Summary

Three major issues are of the outmost importance (although they are not the only important issues) in the path towards a stellarator fusion reactor: optimization of energy confinement, control of the density of fuel particles and avoidance of impurity accumulation. These challenges are intrinsically “multispecies”: the plasmas relevant for us are hot neutral gases mainly consisting of nuclei of hydrogen isotopes and electrons. Impurities are ions different than the fusion reactants that enter the device through interaction of the main plasma with the external wall, and that typically have deleterious effects on energy confinement. With “species” we refer to each type of particle present in the plasma, characterized by its electric charge and mass.

Confinement of a deuterium and tritium plasma by means of magnetic fields constitutes a promising strategy towards energy generation based on nuclear fusion reactions. The most developed concept for future magnetic confinement fusion reactors is the tokamak, but the stellarator is a promising alternative which exhibits a fundamental advantage: whereas a significant fraction of the tokamak magnetic field is produced by driving a large electric current in the plasma, the magnetic field of the stellarator is created by external coils only, which makes steady state operation easier and avoids current-driven instabilities. However, generating the magnetic field by means of external coils requires that the magnetic configuration be three-dimensional (unlike the tokamak magnetic field, which is axisymmetric). Therefore, stellarator design is more complicated, and stellarator-specific theories and modeling tools have to be employed to study them.

Two different and complementary theories are needed to describe the transport of energy, fuel particles and impurities in a stellarator: neoclassics and gyrokinetics. The former accounts for transport caused by magnetic geometry inhomogeneities and collisions; the latter describes transport caused by turbulent fluctuations in the plasma. The goal of this project is to take big steps forward in the understanding of neoclassical and turbulent transport in fusion-grade, multispecies stellarator plasmas. With this objective, we intend to address theoretical problems, and to develop and apply numerical tools that may potentially change the paradigm of transport simulations in stellarators.

In this project:

  • We will extend recently introduced theoretical techniques in neoclassical theory to the description of more general transport regimes of low-collisionality main ions and electrons, and we will derive analytical expressions of the impurity flux in potentially relevant situations.
  • We will develop and benchmark a neoclassical code for the fast calculation of transport in multispecies stellarator plasmas of arbitrary geometry.
  • We will use this code to improve the current strategies for the optimization of energy confinement and to try to discover scenarios free of accumulation of collisional impurities.
  • We will determine the correct computational approach to study certain turbulenceproblems in multispecies stellarator plasmas. Namely, we will assess the relevance of neoclassical effects on turbulence, and we will determine the minimal computational domain required for an accurate description of linear and non-linear turbulent calculations.
  • We will study the relative weights of the neoclassical and turbulent branches of impurity transport in stellarators, as a function of the charge and mass of the impurity and other plasma parameters.
  • We will validate our theory developments and modelling tools against experiments at the stellarators Wendelstein 7-X (Max Planck Institute for Plasma Physics, Germany), TJ-II (CIEMAT, Spain) and Large Helical Device (National Institute for Fusion Science, Japan).

 

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