The plasma generator PSI run by the Berlin (Germany) division of Association Euratom- IPP is a stationary high current arc discharge in an axial magnetic field. This facility allows the production of a stationary plasma column of about 2 m in length and 8 cm in diameter. It is used for basic as well as applied research in the field of plasma physics. A number of technical investigations have been conducted in close contact with the EFDA group in Garching (Germany). The problem of chemical erosion of graphite materials being foreseen for ITER has been studied in a joint project. In this context the chemical erosion yield of various CFC materials was studied in hydrogen and deuterium discharges. It was observed that with moderate or low flux densities (about 1021 ions /m2 s) each hydrogen ion hitting the target has a chance of nearly 10% of releasing a methane molecule (or some other carbon-consisting molecule) from the surface with flux densities of 2 • 1023 ions /m2 s the erosion decreases to values of about 0.5%. This is constant with observations on tokamaks.

Carbon Fiber Reinforced Material (CFC):

Such materials were chosen for the bottom parts of the ITER divertor vertical target due to their resistance to the high heat fluxes present in that area. However, this important advantage is counterbalanced by other problems, such as erosion and codeposition. The experimental results described on this page mitigate the problem of erosion. In addition, with respect to film formation the observations made contain a positive message but are less convincing because of lacking knowledge on the ITER divertor conditions.

An upgraded version of the plasma generator (PSI-2) was used to study the erosion and deposition behavior of hydrocarbons under conditions similar to that in some areas of the ITER divertor. The hydrocarbons were generated either by chemical erosion of a graphite target or by injections into the plasma through a nozzle. Two processes are at the basis of the two phenomena: deposition of all neutral hydrocarbons which have a high sticking probability and simultaneous erosion of the films by atomic hydrogen. Whereas deposition proved to be nearly independent of the surface temperature, erosion strongly increases with rising temperature.

Therefore there is a critical collector temperature where erosion and deposition are in equilibrium and the thickness of a layer is unchanged. Beyond this temperature erosion prevails and films which have been produced previously are dissolved.

The critical temperature is not a universal quantity but increases with the ratio of hydrocarbon to hydrogen fluxes. The critical temperatures are inferred immediately as those where the curves cut the temperature scale (fig. 2). This is seen to happen at T = 360 K and 380 K wall temperatures. However, the flux ratio requires further investigations. Numerical simulations and additional experiments are under way to assess this issue.