Periodical Discharge Regime Transitions Under Repetitive Nanosecond Pulses in N2-O2 Mixtures

Nanosecond repetitively pulsed (NRP) gas discharge plasmas have illustrated unique enabling capabilities in various plasma-assisted applications, such as combustion, ignition, cancer immunotherapy, energy conversion. The discharge regime, a fundamental parameter representing distinct plasma properties, is critical for temperature-sensitive applications, especially for medical-related applications where the spark is intolerable. Conventionally, the discharge regime under repetitive nanosecond pulses consists of three stable types, i.e., corona, glow, and spark, which are classified based on temperature and electron density. However, unexpected periodical transitions of the discharge regime under repetitive nanosecond pulses in N 2 -O 2 mixtures have been experimentally observed. Complicated discharge instabilities are characterized by gradual corona-to-spark transition and repeated spark interruptions. The addition of O 2 dramatically boosts the duration length of the spark regime. The discharge polarity effect is also evident during the periodical transition, although the polarity effect is conventionally regarded as indistinguishable in pulse-periodical discharge due to pronounced memory effect. Discharge feedback mechanisms and residual-conductivity-related screening effects are qualitatively analyzed. In-depth understanding of operation regimes of NRP discharge plasma could be reached. The completion of the operation map by adding unstable discharge regimes are important for designing high-performance plasma reactors.