Speaker
Description
The adhesion of dust on plasma-facing components (PFCs) plays a pivotal role in remobilization, wall mechanical impacts, resuspension during loss-of-vacuum accidents, collection activities and removal techniques [1]. Theoretical evaluations are possible with: (i) the van der Waals approach with adhesion assumed to emerge from the cumulative interaction between instantaneously induced multipoles [2]; (ii) elasticity theory with adhesion incorporated via the surface energy due to metallic bonding [3]. For metallic contacts, as in W-on-W, metallic bonding adhesion is nearly 100 times stronger than van der Waals adhesion. However, the experimental adhesion force agrees with the van der Waals result, since nm-scale roughness suffices to exceed the 0.3nm distance required to establish the metallic bond [4].
Experiments have quantified the effect of surface roughness, fusion-relevant coatings, atmospheric impurities and heat treatment [4,5,6]. In particular, electrostatic detachment measurements combined with vacuum furnace exposures have revealed that moderate but prolonged temperature excursions below the W recrystallization range can increase adhesion by orders of magnitude [6]. Unfortunately, dielectric breakdown at high field strengths prevented accurate adhesion measurements and only permitted lower bound estimates.
We surpass the limitations of earlier measurements employing an optimized electrostatic detachment set-up that features: (i) a HV power supply with a 60kV maximum (compared to 30kV) so that higher electrostatic fields are produced, (ii) 50um W dust (compared to 14um) so that higher electrostatic forces can be exerted, (iii) re-design of the insulation and cabling so that higher electrostatic fields can be tolerated prior to arcing. Thus, adhesion increase due to prolonged thermal treatment could be measured for the first time.
The results reveal that there is a sharp transition from the weak van der Waals value to the nearly 100 times stronger metallic bonding value with the transition point depending on the duration and strength of the thermal treatment. There is a switching of the dominant W-on-W interaction from long-range weak van der Waals to short-range strong metallic bonding which is enabled by atomic diffusion that slowly eliminates the nm-scale roughness.
[1] S. Ratynskaia et al., Rev. Mod. Plasma Phys. 6, 20 (2022).
[2] J. Israelachvili, Intermolecular and Surface Forces, Academic Press (2011).
[3] K. Johnson , Contact Mechanics, Cambridge University Press, (1985).
[4] G. Riva et al., Nucl. Mater. Energy 12, 593 (2017).
[5] P. Tolias et al., Nucl. Mater. Energy 15, 55 (2018).
[6] P. Tolias et al., Nucl. Mater. Energy 24, 100765 (2020).