First-wall, impurities, effects

Plasma impurities may also have a beneficial effect by cooling the plasma edge in the vicinity of the first wall. But this is partially offset by the fact that the sputtering yields due to impurities are higher as compared with that of hydrogen. Consequently, a relatively small concentration of impurities can significantly increase the wall erosion and plasma contamination. 

Although observed quite early in the development of plasma devices, unipolar arcing has only recently gained widespread attention as both a major source of plasma impurities and as a potentially severe erosive effect for first walls, limiters and other material surfaces in close proximity to Tokamak plasmas. 

In the most recent reactor studies a way out of this dilemma was sought by applying the cold plasma mantle method of impurity control. Unfortunately, the experimental basis is not yet adequate for technological problem identification. But there is one thing that can already be stated, i,e. that removal of the plasma and alpha particle energies in the case of the cold plasma mantle has to be performed very probably via the first wall, this having the previously mentioned negative effects on its neutronic load capacity and lifetime. 

The principal impurities in item (1) were the undesirable isomers of TNT DNT their presence lowered the mp (Setting Point) of a-TNT from 80.75° to 80.2° (Specification grade) and could lower it much more. On castin g TNT in the shell, the outside of the cast solidified first because or the cooling effect produced by the metal walls of the shell, while the center portion of the cast solidified last, because it was somewhat insulated. As the result of this cooling, the mixture that sol i di f i ed - a C the outside contained a large proportion of high melting components, while the portion that solidified in the center core contained the bulk of the impurities because they solidified at a lower temperature. 

Simulating erosion and re-deposition processes in fusion devices lead to a better understanding of the processes involved. The 3-dimensional Monte-Carlo code ERO-TEXTOR [35,36] has been developed to model the plasma-wall interaction and the transport of eroded particles in the vicinity of test limiters exposed to the edge plasma of TEXTOR. Important problems concerning the lifetime of various wall materials (high Z vs. low Z) under different plasma conditions and the transport of eroded impurities into the main plasma can be treated with the ERO-TEXTOR code. Recently, the divertor geometries have been implemented to carry out simulations for JET, ASDEX and ITER [37], In addition, first attempts have been made to simulate erosion and re-deposition processes in the linear plasma device PISCES to analyze the effect of beryllium. 

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