3D physics, turbulence and transport in the plasma boundary of RFX-mod

The increasing demand for new energy production has become the constant leitmotif of the society we live in. The impossibility of meeting such request in an economically feasible and environmentally friendly manner within the existing portfolio of options is now a global self-awareness. The International Energy Agency has been thoroughly documenting through its reports during the last years that the known reserves of natural gas and oil will be exhausted in decades, due to overpopulation and increasing energy demand. Consequently, a crucial issue will soon concern supply problems. This is where the research on fusion as energy source enters the picture. In order to demonstrate the feasibility of fusion as an energy source, the international scientific community has devoted countless efforts to the research on controlled thermonuclear fusion. The main and biggest experiment under construction, ITER, is the result of the cooperation among many countries throughout the world and it will be a test bench for fusion physics and fusion engineering. Among the different magnetic configurations suitable for fusion devices, the reversed-field pinch has demonstrated to be an excellent tool for plasma physics studies, although it is not accounted as a viable device for commercial energy production. The RFX-mod experiment hosted by Consorzio RFX in Padova is the biggest RFP device in the world. The research activity that will be presented in this thesis has been developed mainly at RFX-mod in Padova but has also involved the participation to an experimental campaign on the COMPASS tokamak in Prague. The research activity that will be presented in this thesis focuses on the characterization of 3D effects on transport mechanisms at the edge region of fusion plasmas in RFX-mod. The device is highly versatile: it can be operated both in reversed-field pinch and in tokamak magnetic configuration. A detailed description of transport properties at the plasma edge in both configurations will be given in the development of the thesis. The approach that will be followed can be described in terms of a twofold itinerary shared between the investigation of electrostatic fluctuations of transport properties through insertable probes and the study of magnetic topology modifications due to spontaneous 3D processes. The thesis is organized as follows: Part I, Introduction. In the first part of the thesis, all the background information needed for the development of the work will be introduced. In chapter 1 the concept of plasma and fusion physics will be discussed together with a detailed description of the two magnetic configurations for fusion devices relevant for analyses: reversed-field pinch (RFP) and tokamak. In chapter 2 the RFP dynamics will be described in details by introducing Taylor relaxation theory and the two main topologies also experimentally observed: the Single Helicity (SH) and the Multiple Helicity (MH) states. Then, RFX-mod edge region will be specifically described and the main tools used in the analyses will be introduced. The chapter will close with the introduction of a comparison between RFP and tokamak’s edge region through the analogy of a common 3D structure when external magnetic perturbations are applied. Part II, Transport analysis. In chapter 3 the two experimental configurations in which RFX-mod operates will be presented. The insertable U-probe used in the experiments will be described together with the theory upon which the collection of measurements relies. The investigation of electrostatic fluctuations of transport properties at the edge will be discussed. Studies on transport mechanism around different topological regions in presence of an externally applied magnetic perturbation will be shown for both reversed-field pinch and tokamak configuration in the RFX-mod device. Part III, Topology analysis. The study of magnetic topology modifications develops through analyses of spontaneous magnetic reconnection events in reversed-field pinch magnetic configuration. In chapter 4 reconnection models will be briefly discussed. Then, crash events in RFX-mod will be presented and the adopted analysis technique will be thoroughly explored. In the second part of the chapter the ion energy analyzer will be described together with its application in measurements of ion temperature profile in COMPASS tokamakand in RFX-mod. Part IV, Conclusions. In chapter 5 the results of the thesis are collected and discussed. In conclusion, a section entitled Summary and future perspectives is added where the main results are tautly summarized in view of future perspectives of research. Part V, Appendixes. In Appendix A a detailed description of MagnetoHydroDynamics framework for plasma physics is given. In Appendix B transport equations will be treated in details.