A significant effect of these materials on the absolute distribution of the vibrationally states has been found above v″ > 3. quartz, tantalum, tungsten, and stainless steel, in order to compare their relative impact in molecular excitation through recombinative desorption from surfaces. This is performed for different plasma-facing materials, i.e. The high resolution VUV Fourier Transform spectrometer of the DESIRS beam line (SOLEIL synchrotron) is applied to directly scrutinize the ro-vibrationally excited levels of the D2 ground state from v″ = 0 to 10 and J″ up to 8 in an electron cyclotron resonance (ECR) cold plasma. These molecules produced both in a plasma volume and on surfaces are no longer in their electronic ground state X 1∑g+(v”⩾0,J”) and populate non-uniformly different vibrational (v″) and rotational levels (J″). Ro-vibrationally excited molecules of deuterium are involved in non-equilibrium chemical reactions in divertor region of tokamak (see molecular assisted recombination) or in neutral beam injector to produce negative ion (see dissociative electron attachment) for fusion plasmas. The difference to caesiation can be attributed to the higher work functions of MoLa and LaB6 which are expected to be about 3 eV for both compared to 2.1 eV of a caesiated surface. In contrast, both MoLa and LaB6 produced systematically higher densities (MoLa: ×1.60 LaB6: ×1.43). Compared to pure volume production, none of the diamond samples did exhibit a measurable increase in H⁻ densities, while showing clear indications of plasma-induced erosion. Negative ion densities were measured above the samples by means of laser photodetachment and compared with two reference cases: pure negative ion volume formation with negative ion densities of about 1 × 10¹⁵m⁻³ and the effect of H⁻ surface production using an in situ caesiated stainless steel sample which yields 2.5 times higher densities. Promising candidates, like low work function materials (molybdenum doped with lanthanum (MoLa) and LaB6), as well as different non-doped and boron-doped diamond samples were investigated in this context at identical and ion source relevant parameters at the laboratory experiment HOMER. In view of a future DEMOnstration power plant, a suitable material should provide a high negative ion formation efficiency and comply with the RAMI issues of the system: reliability, availability, maintainability, inspectability. Inherent drawbacks with the usage of this reactive alkali metal motivate the search for Cs-free alternative materials for neutral beam injection systems in fusion research. Caesium (Cs) is applied in high power negative hydrogen ion sources to reduce a converter surface's work function and thus enabling an efficient negative ion surface formation.
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