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

Abstract Fourteen ions (Ar + , Ar 2 + , ArH + , H 2 O + , H + (H 2 O), H + (H 2 O) 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 + , H 2 O + ) 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 H 2 O + exhibit their maxima at high-energies due to low-energy threshold for three-body collision reactions.