Measurement of spatio-temporal Ar metastable atom density and emission spectrum in an Ar low-frequency microplasma jet

Summary form only given: Recently low-frequency (LF: 10-100 kHz) dielectric-barrier-discharge (DBD)-driven micro-plasma jet proposed by Teschke et al. is attracting interests. In this plasma jet, emission from the jet is not continuous: bright part like a "bullet" propagates along the plasma jet and its velocity is about 1000 times larger than that of gas flow. In our previous work, the authors observed same phenomenon and reported that the emission of N₂ 2nd positive band (SPB) is enhanced towards the downstream, which would be attributed to the excitation energy transfer from argon metastable level (Ar^m) to N₂ C state (upper state of the SPB). However no direct measurement of metastable atom behavior has been done. Consequently, spatial distribution of metastable atom density of Ar^m (1s₅) was measured and the relationship between its behavior and emission characteristics was studied. Thin Pylex glass tube with inside diameter of 1.25 mm was used for the nozzle and Ar gas was flowed out with flow rate of 1 to 30 L/min. Ar plasma was initiated at the end of the nozzle with DBD using nickel wire electrodes wound on the outside of the nozzle tube with distance between anode and cathode of 10 mm. Applied voltage waveform was rectangular and the typical frequency was 90 kHz and peak voltage was 1.9 kV. Visible length of the plasma jet was about 1 to 2 cm. Ar^m density was calculated from measured absorbance values of IR laser irradiation (811.53 nm: 1s₅-2p₉). Under the typical condition of 2 L/min, 90 kHz, the laser absorption signal was observed at the fall and the rise edges of the applied voltage within a range of 0 to 2 mm (fall edge) or 7 mm (rise edge) from the nozzle tube end (position of 0 mm) towards downstream. It is shown that dense spot of metastable atom propagates at a speed of a few to dozens km/s, which is about same speed of "bullet" propagation confirmed by time- re- olved ICCD images. Moreover, ICCD images of UV emission (mainly N2 SPB) and Ar 811.53 nm emission (1s₅-2p₉) were observed separately using a silica lens and an interference filter. Both UV and Ar have intense emissions at the front edge of the jet (at the "bullet"). On the other hand, at the region between the "bullet" and the nozzle, a few weak striations were observed and their intensity ratio of Ar to UV was larger compared with that for the "bullet". Analysis of the relationship between these emission behavior and Ar metastable distribution is under progress.