Properties of surface arc discharge in a supersonic airflow

An experimental study of a direct-current, surface arc discharge in a Mach 2 cold supersonic airflow is presented. The surface arc discharge is generated with cylindrical tungsten electrodes flush-mounted on a boron-nitride ceramic plate embedded in the lower wall of the supersonic test section. In the presence of airflow, gas breakdown voltage increases from 1.5 kV in stationary air to 2 kV due to particle number density augmentation in the flow. The surface arc discharge transforms from a continuous mode in stationary air to a pulsed-repetitive mode in the flow. The mean time interval between discharge pulses is about 4.3 ms. For a single pulse, arc discharge occupies only about 60 mu s. The discharge photos taken by a high-speed CCD camera (framing rate 1125 Hz) validate this pulsed-repetitive process and indicate that the plasma channel of the surface arc discharge is blown downstream by the supersonic flow. As the length of the plasma channel increases, the discharge voltage also increases. When the channel length reaches a critical value (similar to 25 mm), the dc power supply (3 kV-4kW) cannot sustain the discharge voltage (similar to 3 kV) and the Joule heating energy cannot balance the dissipation of constrained convection, and hence the discharge quenches immediately. Current and voltage measurements demonstrate that the discharge process in a single pulse can be separated into three distinct phases: strong-pulsed breakdown process, steady discharge process and discharge attenuation process. Finally, the underlying mechanism of the dynamic process of surface arc discharge in supersonic flow is discussed. This paper provides more insights into the mechanism of supersonic flow control (in particular, shock waves) by a surface arc discharge.