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Description
See full abstract here
http://ocs.ciemat.es/EPS2019ABS/pdf/P2.1078.pdf
Collisional particle transport in the presence of field perturbations originating from various MHD activity is examined theoretically on tokamaks (ITER, ASDEX Upgrade, NSTX and DIIID) and the reversed-field pinch RFX-mod [1]. For ITER and ASDEX Upgrade, modes typically leading to a disruption [2] are considered. On NSTX and DIII-D unstable Alfvén modes are investigated. Finally on RFX-mode the effect of saturated tearing modes is studied.
The existence of subdiffusive transport [3] for electrons is found to occur in some cases at very low mode amplitudes. Subdiffusion is also found for ions of high energy. In fact, orbit resonances can produce long time correlations and dynamical traps [4] for particle trajectories at perturbation amplitudes much too small for the orbits to be represented as uniformly chaotic. Besides this, in all devices orbits show a high degree of anisotropy, especially when comparing the angular (toroidal and poloidal) and radial directions. As a consequence, in the presence of field perturbations produced by MHD modes, the use of a traditional diffusive-convective scheme for transport, which is expressed by the Fick's law = -Dn + v n, leading to the well known transport scalings, is questionable. The existence and nature of subdiffusive transport is difficult to determine from first-principle theories, since it is found to depend on the nature of the mode spectrum and frequency, as well as on the mode amplitudes: this fact is mirrored in the different value of the Kubo number found in the devices analyzed in this paper. The connection between subdiffusive transport, Kubo and nonlocal models of transport [5] is also discussed.
References
[1] G. Spizzo et al, Nucl. Fusion 59, 016019 (2019) [
2] M. Maraschek et al, Plasma Phys. Controlled Fusion 60, 014047 (2018)
[3] R. Sanchez and D.E. Newman, Plasma Phys. Controlled Fusion 57, 123002 (2015)
[4] G.M. Zaslavsky, Hamiltonian Chaos and Fractional Dynamics (Oxford University Press) p. 187199 (2005)
[5] D. del Castillo-Negrete, AIP Conf. Proc. 1013, 207 (2008)
