Dust and flakes in the JET MkIIa divertor, analysis and results

After operation of the JET machine with the MkIIa divertor, samples of dust and flakes/deposits have been removed from the divertor region for analysis. Flakes/deposits up to 40 mu m thick have been found in the inner leg of the divertor. The flakes/deposits formed on cold surfaces shadowed from the plasma. The flakes were found to be mostly carbon saturated with deuterium to a D/G ratio of 0.4. Analysis of the material removed has shown that it contains significant fractions (similar to 4%) Of the tritium (produced by D-D reactions) and deuterium introduced in the JET machine during this period of operation. Dust material has also been collected. Vacuum cleaning collected very little material but a significant quantity of material was collected by smears. The analysis of this smeared material has shown it to be of a very similar chemical composition to that of flakes but to contain a significantly lower inventory of tritium. The dust has an average particle size of 27 mu m and a specific surface area of 4 m(2)/g. The vast majority of this material is associated with co-deposited layers in the divertor region. The dust is on average present at a level of 120 mu g/cm(2). (C) 1999 JET Joint Undertaking, published by Elsevier Science B.V. All rights reserved.