Summary of research performed [Tritium inventory in fusion reactors - Princeton Plasma Physics Laboratory, United States]

Studies on dust and deposition in NSTX are presented. The possible formation of blisters in tungsten would reduce thermal contact within the bulk and could result in flaking, melting and evaporation under high heat loads. Generation of tungsten dust and radiological activation/dispersion are safety issues in assessments of plant accidents. Tungsten samples blistered as a result of a high fluence of D-ions were subjected to an elevated heat flux from a scanning continuous wave Nd laser. Images taken during the laser heating revealed hot spots at the location of the blisters, showing that blister caps will experience much higher temperature excursions than an unblistered surface. Another segment of the work focused on measurements of temperature rise and morphological changes of graphite and CFC samples during exposure to a high heat flux. A scanning Nd laser was used to deliver pulses of high heat flux at pulse lengths of 1 to 200 ms. One key observation was that the thermal response of tokamak-generated co-deposits showed striking differences to the manufactured material. A third area of study involved the monitoring of dust deposits in NSTX by means of two quartz crystal microbalances. Time-resolved measurements were carried out over a 4-week period in which the net deposited mass of 13.3 μg cm-2 matched a value of 13.5 μg cm-2 measured independently by ion beam analysis. Monte-Carlo modelling suggests that transient processes are likely to dominate the mode of deposition. Since the amount of dust generated in next-generation machines is likely to scale up by two or three orders of magnitude as the erosion increases, dust will become a major factor in safety assessments, site licensing and tokamak operations. Dust particles that had accumulated on a view-port at the bottom of the NSTX vessel were examined with a digital optical microscope - Raman spectrometric analysis showed the particles to be disordered graphitic carbon (sp2), easily recognized by the presence of the typical D-band at ∼ 1330 cm-1 and G-band at 1585 cm-1. Preliminary results from a novel electrostatic dust detector were also presented