Hidden influence of external discharges and plasma expansion on electrical diagnostics and microdischarge behavior in coaxial air dielectric barrier discharges

Abstract

Phenomenon of concurrent external discharges and plasma expansion can occur in practical coaxial dielectric barrier discharge (DBD) systems, yet their impact on electrical diagnostics and discharge behavior remains insufficiently understood. To investigate these effects, a coaxial atmospheric-pressure air DBD operated at 21–27 kV and 21 kHz was studied using both insulated and uninsulated mesh ground electrodes. Lissajous analysis showed that electrode insulation significantly alters equivalent-circuit parameters and power dissipation. In the insulated configuration, the effective dielectric capacitance exceeded the geometrical value at applied voltage above 24 kV, indicating over-discharge and possible virtual electrode formation. The resulting over-discharge capacitance reached 2.1 pF, corresponding to an effective discharge extension of up to 3.6 mm. In contrast, the uninsulated configuration revealed substantial power redistribution to external discharges, limiting the plasma volume within the reactor. At 27 kV, a minimum parasitic power loss of 7.5 W was estimated in the absence of insulation on the ground electrode, corresponding to an approximately 18% increase compared with the insulated configuration. Statistical analysis of 120 current waveforms per operating condition showed that external discharges in the uninsulated configuration strongly influence internal microdischarge behavior. While microdischarge amplitudes remained largely unchanged, their lifetimes decreased by up to 2 ns. Under uninsulated conditions, temporal distributions became strongly polarity-dependent, whereas they remained similar for both polarities in the insulated case. Consistently, the positive-to-negative microdischarge ratio approached symmetry under insulation (0.50 at 27 kV) but remained asymmetric without insulation. Phase-shift analysis further indicated an increased resistive contribution associated with external discharges. These findings demonstrate that external discharges not only dissipate energy but also modulate internal microdischarge dynamics in a phase-dependent manner, providing important guidance for the interpretation of electrical diagnostics and the design of high-efficiency plasma reactors.