Determining the electric field in a 10-ns pulsed discharge in a CH₄-air mixture using EFISH

Transient plasma ignition (TPI) uses repetitive nanosecond high voltage pulses for combustion ignition, enables extreme lean combustion, and reduces emissions [1]. A recent study on the modes of the discharge during TPI determined that a transient spark mode of the discharge was essential to achieve combustion with a minimal ignition energy [2]. To understand the mode of the discharge, determining the electric field prior to the mode transition is hence critical. In this study, we determine the spatiotemporally resolved electric field in a 10-ns pulsed plasma using a pin-to-pin electrode configuration in the CH₄/air mixture with various equivalence ratios () at atmospheric pressure. The electric field-induced second harmonic generation (EFISH) method was used for the study. After a series of calibrations for both the field strength and the impact of gas composition, the absolute electric field strength ranging from 10 kV/cm to 80 kV/cm was measured for the transient plasma in a lean fuel mixture. The maximum electric field was observed during the rising phase of the voltage pulse and corresponded to the time with 90% of the maximum voltage. During the discharge mode transition, the maximum electric field was found to occur slightly earlier for the richer mixture (e.g., Φ = 0.8) compared to the leaner one (Φ = 0.4). The effects of the discharge mode and the time interval between pulses on the electric field in TPI are discussed.