Studies of Cs dynamics in large ion sources using the CsFlow3D code

Abstract

Large negative ion sources for neutral beam injection rely on surface production of negative ions on a converter surface (plasma grid) in a low temperature plasma environment. To increase the negative ion production efficiency and reduce the amount of co-extracted electrons, the work function of the plasma grid is reduced by using Cs, which is evaporated in the source and redistributed by the plasma on the inner surfaces. In order to avoid a degradation of the plasma grid work function during beam pulses a sufficient flux of Cs onto the plasma grid needs to be maintained, especially for long pulses (several hundred seconds). The Cs flux is influenced by the plasma distribution in the source, which can be affected by a vertical drift due to the magnetic filter field. Simulations of Cs transport performed with the Monte Carlo code CsFlow3D have shown that the plasma drift in ELISE does not lead to an asymmetric distribution of neutral Cs density, as shown also by experimental data from TDLAS (Tunable Diode Laser Absorption Spectroscopy). Different results were obtained for the transport of Cs+ ions, which are affected by the presence of the plasma grid bias potential with the conclusions that Cs+ ions might not have enough energy to overcome the plasma grid bias potential. An alternative configuration of Cs ovens relies on the direct evaporation close to the plasma grid and can help in delivering the required Cs flux for long pulses at ELISE is suggested. Furthermore, CsFlow3D was extended to the full size ITER-NBI source at SPIDER and simulations for long pulses have indicated the evaporation rate needed in this large source and show a strong temporal dynamics of the Cs flux.