Time-resolved mass spectroscopic studies of an atmospheric-pressure helium microplasma jet

Jun-Seok Oh (1) , Yolanda Aranda-Gonzalvo (2) , James W Bradley (1)

1 Department of Electrical Engineering and Electronics
2 Hiden Analytical Ltd

Using molecular beam mass spectroscopy, time-resolved measurements of the ionic species 12 in the plasma plume of an atmospheric-pressure helium microplasma jet have been made for 13 a range of excitation frequencies (5, 10 and 25 kHz) and source-instruments orifice distances 14 (1, 7 and 11 mm). Ionic species can only be observed in the visible plasma plume, with the 15 main positive species being N 2 + (65.26%) and O 2 + (21.11%), and few percentages of N +, O +, 16 NO + and He +. For the negative ions, the majority species are O 3 - (22.68 %), O 2-(H 2 O) 17 (10.49 %) and a large range of minority species observed namely, O n-, OH n-, (H 2 O) n-, CO n- 18 and clusters, O m - (H 2 O) n, (OH) m - (H 2 O) n, O n(CO 3) and CO 3-(H 2 O) n. The flux of ions created 19 from air species such, NO + and O 3-, are seen to be maximized a distance of several mm from 20 the nozzle, whereas the He + concentration continually decreases with distance from the exit 21 orifice. The time-resolved measurements (time-resolution down to 2 µs) show positive ions 22 appear twice in one full period of the voltage waveform, correlated directly with positive and 23 negative current peaks. The rise and fall times of the positive ions are typically tens of µs. In 24 contrast, the appearance of negative ions is correlated only with the negative part of discharge 25 current, with one main peak in the detected ionic flux seen per cycle. The rise time of the 26 negative ions is about 10 µs, independent of mass, however we observed longer decay times 27 from 100 to 150 µs increasing with mass. With increased driving frequency, the time 28 2 modulation in the ionic fluxes is reduced, particularly for the negative species that show 1 almost constant fluxes at 25 kHz throughout the cycle. The observations can be understood 2 through a simple picture of the interaction of the He jet and the moist ambient air. The results 3 indicate that the discrete plasma " bullets " and their afterglow tail, that forms the jet, carry an 4 associated positive or negative current depending on the time of their creation in the voltage 5 cycle. 6 7 3

Mots clés: Physical Sciences

Jun-Seok Oh, Yolanda Aranda-Gonzalvo, James W Bradley. Time-resolved mass spectroscopic studies of an atmospheric-pressure helium microplasma jet. Journal of Physics D: Applied Physics, 2011, 44 (36), pp.365202. ⟨10.1088/0022-3727/44/36/365202⟩. ⟨hal-00651638⟩