Plasma Wall Interactions

When an impacting particle transfers energy to a near surface carbon atom in an amount sufficient to overcome the lattice bond energy or surface binding energy, some carbon atoms may be displaced and move in a direction defined by the angle 

The quantitative effect of the mass, energy, and angle of impact on the sputter yield for impacting deuterium ions is shown in Figs. 12a and b. As the kinetic energy 

For intermediate temperatures from 400-1000°C (Fig. 11), the volatilization of carbon atoms by energetic plasma ions becomes important. As seen in the upper curve of Fig. 11, helium does not have a chemical erosion component of its sputter yield. In currently operating machines the two major contributors to chemical erosion are the ions of hydrogen and oxygen. The typical chemical species which evolve from the surface, as measured by residual gas analysis [37] and optical emission [38], are hydrocarbons, carbon monoxide, and carbon dioxide. 

Chemical erosion can be suppressed by doping with substitutional elements such as boron. This is demonstrated in Fig. 14 [47] which shows data for undoped pyrolitic graphite and several grades of boron doped graphite. The mechanism responsible for this suppression may include the reduced chemical activity of the boronized material, as demonstrated by the increased oxidation resistance of B doped carbons [48] or the suppressed diffusion caused by the interstitial trapping at boron sites. 

The limiting temperature for graphite use in fusion systems is defined by thermal sublimation ( 1500-2000°C). However, a process which is very similar to thermal sublimation (in cause and in effect) appears to define the current temperature limit. This phenomenon, which is known as radiation enhanced sublimation (RES), is not clearly understood but dominates above a temperature of about 1000°C and increases exponentially with increasing temperature. 

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The plasma-wall interaction of the neutral particles is described by a so-called sticking model [136, 137]. In this model only the radicals react with the surface, while nonradical neutrals (H2, SiHa, and Si H2 +2) are reflected into the discharge. The surface reaction and sticking probability of each radical must be specified. The nature (material, roughness) and the temperature of the surface will influence the surface reaction probabilities. Perrin et al. [136] and Matsuda et al. [137] have shown that the surface reaction coefficient of SiH3 is temperature-independent at a value of = 0.26 0.05 at a growing a-Si H surface in a... 

The boundary conditions for the density balance equation of neutrals are given by the plasma-wall interaction (for instance by deposition processes), which is... 

FIG. 14. Schematic representation of plasma-wall interaction for neutrals. (Adapted from G. J. Nien-huis, Ph.D. Thesis. Universiteit Utrecht. Utrecht, the Netherlands. 1998.)... 

A sticking model is used for the plasma-wall interaction [137]. In this model each neutral particle has a certain surface reaction coefficient, which specifies the probability that the neutral reacts at the surface when hitting it. In case of a surface reaction two events may occur. The first event is sticking, which in the case of a silicon-containing neutral leads to deposition. The second event is recombination, in which the radical recombines with a hydrogen atom at the wall and is reflected back into the discharge. 

In addition to a spatial variation, the expected plasma flux and consequent plasma-wall interactions on first-wall components... 

Finally the plasma-wall interactions result in erosion of the first wall which can, under long term operation, lead to significant changes of the wall thickness and composition. With primary particle fluxes of the order of 1019-1020 ions m 2 s 1 and a sputtering yield of 0.02, erosion rates of the first wall up to several mm per year are expected36 . Such values appear to be too high for reliable long term operation of a fusion reactor. 

In concluding this section we should like to give a brief overview of the processes involved in the release of impurities due to plasma-wall interactions. These processes will be discussed in some detail in Sect. 5. 

The control of impurity release and transport requires a better understanding of the complex phenomena of plasma-wall interactions including the processes occuring in the scrape-off layer in the limiter shadow. In order to establish the feasibility of suggested solutions such as divertors or surface modifications, experiments have to be performed not only in the laboratory but also in-situ in fusion devices. The latter... 

Proc. Int. Symp. on Plasma Wall Interaction (Jiilich 1976). Oxford Pergamon Press 1977... 

It, rather, represents an ion-conducting wall of the plasma at a floating potential and reactions are motivated by the plasma-wall interactions described earlier. It is feasible to introduce a third electrode to the system, placing it in contact with the electrolyte, but not with the plasma, and therefore gaining some control over the potential difference between the electrolyte and the plasma. In the case of purely ion-conducting electrodes, the electric current offers information about the reaction rate at the plasma/electrolyte interfaces. 

Plasma—Wall Interaction Status and Data Needs... 

The second requirement is common to all confinement devices and is related to the erosion and re-deposition processes of wall materials and to high heat loads, both determined by plasma-wall interaction. An overview on the key issues in this field is given in Sect. 1.2. The damage to wall components... 

The design solutions for heat and power exhaust depend on complex processes. For an optimized design it is not sufficient to have empirical data, like it is the case for energy confinement. The major processes have to be understood and only with extensive numerical modeling we may achieve reliable predictions about the life time of wall components and obtain tools for optimization of the wall and divertor design. In Sect. 1.4 the crucial processes in plasma-wall interaction are discussed, addressing the critical issues and the related needs for improved atomic and plasma-material interaction data. 

In this section the crucial problems of plasma-wall interaction are discussed, which need to be solved in order to achieve a high availability of a fusion power plant. 

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