High-Z Impurities Molybdenum and Tungsten

Initially the molybdenum fluxes in [55] measured via atomic Mo-lines (379.8 nm, 386.4 nm and 390.3 nm) led to unreasonably high flux values. This was the case because for the calculation of the respective excitation rates the formula of van Regemorter was used [56], as these lines are optically coupled to the ground state (resonance lines). Therefore, experiments were performed to measure the excitation and ionization rates directly both in a linear plasma machine [57] and in a crossed beam experiment with a thermal molybdenum emitter [58], The results can be seen in Fig. 6.17 and show that the experimental values for excitation are about a factor of 5 larger than those from the van Regemorter formula, which leads to a reduction of the flux by the same order according to the smaller S/XB. More refined R-Matrix calculations have later confirmed the same factor and are included in Fig. 6.3. 

Tungsten is presently one of the favorite metals for a PFC in a fusion reactor as it even exceeds the properties of molybdenum concerning melting and sputtering. In [59] the inner wall and divertor were covered with tungsten-coated carbon tiles without a serious confinement in plasma opera- 

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