engleski

Helium metastable density within atmospheric pressure plasma jet during treatment of various samples

The content of an atmospheric pressure plasma jets (APPJs) is being investigated due to large area of applications in plasma medicine, biotechnologies and materials modifications [1]. In conjunction to our previous work [2, 3], where optical emission spectroscopy was used as a diagnostics tool, here 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. For this purpose a pulsed cavity ring-down spectroscopy (CRDS), as a direct absorption, highly sensitive, versatile technique was applied for absorption measurements at 388 nm transition in helium atom. 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 [2]. For the purpose of the present experiment the 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 a 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 [4]. When treating various samples with APPJ, He(3S1) number densities were up to 10 times higher and their distribution changes depending on distance of APPJ orifice to surface, its dielectric constant and its surface area. [1] S. A. Norberg, E. Johnsen, M. J. Kushner, Journal of applied physics, 118, 013301 (2015) [2] R. Zaplotnik, Z. Kregar, M. Bišćan, A.Vesel, U. Cvelbar, M. Mozetič, S. Milošević, EPL, 106, 2500 (2014). [3] R. Zaplotnik, M. Bišćan, Z. Kregar, U. Cvelbar, M. Mozetič, S. Milošević, Spectrochemica Acta B, 103, 124-130 (2015). [4] R. Zaplotnik, M. Bišćan, N. Krstulović, D. Popović, S. Milošević, Plasma Sources Science and Technology, 2015 , in press

helium metastable density ; atmospheric plasma jet ; cavity ringdown spectroscopy

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