Negative Hydrogen and Deuterium Ion Density in a Low Pressure Plasma in Front of a Converter Surface at Different Work Functions

Abstract

Negative ion sources of neutral beam injection (NBI) systems for future fusion devices like ITER (“The Way” in Latin) rely on the surface conversion of hydrogen (or deuterium) atoms and positive ions to negative ions in an inductively coupled plasma (ICP). The efficiency of this process depends on the work function of the converter surface. By introducing caesium into the ion source the work function decreases, enhancing the negative ion yield. In order to study the isotope effect on the negative ion density at different work functions, fundamental investigations are performed in a planar ICP laboratory experiment where the work function and the negative ion density in front of a sample can be simultaneously and absolutely determined. For work functions above 2.7 eV, the main contribution to the negative hydrogen ion density is solely due to volume formation, which can be modeled via the rate balance model YACORA H−, while below 2.7 eV the surface conversion become significant and the negative ion density increases. For a work function of 2.1 eV (bulk Cs), the H− density increases by at least a factor of 2.8 with respect to a non-caesiated surface. With a deuterium plasma, the D− density measured at 2.1 eV is a factor of 2.5 higher with respect to a non-caesiated surface, reaching densities of surface produced negative ions comparable to the hydrogen case.