Post-mortem and in situ evidence is presented in favor of the generation of high-velocity solid dust during the explosion-like interaction of runaway electrons with metallic plasma-facing components in FTU. The freshly-produced solid dust is the source of secondary de-localized wall damage through high-velocity impacts that lead to the formation of craters, which have been reproduced in dedicated light gas gun impact tests. This novel mechanism, of potential importance for ITER and DEMO, is further supported by surface analysis, multiple theoretical arguments and dust dynamics modeling.

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.

High temperature excursions have the potential to strongly enhance the room temperature adhesion of tokamak dust. Planar tungsten substrates containing adhered nearly monodisperse spherical tungsten dust have been exposed to linear plasmas and vacuum furnaces. Prolonged thermal treatments of varying peak temperature and constant duration were followed by room temperature adhesion measurements with the electrostatic detachment method. Adhesive forces have been observed to strongly depend on the thermal pre-history, greatly increasing above a threshold temperature. Adhesive forces have been measured up to an order of magnitude larger than those of untreated samples. This enhancement has been attributed to atomic diffusion that slowly eliminates the omnipresent nanometer-scale surface roughness, ultimately switching the dominant interaction from long-range weak van der Waals forces to short-range strong metallic bonding.

Experimental evidence of the pre-plasma remobilization of ferromagnetic dust in FTU is presented. Thomson scattering data and IR camera observations document the occurrence of intrinsic dust remobilization prior to discharge start-up and allow for a rough calculation of the average mobilized dust density. Exposures of calibrated extrinsic non-magnetic and ferromagnetic dust to sole magnetic field discharges reveal that the magnetic moment force is the main mobilizing force, as confirmed by theoretical estimates. Pre-plasma remobilization probabilities are computed for varying dust sizes. The impact of prematurely remobilized dust on the breakdown and burn-through start-up phases is investigated together with the discharge termination induced once the plasma plateau is established.

The adhesion of tungsten dust is measured on plasma-exposed and non-exposed tungsten substrates with the electrostatic detachment method. Tungsten substrates of comparable surface roughness have been exposed to the deuterium plasmas of the GyM linear device and the argon plasmas of rf glow discharges under conditions which invariably modify the surface composition due to physical sputtering. The adhesion has been systematically characterized for different spherical nearly monodisperse dust populations. Independent of the dust size, an approximate 50% post-exposure reduction of the average and spread of the adhesive force has been consistently observed and attributed to surface chemistry modifications.

Tungsten (W) substrates with adhered beryllium (Be) proxy dust - copper, chromium, aluminium - have been exposed in the Magnum-PSI linear device. Their interaction with transient and stationary plasmas has been systematically studied under varying heat fluxes and magnetic field topologies. The dust remobilization activities, macro-morphological changes and chemical modifications induced by the plasma incidence are documented. Aluminium is identified to be the most suitable surrogate material due to the similar binary phase diagram and nearly identical evaporation rates. Extrapolation suggests that Be dust cannot survive on hot W surfaces but it can trigger mixed Be/W effects prior to its plasma removal.

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Castellated substrates with adhered micron dust have been exposed in the outer ASDEX-Upgrade divertor to ELMy H-mode discharges. Beryllium proxy (chromium, copper) and refractory metal (tungsten, molybdenum) dust has been deposited on the plasma-facing and plasma-shadowed sides of the monoblocks as well as the bottom of the gaps. Interaction with time-averaged transient heat loads up to 5 MWm(-2) led to dust remobilization, clustering, melting and wetting-induced coagulation. The amount of dust released in the vessel has been quantified and remobilized dust trajectories inferred. Gaps can efficiently trap locally adhered dust, but dust detaching from adjacent monoblocks does not preferentially move inside the gaps implying that they do not constitute a dust accumulation site. Heat transfer simulations of melting events are also reported taking into account heat constriction due to the finite contact area and the presence of surface roughness.

Comprehensive measurements of the adhesive force for tungsten dust adhered to tungsten surfaces have been performed with the electrostatic detachment method. Monodisperse spherical dust has been deposited with gas dynamics techniques or with gravity mimicking adhesion as it naturally occurs in tokamaks. The adhesive force is confirmed to follow the log-normal distribution and empirical correlations are proposed for the size-dependence of its mean and standard deviation. Systematic differences are observed between the two deposition methods and attributed to plastic deformation during sticking impacts. The presence of thin beryllium coatings on tungsten surfaces is demonstrated to barely affect adhesion.

Dust remobilization is one of the not yet fully understood mechanisms connected to the prompt erosion ofmaterial from plasma facing surfaces in fusion devices. As a part of a newly initiated cross-machine study,dust remobilization experiments have been performed on the COMPASS tokamak. Tungsten sampleswith well-defined deposited tungsten dust grains, prepared using a recently developed controlled pre-adhesion method, have been exposed to ELMy H-mode discharges as well as L-mode discharges withforced disruptions. Here we report on the technical aspects of the experiment realization as well as on theexperimental results of dust remobilization. The latter is discussed in the light of data from other machinesand a physical interpretation is suggested for the observed spatial localization of the dust remobilizationactivity. Evidence of rearrangement of isolated dust into clusters and strings is also presented.© 2017 Elsevier B.V. All rights reserved

Remobilization is one of the most prominent unresolved fusion dust-relevant issues, strongly related to the lifetime of dust in plasma-wetted regions, the survivability of dust on hot plasma-facing surfaces and the formation of dust accumulation sites. A systematic cross-machine study has been initiated to investigate the remobilization of tungsten micron-size dust from tungsten surfaces implementing a newly developed technique based on controlled pre-adhesion by gas dynamics methods. It has been utilized in a number of devices and has provided new insights on remobilization under steady-state and transient conditions. The experiments are interpreted with contact mechanics theory and heat conduction models.

Studies of tungsten dust remobilization from castellated plasma-facing components can shed light to whether gaps constitute a dust accumulation site with important implications for monitoring but also removal. Castellated structures of ITER relevant geometry that contained pre-adhered tungsten dust of controlled deposition profile have been exposed in the Pilot-PSI linear device. The experiments were performed under steady state and transient plasma conditions, as well as varying magnetic field topologies. The results suggest that dust remobilization from the plasma-facing monoblock surface can enhance dust trapping in the gaps and that tungsten dust is efficiently trapped inside the gaps.

The first experimental determination of the pull-off force for tungsten dust adhered to tungsten surfaces is reported. Dust deposition is conducted with gas dynamics methods in a manner that mimics sticking as it occurs in the tokamak environment. Adhesion measurements are carried out with the electrostatic detachment method. The adhesion strength is systematically characterized for spherical micron dust of different sizes and planar surfaces of varying roughness. The experimental pull-off force is nearly two orders of magnitude smaller than the predictions of contact mechanics models, but in strong agreement with the Van der Waals formula. A theoretical interpretation is provided that invokes the effects of nanometer-scale surface roughness for stiff materials such as tungsten.

Transient heat load tests have been conducted in the lower divertor of DIII-D using DiMES manipulator in order to study the behavior of dust on tungsten Plasma Facing Components (PFCs) during ELMy H-mode discharges. Samples with pre-adhered, pre-characterized dust have been exposed at the outer strike point (OSP) in a series of discharges with varied intra-(inter-) ELM heat fluxes. We used C dust because of its high sublimation temperature and non-metal properties. Al dust as a surrogate for Be and W dust were employed as relevant to that in the ITER divertor. The poor initial thermal contact between the substrate and the particles led to overheating, sublimation and shrinking of the carbon dust, and wetting induced coagulation of Al dust. Little modification of the W dust was observed. An enhanced surface adhesion and improvement of the thermal contact of C and Al dust were the result of exposure. A post mortem "adhesive tape" sampling showed that 70% of Al, <5% of W and C particles could not be removed from the surface owing to the improved adhesion. Al and C but not W particles that could be lifted had W inclusions indicating damage to the substrate. This suggests that non destructive methods may be inefficient for removal of dust in ITER.

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The first study of the interaction of metallic dust (tungsten, aluminum) adhered on tungsten substrates with transient plasma heat loads is presented. Experiments were carried out in the Pilot-PSI linear device with transient heat fluxes up to 550 MW m(-2) and in the DIII-D divertor tokamak. The central role of the dust-substrate contact area in heat conduction is highlighted and confirmed by heat transfer simulations. The experiments provide evidence of the occurrence of wetting-induced coagulation, a novel growth mechanism where cluster melting accompanied by droplet wetting leads to the formation of larger grains. The physical processes behind this mechanism are elucidated. The remobilization activity of the newly formed dust and the survivability of tungsten dust on hot surfaces are documented and discussed in the light of implications for ITER.

The first combined experimental and theoretical studies of dust remobilization by plasma forces are reported. The main theoretical aspects of remobilization in fusion devices under steady state conditions are analyzed. In particular, the dominant role of adhesive forces is highlighted and generic remobilization conditions - direct lift-up, sliding, rolling - are formulated. A novel experimental technique is proposed, based on controlled adhesion of dust grains on tungsten samples combined with detailed mapping of the dust deposition profile prior and post plasma exposure. Proof-of-principle experiments in the TEXTOR tokamak and the EXTRAP-T2R reversed-field pinch are presented. The versatile environment of the linear device Pilot-PSI allowed for experiments with different magnetic field topologies and varying plasma conditions that were complemented with camera observations.

Dust remobilization plays a key role in diverse plasma-wall interaction issues. The most important aspect of remobilization is connected with the identification of in-vessel dust accumulation sites and consequently the development of dust removal techniques. In ITER, PFCs will be castellated, and shaped, and gaps have been assumed to be a preferred dust accumulation site. Despite its importance, remobilization of dust grains in normal operating conditions has only been recently addressed. A novel experimental technique for its study in fusion plasmas has been developed and its main theoretical aspects have been analyzed [1]. The technique is based on controlled adhesion of dust grains on tungsten surfaces combined with detailed mapping of the dust deposition profile prior to and post plasma exposure. Experiments with planar tungsten surfaces have been carried out in Pilot-PSI [1]. Here we report on the recent experiments carried out in Pilot-PSI with the aim to study the effect of ELM-like pulses on remobilization under normal and glancing B field, as well as remobilization from castellated PFCs under steady-state plasma conditions. The observations by two fast cameras with a resolution down to ten micrometres per pixel allowed us for the first time to resolve the details of dust motion in the sheath. Discussion on the difference of the dust remobilization activity and release velocity under normal and ELM-like conditions is provided along with the first results of remobilization of W dust from ITER like castellated structures. We also present evidence of a particular phenomenon taking place under interaction of ELM-like plasma with W dust particles residing on the W surface - multiple molten grains tend to coagulate into a single nearly spherical grain.