The change of scale introduced by the Iter fusion reactor poses many difficulties. One of the challenges is to predict heat fluxes coming out of the fusion plasma (amounting to 100 megawatts) and to promote their dissipation before they settle on the reactor wall, on a nearby surface close to a square meter. Until now, the description of these processes has depended on empirical laws.

This is why the IRFM, in collaboration with the University of Aix-Marseille, has developed the TOKAM3X code. This code describes in 3D the turbulence and transport of particles and energy associated with edge plasma, the principal component of heat transport in plasma. This makes it possible, in particular, to explain the spread of the energy flux on the wall. In 2017, the researchers reached a key step in the development of TOKAM3X by addressing the problem of turbulent energy transport in Iter's divertor-type geometry.

Turbulence is structured in filaments. The appearance of a plasma constriction ("X-point") is accompanied by a significant deformation of this turbulent structure, identical to that observed in experiments using ultra-rapid cameras.

Finally, the simulations suggest that it is possible to control the "boundary layer" through which energy leaves the plasma, by adapting the geometry of the divertor. It is thus conceivable to spread more of the energy flux on the walls.