One of the driving forces behind the development of cold plasma sources at atmospheric pressure is an application in the biomedical field. In this respect, the radio-frequency (RF) plasma jets are of particular importance due to possible safe operation on humans and a generation of the high amount of reactive species. For this reason, we designed RF plasma jet in co-axial geometry with the possibility of aerosol introduction, where its characteristics were evaluated by electrical diagnostics, optical emission, and laser scattering spectroscopy. The RF plasma jet operation and stability of diffuse mode were analysed based on energy balance. It was observed that mode diffuse discharge characterized by effluent length up to 5 mm was sustained at power density below 30 W/cm 3 . The gas and rotational temperature were determined by means of spectroscopy methods and compared with results of direct laser scattering. It was established that gas temperature obtained from N2 emission of transition C 3 ПuB 3 Пg (0,2) is highly overestimated whereas the gas temperature estimated from OH transition A 2 Σ+X 2 Пi (0,0) gave a reasonable agreement with both Rayleigh and Raman spectroscopy. Based on Rayleigh scattering method uniform gas temperature distribution in the discharge effluent was found at power below 15 W with average temperature below 340±15 K. The low gas temperature of Ar plasma jet allows using this source in temperature sensitive material applications including skin treatments.
The time behavior of an atmospheric pressure planar discharge sustained in He gas was investigated experimentally and through two dimensional (2D) discharge simulation. The 30 mm long uniform α-mode discharge was observed at radio frequency (RF) input power below 35 W. The gas temperature of 375 ± 50 K in the discharge core was estimated by emission spectroscopy of OH(A-X) emission. A sheath region of about 100-150 μm width near both electrodes was observed during the whole RF cycle. However, there were differences in emission dynamics among various species detected in the discharge. OH(A) emission does not follow the RF voltage temporal variation. Strong He emission was always detected near the cathode, which was consistent with the 2D discharge simulation results. He-excited species production was found mainly due to the electron impact process. The simulation showed that both the electron and ion density vary from 1.88 × 10 17 m −3 to 1.92 × 10 17 m −3 , and the electron temperature was about 1.85 eV in the plasma bulk. The ion temperature stayed close to the rotational temperature of OH radicals, and only increased near the sheath region to 0.65 eV. It was found that the mechanism of the sheath formation in atmospheric pressure discharge strongly correlates with the dynamics of the electron density and electron temperature variation in the gap, and the process is similar to low pressure RF capacitively coupled discharges. The high uniformity of the discharge and the upscale possibility to any desirable size are considered beneficial for industrial applications of the source, which is key for processes of thin coating deposition and polymer modification.
Atmospheric pressure radio frequency helium plasma with two different designs: dielectric barrier discharge (DBD) and discharge with bare electrode (DBE) were investigated by means of optical emission spectroscopy. Both DBD and DBE can work at relatively low temperature and produce abundant electrons facilitating production of reactive species through electron-impact reactions. Stark broadening method of Hydrogen Balmer beta (Hβ) line was employed to analyze the electron density. When electron density is below 10 20 m-3 , fine-structure fitting was used to improve the accuracy of electron density estimation. At power ranged 4-20 W, DBD and DBE showed electron density 4.1-6.1 × 10 19 m −3 , and 3.6-8.6 × 10 19 m −3 , respectively. The DBD is more suitable than DBE for biomedical applications due to the wider working power range and lower gas temperature in the range of 316-344 K, depending on the power.
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