Influence of H2 and D2 plasmas on the work function of caesiated materials

Abstract

Caesium-covered surfaces are used in negative hydrogen ion sources as a low work function converter for H–/D– surface production. The work function χ of the converter surface is one of the key parameters determining the performance of the ion source. Under idealized conditions, pure bulk Cs has 2.14 eV. However, residual gases at ion source background pressures of 10−7–10−6 mbar and the plasma surface interaction with the hydrogen discharge in front of the caesiated surface dynamically affect the actual surface work function. Necessary fundamental investigations on the resulting χ are performed at a dedicated laboratory experiment. Under the vacuum conditions of ion sources, the incorporation of impurities into the Cs layer leads to very stable Cs compounds. The result is a minimal work function of χvac ≈ 2.75 eV for Cs evaporation rates of up to 10 mg/h independent of substrate material and surface temperature (up to 260 °C). Moreover, a distinct degradation behavior can be observed in the absence of a Cs flux onto the surface leading to a deterioration of the work function by about 0.1 eV/h. However, in a hydrogen discharge with plasma parameters close to those of ion sources, fluxes of reactive hydrogen species and VUV photons impact on the surface which reduces the work function of the caesiated substrate down to about 2.6 eV even without Cs supply. Establishing a Cs flux onto the surface with ΓCs ≈ 1017 m−2 s−1 further enhances the work function obtaining values around 2.1 eV, which can be maintained stable for several hours of plasma exposure. Hence, Cs layers with work functions close to that of pure bulk Cs can be achieved for both H2 and D2 plasmas. Isotopic differences can be neglected within the measurement accuracy of about 0.1 eV due to comparable plasma parameters. Furthermore, after shutting down the Cs evaporation, continuing plasma exposure helps against degradation of the Cs layer resulting in a constant low work function for at least 1 h.

Simultaneous measurements of work function and H‒ density including caesiation of a converter surface

“…Furthermore, the lowtemperature hydrogen plasma interacts with the converter surface, resulting in a redistribution of the Cs atoms. Additionally, this interaction can lead to a cleaning effect of the Cs layer performed by plasma particles, like neutral hydrogen atoms and hydrogen positive ions, probably together with the UV light emitted by de-excitation of neutral hydrogen, producing a decrease of the work function [8,9].…”

Efficiency of Cs-free materials for negative ion production in H2 and D2 plasmas

“…Thus, the volume production of H − should not be influenced by the presence of Cs in the plasma and the additionally detected negative ions can be attributed to surface conversion at the caesiated V2A surface. Measurements of the work function under comparable plasma parameters at the laboratory experiment ACCesS have shown values down to 2.1 eV for caesiated surfaces [23][24][25]. Hence, it can be concluded that in this system formation of H − at a surface with 2.1 eV work function leads to an increase of the negative ion density by about a factor of 2.5 in maximum.…”

Work function of Cs-free materials for enhanced H− surface production

“…Determination of the work function of different materials is performed at the laboratory setup ACCesS (planar ICP, ∅ = 15 cm, h = 10 cm, 600 W max.) [5,9,10]. Influence of plasma exposure on the WF is investigated by measuring the WF in short intermediate gas phases (around 3 min) alternating with long plasma exposure times (minutes up to hours).…”

“…However, in addition, Cs is the most reactive alkali metal and its high vapor pressure leads to a high volatility of the surface layer. Hence, under conditions of negative ion sources with non-negligible amounts of impurities from the background gases (vacuum of 10 −6 mbar) and interaction with the low pressure hydrogen plasma in front of the surface, high dynamics including deterioration [5] and redistribution [6] occur. The expected Cs consumption as well as a desired stable and homogenous work function thus drive the question for alternative materials to caesium [7].…”

“…Under hydrogen plasma exposure, WF values around those of a virtually pure caesium layer, i.e. 2.0-2.2 eV, are reached [5]. In order to rate the determined results of alternative materials on the negative ion densities, dedicated measurements of the work function are performed at a separate setup with comparable conditions.…”