Plasma contamination

The gas temperature of the Ar plasma in the range of 600 100 K effects temperatures of water or LN2 cooled samples of 350 30 or 200 30 K, respectively. In addition, C, N, and O species are desorbed from the chamber walls and introduced samples, effecting a plasma contamination level in the 0.01-0.1% range. 

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. 

Theoretical estimates of the erosion of the first wall and plasma contamination due to sputtering requires a knowledge of particle and photon fluxes to the wall, as well as data on the erosion yields. Sputtering will be discussed in a later part of this chapter. Here we shall briefly summarize some of the calculations done on primary fluxes in future fusion reactors. The calculations are rather uncertain because of the poor understanding of various parameters such as divertor efficiency, refueling, neutral beam heating, plasma temperature and density profiles, including the scrape-off layer in the case of divertor operated Tokamaks. 

However, this pocket explosion effect appears not to be an annoying long lasting nuisance, but presents only a transient character Prolonged exposure to intense atomic particle fluxes leads to its disappearance11S). The presently available experimental data do not allow a final conclusion about the significance of this suggested mechanism of the plasma contamination to be drawn and more study is necessary. 

Single-component irradiations have provided useful information concerning certain projectile/surface interactions which contribute to plasma contaminant release and surface damage and erosion, but they cannot yield any information about interactive (synergistic) surface effects. These arise when two or more plasma radiation components interact simulaneously with surfaces, together producing plasma contaminant release and/or surface erosion either larger or smaller than that expected from a simple summation of the effects caused by the individual radiation components. Interactive effects can also occur after sequential irradiations. 

The material for the first wall has to meet a number of requirements. Besides having a low erosion rate and resulting low plasma contamination, the criteria as listed in Table 3 for selecting wall materials in the near term experimental fusion reactors have also to be considered28. With respect to a long term commercial power reactor the priority list of these criteria is different and the availability of the material resources has to be considered as well, as shown in Table 4. 

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. 

Hot-pressed SiC is the preferred material to replace oxide ceramics for rods fixtures, and punches in high-temperature strength testing equipment. In view of the low level of plasma contamination, the low induced radioactivity, and excellent... 

Jackson, L.C., Improving Adhesion by Gas-Plasma Contaminant Removal , Adhesives Age, Sept. 1978... 

LCFS, these impurities have a very high probability to penetrate the core plasma and contribute strongly, therefore, to plasma contamination. As a result, the limiter concept is no option for future fusion devices. 

Surfaces may be treated in inert gas plasmas. In the early studies of plasma treatment with inert plasmas (CASING - crosslinking by activated species of inert gas), plasma contamination probably resulted in oxidation. The activation that does occur in an inert gas plasma is probably from UV radiation from the plasma, causing bond scission in polymers or the generation of electronic charge sites in ceramics. 

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