Optimization of caesium dynamics in large and powerful RF sources for negative hydrogen ions

The development of large and powerful RF sources for negative hydrogen and deuterium ions is mandatory for the realization of the Neutral Beam Injection system at ITER. Caesium seeding into negative ion sources is necessary to obtain the required ion current with a tolerable level of co-extracted electrons. The caesium dynamics, during both plasma and vacuum phases, was investigated by means of the Monte Carlo transport code CsFlow3D, which is used to simulate the time evolution of the distribution of neutral and ionic caesium in the IPP prototype RF ion source. Simulations were performed for different durations of plasma-on and plasma-off phases, with the purpose of understanding how the duty cycle influences the caesium distribution and hence the source performance. In order to investigate asymmetry effects in the caesium distribution, caused by the positioning of caesium evaporator, the caesium coverage on the top and on the bottom part of the plasma grid was simulated and data were compared to the caesium density measured by laser absorption in the prototype source. The next step will be to introduce in the code the simulation of diagnostics such as laser absorption spectroscopy and optical emission spectroscopy, in order to achieve a direct benchmark of the code with experimental data.