Plasma impurity

The interaction of the plasma with the first wall is, in the current Tokamaks, significantly reduced by the so called limiter which is typically a metallic ring mounted inside the torus in order to keep the plasma away from the first wall. In turn, the interaction of the plasma with the limiter is a significant source of plasma impurities. In many of today s Tokamaks the plasma impurity level is dominated by limiter erosion. 

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. 

The previous discussion has shown that plasma impurities present a complex set of problems whose solution is crucial to the successful operation of fusion reactors. The many and often subtle factors that govern plasma-materials interactions are still only partially understood. Consequently, the methods used today to control impurities are to a degree empirical in nature and cannot yet be precisely defined. It is likely that in the end a variety of approaches will be used to keep plasma impurities at minimal levels. The techniques are conveniently divided into divertor and non-divertor methods. The latter depend on modifications of one sort or another of the composition or structure of the surfaces facing the plasma. As will be seen in Sect. 6.5., meth-... 

Plasma impurities are recognized to be a limiting factor in the performance of present day Tokamaks. Unless solutions to the impurity problem can be found, further progress towards fusion as an energy source will be seriously jeopardized. 

Fig. 9.2. Calculated energy dependence of the sputtering yield of carbon by plasma impurity ions Be, C and W. Experimental data for O sputtering are shown for...

In order to investigate the influence of beryllium as a plasma impurity on the chemical erosion of carbon tiles at the divertor strike point, various types of carbons were exposed to a beryllium-seeded, steady state D-plasma in the PISCES-B facility (typical exposure parameters are ne 1-5 x 1018m-3, Id 1-5 x 1022m2 x s-1, Tsampie 330-970K and a total exposure time... 

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