Understanding glow-to-arc transition in microwave-enhanced spark discharge through comprehensive plasma diagnostics

Abstract The microwave-enhanced spark discharge (MESD) has gained significant attention for its applications in emission spectrometry and ignition enhancement. However, the underlying mechanisms governing the transition between discharge regimes and the interaction between metals (from electrode material) and microwave energy remain inadequately understood. This study comprehensively investigates a MESD through power diagnostics, schlieren imaging, and optical emission spectroscopy (OES) under various pressure conditions, focusing on plasma properties in both glow and arc discharge regimes. The experimental results reveal a distinct transition from glow to arc discharge with increasing microwave energy input. In the glow regime, the plasma exhibits diffusive behavior with pressure-dependent energy absorption, while the arc regime demonstrates bright plasma with pressure-independent energy absorption. There is a leap in energy absorption during the glow-to-arc transition. OES confirms that gas excitation governs the glow regime, whereas metal ionization dominates the arc regime. The rotational temperature of N 2 (C–B) serves as a reliable gas temperature indicator, showing a significant surge to approximately 8000 K during the transition, with the arc plasma achieving maximum heating efficiencies exceeding 90% under atmospheric pressure. Furthermore, while the electron temperature remains relatively stable (∼0.7 eV), the electron number density increases substantially with absorbed energy in the arc regime. The overall order of magnitude of electron number density is approximately 10 22 m −3 . The transition mechanism is attributed to enhanced electron generation through secondary electron emission and thermionic emission from electrode materials with the aid of microwave field. The additional electron production creates a self-reinforcing feedback loop, sustaining the arc discharge by supporting metal evaporation and microwave heating.