Optical Observation of Emission Spectra from a Hybrid Plasma Triggered by a Shunting Arc Discharge using a Positively Biased Electrode

Abstract

A hybrid plasma consisting of metallic and gaseous species is produced in a simple way using a shunting arc discharge. A gaseous plasma is triggered by a pulsed carbon shunting arc discharge. A positively biased electrode (80-mm diameter) is set at a distance of 100 mm from the shunting arc plasma source. Methane, nitrogen, or argon gas is used as an ambient gas at a pressure of up to 150 Pa. The plasma species, both from the carbon shunting arc and from the induced gaseous plasma, are mixed to form a hybrid plasma, that is, the shunting arc discharge plays not only the role of a triggering source for generating the gaseous plasma, but also that of a supplier of carbon species. This system has the following features: (1) The metallic and gaseous plasma species are easily mixed in activated states (ionization, excitation, etc.) in a large volume; (2) The induced gaseous plasma is generated over a wide range of pressures from 0.05 to 140 Pa. Feature (2) is favorable for reactive deposition of compound films under an appropriate gas condition. The mixing of the different species is confirmed by the observation of the emission spectra. When argon is used as an ambient gas, the induced hybrid plasma includes argon and carbon-related species. The emission spectra are typically 434.5 nm (Ar I), 440.1 nm (Ar II), and 426.7 nm (C II). These are simultaneously observed at the same position. In the methane gas environment, carbon and CH spectra are commonly observed. Carbon ions (CII) with a wavelength of 426.7 nm and CH molecules with a wavelength of 431.4 nm are mainly observed in the visible light region. The spectral intensity increases in the presence of methane gas as compared to the case where methane is absent. It is found that the plasma structure changes by the ambient gas pressure. In a pressure region as low as 1 Pa, a glow-like hybrid plasma is uniformly generated, while the glow-like plasma is transferred to a beam-like plasma at pressures higher than 1 Pa. The transferring is due to a sudden change of the space potential brought by termination of the shunting arc discharge. The intensity of the emission spectrum increases with the current of the glow-like plasma. In the beam-like plasma, the life of the emission at the surface of the positively biased electrode is shorter than that of the beam-like plasma current, which suggests a strong flow of the plasma.