Study and control of turbulent transport in the boundary plasma region of the tj-ii stellarator and the jet tokamak
- Autores: Ulises Losada Rodríguez
- Directores de la Tesis: Juan Carlos Hidalgo Vera (dir. tes.), Eduardo de la Cal (codir. tes.), Luis García Gonzalo (tut. tes.)
- Lectura: En la Universidad Carlos III de Madrid ( España ) en 2019
- Idioma: español
- Tribunal Calificador de la Tesis: Luis Conde López (presid.), Isabel Garcia Cortes (secret.), Alexander Vladimirovich Melnikov (voc.)
- Programa de doctorado: Programa de Doctorado en Plasmas y Fusión Nuclear por la Universidad Carlos III de Madrid
- Materias:
- Enlaces
- Tesis en acceso abierto en: e-Archivo
- Resumen
Harnessing the energy of thermonuclear fusion has been proposed as a clean and sustainable method to cope with future energy demands. However, scientific and technical challenges still have to be overcome to make the operation of a nuclear fusion reactor viable. One of the biggest challenges is the understanding of the plasma physics that takes place in fusion reactors as well as the technological optimization of the reactors themselves.
The performance of future fusion reactors is limited by particle and heat losses. Both phenomena are governed by the non-linear interaction between plasma turbulence and large-scale flows, known as Zonal Flows. On the other hand, the physics of the spontaneous transitions between regimes of different level of confinement, the isotopic effect and its implications, the mechanisms of non-linear saturation of the turbulence as well as the evacuation of the heat fluxes and the physics of the interaction between the plasma and the wall still needs to be understood.
This thesis describes the empirical investigation of the interaction between the different scales of plasma turbulence in the TJ-II stellarator and the influence of the magnetic configuration on the physics of the plasma-wall interaction in the JET tokamak. To this effect, two diagnostics have been used: electrostatic probes (known as Langmuir probes) and rapid spectroscopy in the visible spectrum.
A profound understanding of the mechanisms that lead to the self-regulation of turbulence by the action of the Zonal Flows (ZF) is of utmost importance. In this regard, it has been shown that the evaluation of long-range correlations (LRC) at the plasma edge, using the Langmuir probe systems, is a powerful strategy to study the interaction between ZF and turbulence. Improvements in the experimental strategy to characterize LRC have been applied to study the interaction between ZF neoclassical radial electric fields during the transition to improved confinement regimes in the TJ-II stellarator.
The experimental studies carried out reveal the role of neoclassical radial electric fields in the control of the amplitude of the zonal flows that result in the development of scales of long (neoclassical) and short radial electric fields (due to zonal flows), with an important implication in the physical understanding of the mechanisms of self-regulation radial transport. Here we provide a complete description of the influence of plasma scenarios on the radial width of the ZF, with a special focus on its dependence on heating and isotopic mass.
For the first time, the characterization of these global fluctuations of low frequency and radial electric fields has been studied experimentally during the L-H transition in hydrogen plasmas and deuterium in the TJ-II stellator. No evidence of isotope effect was observed in the L-H transition dynamics. These observations emphasize the critical role of the neoclassical (or equilibrium) electric field and low-frequency large-scale flows to stabilize turbulence during the initiation of the L-H transition in toroidal, magnetically confined plasmas. This shows that there are different ways to achieve the LH transition with an impact on the conditions of access to the improved confinement regime.
In addition to the relevance of studies performed in stellarators, the experimental validation of relevant plasma scenarios in large tokamaks constitutes the fundamental test bed for future fusion reactors such as ITER. For example, in recent years it has been shown that, in the JET tokamak, with the new wall, which is identical to the one that will be installed in ITER, the global confinement of the plasma is strongly linked to the topology of the divertor, which influences in physics and in the characteristics of plasma edge and plasma-wall interaction. In this thesis, we show a study on how neutral particle fluxes are affected by the magnetic configuration of the divertor and, consequently, how the outermost part of the plasma changes, in which the field lines are not closed on themselves , but they are closed through the metallic elements of the device, known as "Scrape-Off Layer". We also present a detailed preliminary analysis of the dynamic behavior of ion and neutral fluxes during ELM-type instabilities. The results point to the recycling coefficient, which varies significantly within this short time scale, something that could have important implications in the understanding of high confinement performance.
