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Diagnostics of atmospheric pressure plasma jet by cavity ring-down spectroscopy

Pulsed cavity ring-down spectroscopy (CRDS), as a direct absorption, highly sensitive, versatile technique has been applied for diagnostic of atmospheric pressure plasma jet (APPJ). The emphasis is given on temporal resolution and pulsed character of plasma sources. In conjunction to our previous work [1, 2] we have studied the influence of various target materials on the content of the plasma, namely concentrations of the helium metastables in the 2s 3S1 state. The APPJ electrode was a copper wire with a diameter of 0.1 mm inserted inside of a 50 mm long and 1.5 mm outer diameter borosilicate glass capillary tube [1]. For the purpose of the present experiment power source has been modified. Single electrode APPJ was connected to a direct current (DC) power supply through high voltage (HV) switch. The HV switch was triggered with function generator, with a 50 % duty cycle square signal at a repetition rate of 10 kHz. The helium flow (purity 99.996%) of 2 l/min was controlled with mass flow controller. The CRDS measurements were performed with APPJ placed in a centre of an 83 cm long cavity with cavity mirrors of 99.95 % reflectivity and mirror radius of curvature of 1 m. Dye laser used for CRDS was operating with BiBuQ (LC 3860) laser dye and was pumped with ns excimer laser. The laser beam waist was around 1 mm. Photomultiplier tube was used to detect ring-down signal, which was sent over a 8-bit digital oscilloscope to a computer. The HV pulses used for APPJ discharge need to be synchronized with the laser pulses used for CRDS. In order to achieve this, the delay between trigger signals for excimer laser and for function generator was controlled with delay generator. Spatially and temporally resolved measured He(3S1) number densities ranged from 6.5·1011 cm-3 near the capillary orifice to 5·1010 cm-3 at 3.5 mm downstream. When treating samples with APPJ, He(3S1) number densities were up to 10 times higher. Application of method to various APPJ applications will be discussed [3]. [1] R. Zaplotnik, Z. Kregar, M. Bišćan, A.Vesel, U. Cvelbar, M. Mozetič, S. Milošević, EPL, 106 2500 (2014). [2] R. Zaplotnik, M. Bišćan, Z. Kregar, U. Cvelbar, M. Mozetič, S. Milošević, Spectrochemica Acta B, 103, 124-130 (2015). [3] A. Režek Jambrak, T. Vukušić, V. Stulić, J. Mrvčić, S. Milošević, M. Šimunek, Z. Herceg, Food and Bioprocess Technology: An International Journal, 8(4), 791-800 (2015).

cavity ring-down spectroscopy ; atmospheric pressure plasma jet ; cold plasma

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