Ultra-low work function of caesiated surfaces and impact of selected gas species

Abstract

Modern high-power negative hydrogen ion sources rely predominantly on the surface production of negative hydrogen ions. Hence, low work function converter surfaces are mandatory, for which the alkali metal Cs is commonly evaporated into the ion source to lower the work function of refractory metals by surface adsorption. To study the work function behaviour upon caesiation under the typically given non-ultra-high vacuum conditions, investigations are performed at a dedicated laboratory experiment. In a vacuum environment dominated by water vapour, the work function evolution is found to be dependent on the flux ratio of Cs to H2O onto the surface. For sufficiently high flux ratios, ultra-low work functions in the range of 1.25±0.10 eV are generated with excellent reproducibility. In the absence of Cs evaporation, the work function gradually increases under the influence of the residual gases, and re-caesiation processes lead to lower quantum efficiencies and higher work functions of typically 1.9–2.1 eV. While the addition of hydrogen and deuterium gas at several Pa as well as the leakage of inert gases (argon and nitrogen) into the vacuum system have a negligible influence on the caesiation process, small amounts of oxygen with partial pressures of ∼10-2–10-1 Pa lead to an instant reduction of the Cs density in the gas phase by several orders of magnitude and to an increase in the work function of the order of 1 eV. After the oxygen exposure is terminated, however, the Cs density and work function fully recover within several minutes.

Contributions of the extended ELISE and BATMAN Upgrade test facilities to the roadmap towards ITER NBI

“…Insights into the high dynamics of the work function at source parameters similar to the ion source are gained from the laboratory experiment ACCesS [38][39][40]. The most recent measurements revealed an ultra-low work function below 1.5 eV attributed to the formation of compounds of caesium, hydrogen and oxygen, the latter representing an impurity (essentially water) at base pressures of the order of 10 −4 Pa [41]. Initial measurements of the work function at the ion source BUG confirmed the ultra-low work function in the vacuum phase after several caesium conditioning pulses [42].…”

Summary of caesium evaporation and deposition during SPIDER first campaign

“…Depositing Cs on a metal surface, Mo for SPIDER case, lowers the work function of the surface near the extraction region, i.e. the PG, typically down to 1.5 eV from the original value of 4.5 eV of Mo [3] even though in recent experimental campaign values of 1.2-1.3 eV have been achieved [4]. The surface mechanism is much more efficient compared to the volume mechanism.…”