Measurement of the electron density in Transient Spark discharge

This paper presents our measurements of the electron density in a streamer-to-spark transition discharge, which is named transient spark (TS), in atmospheric pressure air. Despite the dc applied voltage, TS has a pulsed character with short (similar to 10-100 ns) high current (>1A) pulses, with a repetition frequency on the order of kHz. The electron density n(e) similar to 10(17) cm(-3) at maximum is reached in TS with repetition frequencies below similar to 3 kHz, using relatively low power delivered to the plasma (0.2-3 W). The temporal evolution of n(e) was estimated from the resistance of the plasma discharge, which was obtained by a detailed analysis of the electric circuit representing the TS and the discharge diameter measurements using a fast intensified charge-coupled device (iCCD) camera. This estimate was compared with ne calculated from the measured Stark broadening of several atomic lines: H-alpha, N at 746 nm, and O triplet at 777 nm. Good agreement was obtained, although the method based on the plasma resistance is sensitive to an accurate determination of the discharge diameter. We have found that this method is also limited for strongly ionized plasmas. On the other hand, a lower n(e) detection limit can be obtained by this method than from the Stark broadening of atomic lines.