Plasma surface interaction

Plasma–Surface Interaction (PSI) studies study the interaction at the interface between plasma and materials. Focus of the research lies on providing both theoretical and experimental support to the design and validation of plasma facing materials for the fusion experiment ITER and future devices.

Plasma-surface interactions refer to the complex physical, chemical, and mechanical processes at the interface between plasma and solid surfaces. These interactions are a critical issue for fusion reactors, as they determine the lifetime and performance of the plasma-facing components (PFCs), which are exposed to high heat and particle fluxes. Mitigating the effects of plasma-surface interactions is essential for the practical realization of nuclear fusion as a clean, sustainable, and safe energy source.

Plasma-surface interactions involve several phenomena, including sputtering, ion implantation, radiation damage, erosion, deposition, and re-deposition of material. The energy and particle fluxes in the plasma can modify the surface properties, such as composition, structure, roughness, and temperature, which in turn affect the plasma behavior. The interactions between plasma and PFCs are strongly influenced by the plasma parameters, such as density, temperature, and particle flux, as well as the material properties of PFCs, such as composition, surface area, and geometry.

In magnetic confinement fusion devices, such as tokamaks and stellarators, the plasma-surface interactions mainly occur in the divertor region, where the plasma is exhausted from the confinement volume to reduce the heat and particle loads on the PFCs. The divertor region is designed to enhance the radiative losses of the plasma by introducing low-Z and mid-Z impurities, such as nitrogen, neon, argon, boron, or lithium, which emit radiation in the visible and ultraviolet range. The radiative losses cool the plasma and create a buffer gas layer, which reduces the heat and particle fluxes on the PFCs. The choice of impurities depends on their radiation characteristics and compatibility with the fusion environment.

Plasma-surface interactions are still not fully understood, and several challenges remain in predicting and controlling their effects. The plasma-surface interface is a complex and dynamic system that involves multiple scales, from atomic to macroscopic, and a wide range of physical and chemical processes. The optimization of the plasma-surface interaction regimes and the development of advanced PFCs require the development of accurate models and diagnostics.

Wall conditioning in magnetic confinement fusion devices serves to manage impurities and control the fuel gas from plasma-facing components (PFCs). Currently, glow discharge boronization (GDB) is a prevalent technique, using boron-rich gases to deposit boron coatings on device walls, thereby enhancing performance. Nonetheless, for long-pulse, next-step fusion devices, alternative strategies are being explored due to the limitations of GDB.

A significant effort is being devoted to experimental and theoretical research in plasma-surface interactions in laboratory devices and large-scale fusion experiments, such as ASDEX Upgrade, JET, DIII-D, EAST, or Wendelstein 7-X.^[1][2][3]

References

1. Dux, R.; et al. (2005). "Plasma surface interaction with tungsten in ASDEX Upgrade". Journal of Nuclear Materials. 337–339: 852–856. Bibcode:2005JNuM..337..852D. doi:10.1016/j.jnucmat.2004.10.105.
2. Brezinsek, S.; et al. (2015). "Plasma-surface interaction in the Be/W environment: Conclusions drawn from the JET-ILW for ITER". Journal of Nuclear Materials. 463: 11–21. Bibcode:2015JNuM..463...11B. doi:10.1016/j.jnucmat.2014.12.007.
3. Brezinsek, S.; et al. (2022). "Plasma–surface interaction in the stellarator W7-X: conclusions drawn from operation with graphite plasma-facing components". Nuclear Fusion. 62 (1): 016006. Bibcode:2022NucFu..62a6006B. doi:10.1088/1741-4326/ac3508. S2CID 240484560.