PAVLENKO I, Dept of Physics and Technology, Kharkiv National University, Kharkiv (UA);PETRISOR E, Department. of Mathematics, Polytechnic University of Timisoara, Timisoara (RO)
Bibliographic Reference: An article published in: Journal of Physics: Conference Series 7 (2005), pp. 233-238
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