Temperature dependence of the work function of caesiated materials under ion source conditions

Abstract

The key parameter for the performance of negative hydrogen ion sources based on surface conversion is the work function of the converter surface. In order to enhance the negative ion yield caesium is introduced into the source which lowers the converter work function upon adsorption. However, the Cs layer is subject to background pressures of 10 -7 -10 -6 mbar and to H 2 or D 2 low-temperature plasmas which can have a major impact on the resultant work function. In order to determine the work function of caesiated surfaces under ion source relevant conditions and to identify influencing parameters, systematic investigations are performed at the laboratory experiment ACCesS. It is confirmed, that the work function of a sample surface in a vacuum of 10 -6 mbar is decreased upon the evaporation and adsorption of caesium. However, the adsorbed layer is determined by the formation of Cs compounds with residual gases from the background pressure leading to an increased work function by about 0.6 eV compared to pure Cs layers. Elevated surface temperatures are beneficial for cleaning and against degradation in absence of Cs evaporation, but no further enhancement of the work function can be achieved by temperatures up to 260 °C if a Cs flux onto the surface is already present. Thus, the minimally achievable work function of a stainless steel surface in ion source relevant vacuum conditions is 2.75 eV. The influence of the substrate material and plasma surface interaction will be investigated next.