The influence of hydrogen and impurities on the work function of multilayer Cs atoms on the plasma grid: An ab-initio molecular dynamics study about negative hydrogen ion sources for neutral beam injection systems

Abstract

We conducted ab-initio molecular dynamics simulations to investigate the migration of hydrogen and impurities (oxygen or copper) and their impact on the work function of cesiated surfaces under different cesium coverages. To ensure temperature and energy stability, all simulations were divided into pre-equilibrium and equilibrium phases, with more rigorous computations performed on equilibrium configurations. Our findings indicate that as the cesium coverage increases, the effects of hydrogen and impurities on the work function gradually diminish. When the cesium coverage reaches 20/16 θ, the work function without hydrogen and impurities is approximately 2.07 eV, and the presence of hydrogen and impurities causes a work function change of less than 0.05 eV. Additionally, when hydrogen and oxygen impurities are bound to the Mo (001) substrate (deposited underneath the cesium layer), the range of electron redistribution induced by hydrogen and impurities is limited, terminating at 1.03 Å above the dense cesium layer. When the cesium layer thickness is greater than dense layer, the electronic density perturbations caused by hydrogen and impurities vanish. The dynamic simulation with hydrogen and impurities initially positioned above the cesium layer reveals that they quickly enter the disordered Cs phase but stabilizes at a specific height, without penetrating the dense cesium layer. Impurities close to the top of disordered Cs phase cause electronic density perturbations that cannot be fully screened in this case. Furthermore, hydrogen and impurities remaining bound to the Mo (001) substrate are more energetically stable. This is attributed to the electron-rich environment below the dense cesium layer, which is particularly advantageous for oxygen impurities with strong electronegativity.