Dust control in Tokamak environment

During ITER lifetime, dusts will be produced due to the interaction of the plasma with the Plasma Facing Components. During steady state phase, the dust formation is closely linked to erosion of plasma facing materials and the flaking of the co-deposited layers. The growth of small particles in the edge of fusion plasmas from atomic or molecular precursors which are released by physical or chemical erosion can also lead to dust creation. Particles can as well be created during off-normal plasma events by either condensation and growth of the vaporized material, or pressure-driven ejection of melt layer material or explosive brittle destruction by heating of gas bubbles. At last, during maintenance activities, much larger particles can arise from mechanical abrasion during component replacement. In ITER, these dusts will be activated, tritiated and chemically toxic due to the presence of beryllium. ITER has fixed a set of safety limits in order to control the mobile activation product inventory inside the vacuum vessel, to ensure that the dust chemical reactivity is adequately controlled, and to avoid the hazard of dust explosions. This has been traduced into the following project guidelines: the mobilisable dust inside the vacuum vessel should be limited to 100 kg of W, 100 kg of Be and 200 kg of C (limits based on radiological hazard) and the dusts on the ''hot'' surfaces of the divertor should not exceed 6 kg of Be, 6 kg of W and 6 kg of C in order to produce less than 2.5 kg of H2 in the vacuum vessel in case of a stream ingress on hot surfaces. Some calculations have shown that the administrative guideline of 100 kg for tungsten dust could be reached in about 500 plasma pulses, and in any case before the assumed replacement of the divertor. Dust diagnostics and removal methods need thus to be developed for ITER considering the constraints linked to magnetic field, radiation, vacuum and temperature. Concerning dust diagnostics, several techniques could be potential candidates in a Tokamak environment. They can be based on erosion, deposition or dust in suspension measurements using optical, sampling or gravimetric systems. The paper assesses the technical feasibility of the different techniques and the adaptation needed, if any, for an implementation in ITER. It focuses in particular on techniques that are being developed to monitor the integrity of the plasma surface components by measuring defects or erosion and that could be applied to dust/deposit monitoring. For the dust removal, three stages are needed: mobilisation of the dusts, collection of the mobilised materials and transport outside the machine. Many studies have been performed in the past years on the collection and transport of the dusts using for example vibrating or electrostatic conveyor. The paper focuses on the studies currently performed in order to improve the mobilisation step using in particular laser beam. (orig.)