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.
Highlights
The two neutral beam injection (NBI) systems for ITER (“The Way” in Latin) will be based on the acceleration of negative hydrogen or deuterium ions [1,2,3]
The negative ion density decreases with increasing work function, while the electron density increases, as a consequence of the quasi neutrality of the plasma
The formation of negative hydrogen and deuterium ions in negative ion sources based on surface conversion is strictly depending on the work function of the converter surface
Summary
The two neutral beam injection (NBI) systems for ITER (“The Way” in Latin) will be based on the acceleration of negative hydrogen or deuterium ions [1,2,3]. The corresponding negative hydrogen ion sources must deliver a homogeneous beam for pulses of up to one hour, with an extracted negative ion current density of 329 A/m2 for hydrogen and. 286 A/m2 for deuterium, over an ion source area of 1 × 2 m2 and with a ratio of co-extracted electron to extracted negative ion current density below unity. The ion source must operate at a filling pressure of 0.3 Pa. To fulfill the strict requirements, the negative ion source for ITER relies on the surface conversion of hydrogen particles into negative ions by electron transfer from a low work function surface [4]. In order to reduce its work function—around 4.6 eV for Mo [5]—caesium is evaporated in the source [6], since it is the alkali metal with the lowest work function among all stable elements
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