Metal pin-to-dielectric-covered electrode arrangements can be considered as a combination of corona discharge and dielectric barrier discharge. In the current study, the discharge is operated in dry air at atmospheric pressure. Sinusoidal voltage is applied to the dielectric-covered hemispherical electrode, and the metal pin electrode is grounded. Using an ICCD camera and a current probe (Rogowski coil), images and current pulses of single microdischarges (MDs) are recorded simultaneously. In addition, a time-correlated single photon counting technique is used to record the spatio-temporally resolved development of the MDs. The appearance and properties of the MDs in the two polarities of the applied voltage differ significantly. In this contribution, only the results of the negative half-cycle, i.e. anodic pin, are presented. In this half-cycle, for the voltage amplitude being applied, two MDs appear in each applied voltage cycle. The first MDs leave a positive charge on the surface of the dielectric, which has considerable influence on the propagation and properties of the second MDs, namely slower propagation of the cathode directed streamer in the dielectric vicinity, further expansion of the plasma on the dielectric surface and longer presence of the bulk plasma in the gap.

We investigate single microdischarges (MDs) in a sinusoidally operated barrier corona discharge in air. For the voltage amplitude being applied, two subsequent MDs appear in the anodic pin half-cycle. The developments of these subsequent MDs were studied and presented in detail in a previous contribution [Jahanbakhsh et al., Plasma Sources Sci. Technol. 27, 115011 (2018)]. In the present study, the reduced electric field strength (E/n) values of the MDs are determined. In addition, the current pulses are synchronized, with a subnanosecond time resolution, to the spatiotemporally resolved light emission and E/n development of the MDs. It is proposed that the current pulse derivative maximum corresponds to the streamer head arrival on the cathode surface. Therefore, the derivatives of the current pulses are used to synchronize the light emission and current measurements. Based on this synchronization, spatiotemporally resolved light emissions at different positions are compared to the averaged current pulses. Considering the observed correlations, it is proposed that after the arrival of the streamer head on the dielectric (cathode) surface and bulk plasma formation, the ionization processes near the dielectric surface are the dominant source of electron current production. The determination of the E/n is based on the analysis of the time-correlated single photon counting results for the molecular states of the first negative and the second positive systems of nitrogen. The E/n increases during the streamer propagation in the gap, reaching its maximum value at the impact of the streamer on the cathode. The E/n values for the second group MDs are lower only in the vicinity of the dielectric surface, which can be attributed to the positive residual surface charges from the first group MDs.

Microdischarges (MDs) in a sinusoidally driven barrier corona discharge, operated in dry air at atmospheric pressure, are investigated using time-correlated single photon counting (TC-SPC) technique, electrical measurements (applied voltage and current pulses) and ICCD images. In the current contribution, results of the cathodic pin half-cycle are presented. Results of the anodic pin half-cycle were presented in a previous study (Jahanbakhsh et al 2018 Plasma Sources Sci. Technol. 27 115011). In the cathodic pin half-cycle appearance phase and current pulse amplitude of the MDs have an erratic behavior. Hence, a statistical study of the current pulse amplitudes is conducted, and it is correlated to the phase-resolved spatio-temporal development of the MDs, obtained from TC-SPC recordings. This study indicates that the breakdown and development of the MDs is dependent on their inception phase, which is related to the discharge activity in preceding (anodic pin) halfcycles. It is shown that the MDs appearing at lower applied voltages are ignited with a positive streamer starting near the anode (dielectric), similar to single dielectric barrier discharge MDs. On the other hand, the inception of the MDs appearing at higher applied voltages takes place with a double-streamer mechanism starting near the tip of the cathode (pin). These MDs have similarities to transient sparks, in particular much higher current pulse amplitudes compared to single DBDs.

Plasma cathodes are insert free devices that are developed to be employed as electron sources in electric propulsion and ion source applications as practical alternatives to more commonly used hollow cathodes. Inductively coupled plasma cathodes, or Radio Frequency (RF) plasma cathodes, are introduced in recent years. Because of its compact geometry, and simple and efficient plasma generation, RF plasma source is considered to be suitable for plasma cathode applications. In this study, numerous RF plasma cathodes have been designed and manufactured. Experimental measurements have been conducted to study the effects of geometric and operational parameters. Experimental results of this study show that the plasma generation and electron extraction characteristics of the RF plasma cathode device strongly depend on the geometric parameters such as chamber diameter, chamber length, orifice diameter, orifice length, as well as the operational parameters such as RF power and gas mass flow rate.

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