Interpreting the dynamic equilibrium during evaporation in a Caesium environment

Caesium oven prototypes for the prototype source of the ITER neutral beam injectors have been tested in CATS facility. Three types of diagnostics are available to determine the Cs vapour emission rate, the Cs density at various positions in the vacuum chamber and the density integrated over a line of sight. In this paper, we present a model to describe the dynamic equilibrium in the evaporation chamber of CATS during Cs evaporation. The model includes sticking and energy accommodation of the Cs atoms to the walls, calculates the flux density at the surfaces and provides the Cs atom density at the desired locations in the volume. By this model, we correlate the Cs emission with the equilibrium density, measured respectively by Langmuir-Taylor (LT) detector embedded in the Cs oven and by a movable LT detector and by Laser Absorption Spectroscopy (LAS). The experimental data, obtained in a background gas pressure of 10-7 mbar, are well matched with the simulations considering a sticking coefficient of 3%.