Thin films of low Z materials in fusion devices

The vacuum requirements of fusion devices differ in many aspects from those of ultra-high vacuum systems. The power loads on the walls are high. The impurity release during the plasma discharge must be small. The hydrogen exchange between plasma and wall ('hydrogen recycling') must be optimized. Nuclear aspects must be kept in mind. This paper indicates the resulting consequences for the choice of wall and configuration. The plasma-facing surfaces should consist of one single material, otherwise the transport of impurity atoms between components negatively affects the plasma behaviour. Atomic hydrogen from the plasma reacts with the film of oxides and carbon which initially covers the stainless steel or Inconel walls. Water and hydrocarbons, i.e. O and C impurities result. Wall conditioning techniques developed to remove this film are described. However, the surface thereafter releases many metal atoms when auxiliary heating is applied to the plasma. A way out has been to use graphite where high particle fluxes impinge and to cover the other plasma-facing surfaces with a thin film of hydrogen-rich amorphous carbon (a-C:H) ('carbonization'). A recent further improvement in TEXTOR has resulted from the addition of boron to the carbon in the film. The success of 'boronization' may be partly due to the formation of a thin boron-rich film in non-bakable portholes. (author)