Experimental investigation on the discharge modes of a direct-current voltage excited plasma jet in a needle-to-ring geometry

Abstract

Plasma jet can generate a remote non-equilibrium plasma out of its intra-electrode region, which has application potentials in a large variety of fields. Now, it has become a key issue in low-temperature plasma research. It is important to decrease the gas temperature of the plasma plume for industrial applications, especially for biomedical application. In this paper, with ambient air used as working gas, an atmospheric pressure plasma jet in a needle-to-ring electrode geometry is developed to generate a non-thermal plasma plume excited by a direct current voltage, which has a so low temperature that can be touched directly by human body. Through electrical and optical methods, it is found that the discharge can operate in two distinct modes: a pulsed mode or a continuous mode. For the pulsed mode, the discharge frequency increases with increasing the gas flow rate or the power output voltage. For the continuous mode, the voltage-ampere characteristics are investigated for different gas flow rates, which indicate that the discharge is in an abnormal glow regime. Moreover, the gas temperature of the plasma plume is investigated through an optical fiber thermometer. It is found that the gas temperature of the plasma plume increases with increasing the power voltage or decreasing the gas flow rate. Compared with the continuous mode, the pulse mode has a lower gas temperature, so that its plume can be touched directly by a human finger. Therefore, the plasma jet in the pulsed mode is more important for the application in biomedical field. The optical emission spectrum from the two discharge modes is detected to calculate the molecular vibrational temperature. Result indicates that the vibrational temperature as a function of the experimental parameters is similar to the variance of the gas temperature. The physical phenomena mentioned above are discussed and qualitative explanations are given. These results are important for the further investigation of discharge mode and application of direct-current voltage excited plasma jet.