Stabilized Arc Discharges as Harsh Environment Transducers

We describe a new type of compact transducer that can be used to initiate and amplitude modulate low-intensity radio-frequency atmospheric pressure plasma (RF-APP) and measure small variations in its electrical properties related to its mechanical structure. The new transducer consists of a quarter wave transmission line where the ground return path is a coaxial solenoid winding. The electrical properties of the plasma are measured at the low-voltage end of the transducer, eliminating the need to measure kV-level voltages near the plasma discharge. We show results from a transducer prototype suitable for use in a harsh-environment as clearance sensor with best sensitivity between 0.1 - 5.0 mm from a conductive object. Application of RF-APP to other NDE disciplines, such as air-coupled ultrasound generation, plasma spectroscopy and gas flow measurement, is also likely to be advantageous, especially in harsh environments where existing approaches are prohibitively expensive or complex.