Caesium in Negative Ion Sources
In the ITER Neutral Beam Injection (NBI) system, negative hydrogen ions (H⁻) are produced in a radio-frequency driven plasma source. The extraction of H⁻ ions is greatly enhanced by the presence of caesium (Cs), an alkali metal with an exceptionally low work function (~2.1 eV). Cs is evaporated from three dedicated ovens and deposited as a thin layer on the plasma-facing surfaces of the source, in particular on the plasma grid (PG). This Cs coating lowers the work function of the surface, dramatically increasing the surface production of H⁻ ions via resonant electron capture from hydrogen atoms, molecules and ions impinging on the grid. The amount of Cs available in the source is controlled by the temperature of the Cs reservoir Tres: higher temperatures produce a larger evaporation flux ṁ, and consequently a higher neutral Cs density nCs in the source volume. When the RF plasma is active, the neutral Cs density increases by a factor (~4 in this example) due to enhanced redistribution and re-evaporation from the walls. A key figure of merit for the source performance is the co-extracted electron-to-negative-ion ratio e⁻/H⁻: a lower ratio indicates a cleaner, more efficient beam, and it decreases with increasing Cs density. The intra-day evolution of γ = e⁻/H⁻ follows a decaying exponential as a function of the mass m evaporated during the day, with parameters that depend on the total cumulated mass M at the start of that day (the correlations used here are for illustrative purposes only). See for instance B. Pouradier Duteil et al. Phys. Plasmas 33, 053107 (2026) and references therein.
Physics Model · Cs flux (per oven): ṁ = 13 × 10(9.171 − 3830/Tres) [mg/h] · Total (3 ovens): ṁtot = 3ṁ · nCs = 0.15×10¹⁴ × ṁtot [m⁻³] · During plasma, the measured neutral caesium can be quite higher, in this example nCs′ = 3 nCs · The electron to ion ratio rapidly improves during the first pulses of the experimental session: γ(m) = A·e−m/B + C ·