Role of the Sawtooth Crash in the Electron and Impurity Transport in the Tore Supra and JET Tokamaks
In the Tore Supra and JET tokamaks, the plasma density equatorial profile is measured with fast-sweeping reflectometry, in ohmic sawtoothing regime. The profiles are used to reconstruct the 2D density during an (m, n) = (1, 1) kink mode, following the tomography technique described in Ref. . Crescent-shaped structures of amplitude 1%of the total density are observed inside the q = 1 surface immediately following the sawtooth crash. Despite the small amplitude of the structure, it constitutes a significant deviation with respect to the expected post-crash flat profiles. In Ref. , this behaviour was modelled using the 3D non-linear full MHD code XTOR-2F . This code is capable of simulating several self-consistent sawtooth crashes . Despite full reconnection occurring in the simulation, contrary to most experimental cases, the experimentally observed density structures were recovered and it was demonstrated that they arise because of the fast flows at the reconnection layer which first advect the density along the separatrix and then reinjects it inside the q = 1 surface. Thus the redistribution mechanism of the density is fundamentally different from that of the temperature, the evolution of which is governed by the fast diffusive transport along the reconnected field lines. The question of the particle transport induced by the sawtooth instability has been given more importance lately due to recent experiments on the JET tokamak with the ITER like wall , where Tungsten (W) is seen to accumulate in the core despite frequent sawtooth crashes.