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First results of a programme started in the KFA Julich to investigate through simulation experiments possible mechanisms responsible for the early appearance of impurities in Tokamak discharges are described. They stress the dominating influence of manufacturing and prehandling procedures on the surface properties and vacuum behaviour of technical materials frequently used and often considered for the first wall of large experimental devices. In addition, measurements of the tritium retention in carbon indicate that particular attention should be paid to the possible role of such reactions on the tritium inventory when low-Z materials are envisaged for the first wall.

The work function of regularly stepped tungsten single crystal planes has been investigated as function of terrace width and step orientation. The terraces were always formed by (110) planes. One set of samples exhibited step edges running parallel to the [001] direction but different terrace widths. Another set with almost equal terrace width showed step edges of different directions with correspondingly different edge structures. The work function measurements were performed using thermionic emission in the temperature range 2000–2800 K. The work function decreases linearly with step density for a given step orientation. Different step orientations give rise to slightly different work function reductions. The results are interpreted by attributing additional dipole moments to edge atoms. Different edge structures lead to different dipole moments. These findings are consistent with Smoluchowski's “smoothing effect” of the electron charge distribution caused by the structural arrangement of surface atoms. The temperature coefficient of the work function also depends strongly on step density and step orientation.

In order to get information on the vacuum compatibility of some low-Z materials as well as on the ultimate pressure which can be expected in the JET device, a vacuum system was constructed and tested which simulates the vacuum parameters of the JET device, drastically scaled down to a laboratory level. The system was made from Inconel 600 and baked repeatedly at 680 K. Outgassing rates were determined and the residual gas composition recorded. After bakeout an outgassing rate of 2 × 10−12 mbar 1/(sec cm2) was measured for Inconel 600. In this system low-Z samples made from SiC, B4C, Al2O3 and carbon base materials were tested. For these samples outgassing rates of the same order as found for Inconel were observed. Therefore we conclude that ultrahigh vacuum conditions can be maintained in the presence of appropriately treated low-Z materials.