Glow discharge characteristics of non-thermal microplasmas at above atmospheric pressure

Abstract

Normal glow discharges in micrometer scales are being widely studied for a variety of ultrafine processing technologies. DC glow discharge characteristics were investigated at an atmospheric pressure in the range of 0.1–1.65 MPa. The operational discharge characteristics such as voltage–current (V–I) measurement, discharge size, gas temperature, vibrational, and rotational temperatures were obtained for helium (He) and nitrogen (N2) gases. Optical emission spectroscopy studies were carried out to measure rotational and vibrational temperatures using the nitrogen second positive system. Discharge above atmospheric pressure was characterized for the current range of 0.1–1.2 mA and an electrode gap of 5–250 μm. This paper presents evidence for a discharge size as small as 7 μm in N2 and 16.7 μm in He, while operating at a low current of 0.5 mA and at high pressures of 1.65 and 0.34 MPa, respectively. To the best of our knowledge, this is the first time an experimentally obtained normalized current density corrected with effective pressure has been estimated to be close to an analytical value (400 μA cm−2 Torr−2) for N2 using a stainless-steel cathode at above atmospheric pressure. The observation of the breakdown of two different gases (N2 and He), the V–I characteristics, and high current densities indicate that microplasma exhibits a stable glow discharge when a low capacitance external circuit is used. Rotational and vibrational temperatures are obtained in the ranges of 1186–2894 K and 4208–3608 K, respectively, for the pressure range of 0.10–1.65 MPa in N2 gas for a constant discharge current of 0.5 mA and an electrode gap of 150 μm. It is also notable that the transition from non-equilibrium to equilibrium discharge in both rotational and vibrational temperatures was observed in N2 at 0.86 MPa, without the transition to thermal plasma (arc discharge).