The crucial role of molecular ions in the radial contraction of argon microwave-sustained plasma jets at atmospheric pressure

Fourteen ions (Ar+, Ar 2 + , ArH+, H2O+, H+(H2O), H+(H2O)2, O+, O 2 + , OH+, NO+, N 2 + , N+, Ar 2 + , N 4 + ) were detected, by mass-resolved ion-energy measurements when the discharge was operated at 50 W and fluxes of 2.5 and 5.0 slm. The crucial role of three molecular ions ( Ar 2 + , ArH+, H2O+) during the radial contraction, from diffuse (r = (750 ± 50) μm) to contracted plasma jet (r = (500 ± 50) μm), and the relationship between power, gas flow and molecular ions production were pointed out. Ion energy distributions for Ar 2 + , ArH+ and H2O+ exhibit their maxima at high-energies due to low-energy threshold for three-body collision reactions.