Tungsten, with its refractory properties and resistance to atomic sputtering, is chosen as the reference material for plasma-facing components in magnetic confinement fusion devices such as tokamaks. Subjected to intense flows of heat and particles, tungsten is nevertheless progressively eroded by the plasma. These erosion phenomena are critical for two reasons: they determine the lifetime of the materials exposed to the plasma, and once diluted in the confined plasma, these materials constitute a source of intense radiation (due to their high atomic number), which is detrimental to performance. A kinetic approach has been developed as part of a collaboration between the IRFM, the University of Aix-Marseille and General Atomics (US) to gain a better understanding of these phenomena.
Components facing the tungsten plasma in the WEST tokamak (foreground)
In current fusion facilities, the wall facing the plasma is made of tungsten and is subjected to intense flows of heat and particles that will erode it through the exchange of momentum between the ions in the plasma and the atoms in the solid material. While the physics of sputtering has been well known for more than fifty years, the dynamics of heavy ions through the plasma are much less well known. In particular, the trajectories of the sputtered particles through the first few millimetres of the plasma layer in contact with the material (electrostatic sheath) completely determine the probability of the particles reaching the confined plasma, or being redeposited directly on the material. This is due to two mechanisms: heavy metal atoms ionise rapidly in the plasma, i.e. close to the material. Once ionised, they are sensitive to the confining magnetic field and describe helical trajectories that can directly intercept the material. Secondly, the electrostatic sheath supports an extremely intense electric field, which accelerates the ions present in this region towards the material.
Ultimately, the probability of a heavy ion escaping from this transition zone depends very much on its kinetic properties (energy and angle of emission), the properties of the magnetic field (amplitude and angle of incidence with respect to the surface of the material) and finally the ionisation process, which is stochastic by nature (quantum phenomenon). This means that only a kinetic approach can quantify this probability and calculate the fraction of particles actually escaping from the material and re-entering the plasma. In most of the codes currently in use, the plasma is considered to be a fluid. Averaging the energy distribution of the incident particles often results in an underestimate of the net erosion of the component.
Example of particle trajectories: redeposited on the material (blue) or escaping towards the confined plasma (red)
A kinetic approach taking into account the energy distribution of the incident particles and using a Monte Carlo method was developed as part of a collaboration between the IRFM, the University of Aix-Marseille and General Atomics (US) in order to gain a better understanding of these erosion and redeposition phenomena. The figure opposite shows an example of the trajectories calculated: in blue the trajectories of the particles redeposited on the material, in red the trajectories escaping towards the confined plasma.
In this study, the average probability of sputtered tungsten ions escaping is calculated according to to the macroscopic conditions: magnetic field, properties of the plasma in contact with the material (density, temperature, potential, charge of the sputtered ions). The results show that the kinetic description of the energy is important only for incident particles that have been ionised once, as for plasma ions such as deuterium. The contribution of deuterium to the erosion of tungsten is therefore not always negligible when compared with the erosion due to light impurities or the self-erosion of tungsten in the range of plasma parameters studied. The results of this meta-model make it possible to interpret the experiments on the CEA's WEST tokamak in greater detail, and also to provide new boundary conditions for fluid simulation codes dealing with the equilibrium between edge plasma and wall elements.
Référence: L Cappelli, N Fedorczak, J P Gunn, S Di Genova, J Guterl, E Serre, Study of the erosion and redeposition of W considering the kinetic energy distribution of incident ions through a semi-analytical model, Plasma Phys. Control. Fusion 65 (2023)
Last update : 04/01 2024 (935)
