Confinement of a Deuterium and Tritium plasma by means of toroidal magnetic fields represents one of the ways towards electricity generation based on fusion energy.
The most developed type of magnetic confinement fusion device is called “tokamak”. Due to their axisymmetry, tokamaks have problems to operate in steady state. The most important one has its origin in the fact that a large electric current is needed in the plasma to create part of the confining magnetic field. These obstacles are not present in “stellarators”, where the magnetic field is produced only by external coils, something that is possible because the magnetic configuration is intrinsically three-dimensional. On the other hand, the three- dimensional nature of the stellarator introduces new physics and challenges that were absent in tokamaks.
On this website you will find progress made in the framework of the research grant ENE2015-70142-P, Ministerio de Economía y Competitividad, Spain, entitled “Collisional and turbulent transport in stellarators”. This project addresses some of the most critical challenges, with a focus on transport processes that lead to particle and energy losses, and with the goal of contributing to the design of future stellarator reactors:
Transport caused by magnetic geometry inhomogeneities and by collisions is called “neoclassical”. Magnetic configurations whose neoclassical transport level is as low as that in a tokamak are said to be “omnigeneous” and the stellarator is said to be neoclassically optimized. The problem is that exactly omnigeneous stellarators are technically impossible to build. What is more, deviations with respect to omnigeneity cannot be neglected when evaluating neoclassical transport in new stellarator designs. We will study analytically the effect of such unavoidable deviations.
Even in recently designed stellarators, neoclassical transport dominates in the inner region of the plasma. We will employ new theoretical findings and computer simulations to describe neoclassical fluxes in real stellarator discharges. To this end, experiments will be carried out in W7-X (the flagship experiment on the road towards a stellarator fusion reactor. It is planned to start operating in Autumn 2015 in Greifswald, Germany), TJ-II (Madrid) and LHD (Toki, Japan). The aim is to predict and explain operation scenarios, specially in connection to energy confinement and density control.
The term “impurity” refers to atomic species that do not participate in the fusion reaction. Even small concentrations of them can have very negative consequences for confinement. Recently, electrostatic potential and impurity density variations on the flux surface have been identified as a fundamental ingredient for impurity transport and control. In this project, we will study those variations from a theoretical, computational and experimental point of view in W7-X, TJ-II and LHD.
In addition to magnetic geometry inhomogeneities and collisions, turbulent fluctuations in the plasma cause transport as well. Hence, it would be desirable to design stellarators that are optimized not only with respect to neoclassical transport, but also with respect to turbulent transport. With this long-term goal in mind, we will use the gyrokinetic code EUTERPE to simulate instabilities and the process of turbulent saturation in stellarators.
The research team consists of the following members:
From the Laboratorio Nacional de Fusión, CIEMAT:
From other institutions:
Ralf Kleiber (Max-Planck-Institut für Plasmaphysik, Greifswald, Germany),
Felix I. Parra (Massachusetts Institute of Technology, Cambridge, Massachusetts, USA),
Shinsuke Satake (National Institute for Fusion Science, Toki, Japan).