Joaquim Loizu granted the IUPAP 2020 Prize in plasma physics

Nuclear fusion, a promising source of clean and safe energy, consists of creating a very hot plasma, a gas made of highly unstable charged particles, inside an experimental reactor. Scientists use magnetic fields to confine the plasma in a stable equilibrium. Their goal is to avoid energy losses resulting from plasma instabilities, particularly important for DEMO, a future prototype of a commercial reactor aimed at producing electricity. To do this, researchers use magnetohydrodynamics (MHD), a theory in physics that explains the behaviour of electrically conducting fluids. "MHD allows us to predict how the plasma moves and eventually settles, or not, into equilibrium. In other words, we calculate where the plasma positions itself and what shape it takes," explains Joaquim Loizu, a physicist at EPFL’s Swiss Plasma Center. 

Joaquim Loizu’s research contributes to a better understanding of 3D magnetohydrodynamic equilibrium, using theoretical developments and numerical simulations. Another part of his work focuses on interactions between plasma and a reactor's solid walls. It is for these two lines of research that he received the 2020 IUPAP (International Union of Pure and Applied Physics) award in plasma physics. This prize is awarded each year to a young international scientist for outstanding research.

2D versus 3D

3D magnetohydrodynamic equilibria are at work mainly in stellarators. In this type of experimental fusion reactor, magnetic fields that confine the plasma are configured in 3D, and therefore deformed. As a result, these machines are more complex than their cousins, the tokamaks. In the latter, magnetic fields are shaped symmetrically, in 2D, and particles circulate in a more controlled pattern. However, stellarators have a key advantage. "In tokamaks, an electric current is formed inside the plasma, potentially causing hydrodynamic instabilities. The equilibrium in this type of reactor is therefore sometimes unstable. Stellarators eliminate this issue," says Joaquim Loizu. 

While they generate stable equilibrium, stellarators are far more complex in terms of engineering and mathematical prediction. "From a mathematical point of view, complexities emanate, for example, in the form of singularities - infinities -, which are difficult to treat numerically," says Loizu. In his work, Joaquim Loizu contributes to establishing these equilibria and predicting characteristics of these plasmas. In particular, his approach should make it possible to predict the maximum achievable pressure in the plasma, while maintaining a magnetohydrodynamic equilibrium that confines the plasma particles well.

Interaction of plasma and solid walls

The other part of Joaquim Loizu's prize-winning research is related to interactions between plasma and the solid walls of a reactor. It aims to understand how these interactions affect currents and turbulence in plasmas. "A major challenge for controlled fusion, whether in a tokamak or in a stellarator, is the way in which heat is removed from the plasma. This heat escapes because of turbulence. It has to be channeled to specific parts of the reactor and spread over a large enough surface area", explains Joaquim Loizu. His work has contributed to developing a code that enables fusion researchers to simulate turbulence on the edge of tokamaks. With these simulations, researchers can predict on which parts of the reactor walls heat will be deposited.