Tokamak disruptions can generate relativistic runaway electrons (REs) that preserve a significant fraction of the original plasma current. Upon termination on plasma-facing components (PFCs), REs are known to lead to bulk melting [1] and have been recently observed to lead to material explosions as well as to the expulsion of fast solid debris [2]. The solid dust particles are ejected with speeds of the order of km/s and their unavoidable mechanical collisions with the vessel yield further delocalized damage [2,3]. The physics of dust-PFC impact damage have been studied through experiments that launch dust to near supersonic speeds using First Light Fusion's two-stage light-gas gun (12.7 mm diameter bore, 7.5 m long [4]). Spherical molybdenum particles with a diameter of 71 um have been launched inside a 2-part sabot to velocities of ~1-2km/s towards cryogenically cooled TZM targets. The set up mimics recent observations at the FTU tokamak [2]. The crater geometry has been analyzed with surface techniques. The effect of the PFC temperature (room vs cryogenic) on the crater dimensions and crater micro-morphology is discussed. [1] G. F. Matthews et al., Phys. Scr. T167, 014070 (2016) [2] M. De Angeli et al., Nucl. Fusion 63, 014001 (2023) [3] P. Tolias et al., arXiv:2208.02897 (2023) [4] T. J. Ringrose et al., Procedia engineering 204, 344-351 (2017)

Plasma facing components (PFCs) erosion in tokamaks is a well-known problem for a successful integrated tokamak reactor and for future fusion power plants for energy production [1, 2]. In fact the eroded material could migrate inside the plasma core, leading to an increase of Zef f of the plasma itself, and/or in other critical locations inside the vacuum vessel and on sensitive apparatus (such as microwave mirrors, optical windows, probe heads, antennas) rising problems during tokamak operations [3]. Moreover, eroded material can increase the dust inventory in the vacuum vessel leading to safety related problems [4]. The morphological analysis of craters found on FTU limiter tiles [5] has revealed a novel source of material erosion. In particular, energetic runway electrons (REs) strike the vessel wall and limiters causing an explosion-like event that leads to the ejection of fast, about 1km/s, solid dust. This heavy fast dust moves inertially and impinges on nearby PFCs leading to craters generation, cracks formation and the vaporization of co-deposited material. The experimental reproduction of damages, induced by W dust projectile impinging on W targets in the km/s speed range, could be useful to study possible damage in the last generation tokamaks, such as ITER, where REs could be an issue. In this report are present morphology analyses of craters resulting from shooting different sizes of W dust on W targets up to 3 km/s. The results of this investigation could then be used for the empirical formulation of a damage scaling law.

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During the 2013 shut-down of FTU, some tiles of the toroidal limiter were collected. In particular two tiles, located in front of the poloidal limiter, were discovered to be covered by craters surrounded of deposited material (haloed craters). The tiles have been morphologically and chemical analyzed by means of electron microscope and surface analyses techniques showing that the halos were formed, upon impact with Mo dust, inside a co-deposited region of molybdenum oxide MoO3. Based on the results of the analysis and on the information available in literature it seems that the haloed craters are due to the impact of dust ejected by the poloidal limiter surface upon a disruption and hitting the closest available surface. In order to support the above rationale we have conducted an experimental campaign simulating the dust projectile, by means of a light gas gun, reproducing the impacts of Mo grains, with different size, on Mo targets. The results suggested that the responsible of the haloed crated are those particles with size within 60-100?m diameter, travelling at, about, 500m/s.

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