Previous studies of the gliding arc (GA) plasma have shown that the highest degree of non-equilibrium is obtained at the maximum length of elongation of the discharge. In order to better understand the properties of the discharge in this mode of operation, a system was developed wherein the GA is stabilized by a magnetic field. The GA is driven as a single unit in the direction governed by the Lorentz force. The discharge reactor is designed in such a way that the plasma moves continuously without further elongation or extinction, while electrical parameters can be carefully controlled. This paper presents the experimentally measured effects of plasma current on (a) the motion of plasma, (b) the presence of an 'overshooting' regime, (c) voltage characteristics and (d) rotational and vibrational temperatures, when operating in air at atmospheric pressure.
A magnetic field is used to stabilize a gliding plasma discharge for controlled study purposes. The design, development, voltage-current characteristics, rotational and vibrational temperature measurements and the presence of an 'overshooting' regime in the magnetically stabilized gliding arc discharge (MSGAD) were discussed in part I (Gangoli et al 2010 Plasma Sources Sci. Technol. 19 065003). This paper deals with further experimental quantification of the dimensional features, current densities and power dissipated per unit length in the MSGAD. Estimations are made of important plasma properties, such as electric field, average electron energies and densities, that provide insight into the non-equilibrium 'glow-like' nature of the MSGAD. Visual observations of the MSGAD when operated in helium at atmospheric pressure clearly show physical features similar to a non-equilibrium glow discharge.
The stability and uniformity of a radio-frequency (RF) discharge is limited by a critical power density. Beyond this critical power density, instability occurs in the form of physical changes in the plasma (such as contraction due to arcing). The RF discharge used in this study is the non-equilibrium Atmospheric Pressure Plasma Jet (APPJ ® ) developed by Apjet, Inc. This discharge is known to operate uniformly in helium gas. However, for some proposed applications such as surface modification, there is a need to operate with reactive gases such as O 2 . Our experimental studies show that addition of molecular gas to a discharge operating in helium increases its power density (W cm −2 ), until it reaches the critical unstable arcing limit. Moreover, an increase in the frequency of operation (from 13 to 27 MHz) allows the plasma to sustain higher molecular gas concentrations and power densities before instability occurs. Further, it is observed that this critical power density is dependent on the type of molecular gas added. These results provide a motivation for the development of a mathematical model that can provide insight into the causes of instability and potential methods of suppression. The two commonly studied modes of instability are (1) thermal instability (TI) and (2) α-γ -arc mode transition. For the APPJ ® discharge conditions, the development time scales of TI are much longer (∼1 ms) as compared with discharge oscillation period (∼100 ns). Hence, if the instability was indeed thermal, discharge frequency increase would have no consequence, contrary to experimental findings. A 1D fluid model based on the local field approximation is developed to study instability in APPJ ® discharge. The analysis of modeling results confirmed our hypothesis that the instability development actually takes place via breakdown of sheath i.e. α-γ -arc mode transition and not by TI.
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