Resumen de Three dimensional physics in reversed field pinch and stellarator
This dissertation is dedicated to the study of three dimensional transport in toroidal magnetic configurations induced by the presence of the magnetic islands, which break the nested flux surfaces and make the magnetic field intrinsically three dimensional. The methodology is to define a certain symmetry so that the transport study could be performed on 1.5 dimensions. The work has been carried out on both RFX-mod and TJ-II stellarator.
RFX-mod is the largest Reversed Field Pinch (RFP). The RFP configuration is sustained by the so-called dynamo mechanism, which is related to the non-linear interactions among many resonating MHD tearing modes. The transport study on RFX-mod has been performed on three sub-states identified in the Quasi-Single Helicity state, that features one single mode (dominant mode) dominating the mode spectrum, with the rest of the modes (secondary modes) remaining at low amplitudes. In this regime proper flux coordinates can be identified considering an equilibrium built on the underlying axi-symmetric magnetic field plus the contribution from the dominant mode. In this way, 1.5 dimensional transport study is possible by averaging over the flux surfaces. The transport study is focused on the bean-shaped region where nearly conserved flux surfaces have been identified, for which the proof is the formation of steep thermal gradients, interpreted as electron Internal Transport Barrier (eITB). Both the thermal gradients and the thermal diffusivity have been calculated and their behaviors have been discussed within the framework of stochastic transport. Finally, the energy confinement time has also been evaluated, adopting an improved method and the results show a significant improvement.
The transport study on TJ-II stellarator focuses on the calculation of the enhanced non-ambipolar radial electric field due to the presence of the magnetic islands. The main idea is that the magnetic islands could modify the toroidal plasma viscosity, giving rise to an enhancement on the non-ambipolar particle fluxes. This work started with the study of the Neoclassical Toroidal Viscosity developed by K. C. Shaing for tokamak configurations, which ideally possess a toroidal symmetry. Applying this theory, the particle flux can be expressed in a monotonic radial coordinate and thus the transport study could be performed with a 1.5-dimensional approach. A moderate modification on the original theory has been made and the corresponding justification is presented, together with the detailed study in both tokamak and TJ-II configurations. The results show that an 'extra' local radial electric field is indeed induced by the magnetic islands in TJ-II plasmas, which could play a positive role in the plasma confinement properties by affecting the L-H transition, which is believed to be strongly linked to the shear of E X B flow.
