Ion dynamics in a supersonic jet: Experiments and simulations

The authors suggest a model to simulate the dynamics of ions in a supersonic plasma jet. The model relies on experimental argon ion, Ar(+), energy distribution functions measured by a quadrupole mass spectrometer at different positions along the central axis of a supersonic argon plasma jet. The latter is generated by the pressure difference between two vacuum chambers connected through a converging nozzle: a high-pressure chamber (P ≃ 3.20 Pa), where an inductively coupled argon plasma discharge is maintained, and a lower-pressure one (P ≃ 0.11 Pa), where the plasma jet expands. The model is based on the integration of the equations of motion of a single Ar(+), moving along the supersonic jet in a reference system in which neutral species are at rest. Ar(+)-Ar induced dipole interactions are treated using a 12-4 Lennard-Jones potential. The resulting collisions are considered to be purely elastic, and in addition to them, we allow for charge transfer processes. The energy and position of 1000 Ar(+) were calculated, using an integration time step of 10 ps for ion trajectories ranging from 5 mm to 20 mm from the nozzle, well inside the spatial extension of the supersonic jet. The numerically obtained ion energy distribution functions agree remarkably well with the experimental measurements. From our calculations we can draw conclusions about the energy loss and the mean free paths along the jet. In particular, we can distinguish between processes with and without charge transfer, allowing us to determine the effect of charge exchange phenomena in which the ion changes its nature. The calculated mean free paths were used to evaluate the effective cross sections for momentum transfer and charge transfer collisions, valid for ion energies in the range (0.5-10) eV, in very good agreement with those reported in the literature.