Theoretical determination of the microstructure of Cs covering of Mo in negative ion sources for nuclear fusion applications

Abstract

Cs is the most well known catalyst used in negative ion sources for fast neutral beam generation employed in nuclear fusion, where the element is evaporated and deposited on Mo surfaces forming non permanent films. In this paper the interaction of Cs with Mo under conditions of interest for negative ion sources is studied using different methods. Cs-Mo potential has been characterized starting from high level electronic calculations for two atoms. Mo-Mo and Mo-Cs potentials are based on new fits of the literature data. Density functional theory calculations on a reduced cell are used to determine the adsorption energy of Cs on Mo for different sites. Good reproduction of experimental results, when available, is achieved (e.g. Mo crystal data, Cs 2 dissociation energy) and new results for the evaporation energy of Cs from Mo surfaces, CsMo dissociation energy, adatom geometry etc. are reported and tabulated. A functional expression of the Cs-Mo[0 0 1] interaction potential is proposed based on these ab-initio results. The use of this potential is illustrated by classical MD calculations for the morphology for Cs partial layers on Mo[0 0 1]. Calculations show that the interaction between Cs and the surface leads to peculiar morphology of Cs partial layers, to be considered in future studies of Cs role in negative ion sources as well as in the ongoing quest to alternative catalyzers.

Stochastic models of systems for Nanotechnology: from micro to macro scale

“…The geometry calculated for a half-monolayer of cesium on molybdenum h w g h w g f " f " g f h b ff . g f hy Fusion vol.57, no.3, March 2015 [Damone, 2015].…”

“…An example of application, where a multiscale approach was adopted, is the production of Cs/Mo bimetallic surfaces as catalyst in the Neutral Drive concept for the ITER nuclear fusion experiment (Dudnikov 1992, Taccogna et al 2008. The macroscopic models for this system, employing Monte Carlo models of the plasma phase and transport processes, were also supported by rigorous ab initio simulations by methods of quantum chemistry (Damone et al 2015): Cs atoms were launched on a pristine 001 Mo substrate, which allowed to study the partially ordered geometry of a molybdenum surface covered by half a cesium monolayer. This appears to be the most efficient catalyst system for this application, thereby suggesting that the 'random nanostructure' of a partial Cs coverage, leaving part of the substrate metal visible to hydrogen molecules, is beneficial to get the highest reaction yield.…”

Isotopic effect for hydrogen atoms interacting with a cesiated surface

“…The method consists mainly of three different operational steps: (1) determination of the 3D surface model structure and evaluation of the corresponding phonon dynamics; (2) building up of the PES for the reaction under study obtained by ab initio calculations in the framework of the density functional theory (DFT) assuring an accurate description, at the atomistic level, of the interaction under study; (3) propagation of a sufficiently large number of classical trajectories using the semiclassical collisional method [20]. The 3D surface model is that obtained in [8], for which we determined the phonon dynamics by solving the time-dependent Schrödinger equations of motion under the harmonic oscillator approximation. In other words, we assume that a set of M=3N-6 (N is the number of atoms in the 3D surface model) independent harmonic oscillators are perturbed by a linear force exerted between the species approaching the surface from the gas-phase and the solid substrate [9].…”

“…In two recent papers, we determined the work function for a new cesiated surface model proposed in the literature [8] and we presented results on the scattering of hydrogen atoms with a negative charge [9] and the desorption of roto-vibrationally excited H 2 molecules [10] from this surface.…”

Adsorption of H on Cs/W(110): Impact of H on the Stability of Cs on the Surface

“…The electropositive nature of the alkali metal adsorbates results in an induced surface dipole moment, which in turn results in a low work function material [4]. Several notable previous works, both experimental and theoretical, have reported metal surface work function modifications with alkali metal adsorption [5][6][7][8][9][10][11].…”