Mixed-Material Erosion

The measured erosion of a beryllium surface will therefore be dependent not only on the impurity concentration in the incident plasma, but also on the temperature of the sample surface during the plasma exposure. An example of this effect is shown in Fig. 14.5, where the deposition of a carbon impurity 

Erosion during mixed impurity species bombardment of beryllium has also shown unexpected chemical effects that play a dominant role in determining the erosion rate of the substrate material. Bombardment of a beryllium sample with a CO+ ion beam produces an equilibrium surface state consisting of beryllium oxide, elemental carbon and C-0 compounds [13]. The chemical erosion of CO limits the carbon accumulation on the surface and therefore beryllium continues to be eroded. The complicated and interrelated nature of plasma-surface interactions requires measurements to be made in a situation that includes as many of the conditions of the final application as possible. 

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It is important to assess the need to implement further atomic and molecular reactions into the modeling. Our knowledge about the re-erosion yields of deposited layers has to improved urgently. A better understanding of hydrocarbon molecules and radicals is needed, in particular with respect to layer formation and material transport. The atomic data bases needed for the spectroscopic determination of impurity fluxes has to be improved for a critical re-evaluation of erosion yield measurements in tokamaks. The behaviour of mixed material systems (C, Be, W, etc.) deserves special attention. The data base about the dependence of chemical erosion on surface temperature, plasma flow density and ion energies needs to be consolidated. Finally the benchmarks of the numerical models with dedicated experiments must be one of the prime tasks of ongoing experiments. 

The longer pulse duration and cumulative run-time, together with the higher heat loads and more intense disruptions, represent the largest changes in operation conditions compared to today s experiments. Erosion of PFCs over many pulses, and distribution of eroded material, are critical issues that will affect the performance and the operating schedule of the ITER tokamak. Primary effects ensuing from erosion/re-deposition include plasma contamination, tritium co-deposition with carbon (if used in some parts of the divertor), component lifetime, dust, and formation of mixed-materials, whose behavior is still uncertain. 

Co-deposition of tritium with carbon is potentially the major T repository for ITER even if the use of carbon is minimized to the divertor strike plates. Retention by other mechanisms is expected to be low and to contribute only marginally to the in-vessel tritium-uptake (Sect. 12.4). For this reason the focus here is on some recent experimental findings associated with ( ) carbon erosion and deposition patterns in existing tokamaks, ( ) hydrocarbon film formation in areas of the divertor hidden from the plasma, and (in) mixed-material effects. 

ITER. This will remain a major difficulty unless experimentally validated in tokamaks with impurities and relevant wall materials to provide a realistic test-bed which would closely mirror options proposed for the next-step device (e.g., beryllium walls and carbon and/or tungsten divertor proposed for ITER). Such experiments would indeed help answer questions including the magnitudes of erosion and tritium co-deposition, dust formation in the vessel, the ease of tritium removal from mixed-materials, as well as operational aspects (e.g., of using beryllium on the first wall). 

Plasma conditions and wall materials must also enable a sufficient lifetime of the first wall components for economic reasons. Chemical erosion of graphite leads to significant erosion yields even under low-temperature, cold plasma conditions and can seriously limit the lifetime. Since the tokamak is a fairly closed system, most of the eroded material will be re-deposited somewhere inside the machine. The question of tritium retention and overall inventory in the device is closely connected to the chemical erosion and to possible co-deposition as well [6,7]. In order to minimize the net-erosion and optimize the lifetime of wall components, the re-deposition should be concentrated in areas of major erosion. Another way to minimize chemical erosion is the use of mixed materials, which - in laboratory experiments - display a reduced erosion yield in comparison to pure graphite. 

This sol-gel procedure is an elaboration on well established entrapment methods [29], but with the added advantage of stability and better flow properties. Interestingly, none of the examples presented thus far demonstrate competitive behavior between multiple ligands (i.e. displacement) in the FAC analysis of trimethoprim and pyrimethamine a reversed order of elution based on is described, but this could simply be due to the shift towards an on-rate limited situation for higher affinity compounds, as described earlier. Erosion of dynamic competition between ligands could occur if the sol-gel allows convective mixing of the entrapped protein however the bimodal pore structure of these materials would... 

The calculation of Rero, and therefore also of reg, contains quite some uncertainties. Knowledge about the local plasma parameters, erosion yields and sticking coefficients is required [39]. A possible mix of different erosion mechanisms and a surface layer composition with different materials adds some complexity to the problem. However, the extreme case of erosion mentioned above with a layer of 4.5 m eroded per year is unrealistic, since it is only valid for Rep = 0 or Rero = oo. Instead, experiments indicate that Rero does not deviate very much from unity. Indeed, with values typical for carbon, namely S = 0.75, Yr = 0.015, Yj = 0.02-0.5, c = 0.01-0.03, we obtain Rero in the range of Rero = 0.7-2.7. 

A mix of several different plasma facing materials is likely to be used in ITER to meet the requirements of areas with different power and particle flux characteristics. Erosion, and the subsequent transport of impurities, will inevitably lead to a certain amount of material mixing between these materials, whose behavior in a tokamak is uncertain. 

The use of carbon in ITER will lead to tritium co-deposition, and operational availability of the machine will depend on the actual tritium codeposition rate and the availability of reliable tritium co-deposition mitigation and/or removal techniques, which still need to be developed. An important uncertainty, which is being addressed by R D, is the consequence of mixing of eroded materials, which will unavoidably occur and, in particular, whether the deposition of eroded Be from the wall at the target plate could reduce substantially the chemical erosion of carbon near the strike points and, thus, tritium co-deposition. 

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