Speaker
Description
This contribution is based on over 40 years of dust research in several devices with consecutive modifications of plasma-facing materials (PFM):
• JET from full metal wall in 1983-1984, via JET-C with different fuelling schemes (including full D-T in 1997) and three consecutive divertor models, to JET-ILW with Be and W wall components;
• TEXTOR from full metal wall (1983-1984) to the boronised (also siliconized) carbon wall and bulk W test limiters;
• EXTRAP-T2 reversed field pinch with graphite PFM.
Holistic approach to dust studies in fusion devices comprises a broad range of plasma diagnostics (spectroscopy, imaging tools, magnetic sensors), tracer techniques for material migration, development and selection of collection and retrieval methods for secure material transfer to analytical laboratories where over 35 techniques for ex-situ material characterisation were used. A crucial element of the programme was on fuel retention analysis from total amounts of H, D, T to depth profiling, elemental micro-mapping, determination of specific tritium activity and fuel (D, T) quantification in individual dust grains, e.g. distinction of retention between C and metals in mixed samples of dust.
Such integrated efforts have allowed for the determination/ identification of several generation mechanisms including not only disintegration of co-deposits, arcing, brittle destruction and metal splashing but also plasma-chemical effects and metal flaking. In addition, dust-related safety aspects have been experimentally addressed, e.g. particle mobility during in-vessel operations with remote handling, the impact of hot water and/or water vapour on dust generation from affected wall materials, and dust accumulation in filters of the vessel ventilation system
Particular emphasis is given to: (i) boron species, its chemical form, role of boronisation as a marker in the growth of co-deposits; (ii) metal-based or metal-containing dust W, steel, Be in the form of splashes and droplets; (iii) metal oxides, WO$_2$, BeO, whose in-situ formation has been proven. The latter process may lead to quite severe material erosion and transport. The investigation of W presence in dust was often concentrated on searching for loose droplets. No solid evidence for such objects has been obtained either in JET-ILW or in TEXTOR. Very detailed microscopy data clearly indicate that splitting and then flaking of thin metal layers (<0.5 micrometre) occurs in overheated surfaces of bulk W components, e.g. Langmuir probes. This pathway of material loss should not be neglected in the estimates of global erosion.