Discharge mechanisms of a pulsed-modulated microwave plasma jet investigated by time-resolved emission spectroscopy

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Abstract This study employs a λ /4 coaxial resonant cavity to generate a pulsed-modulated microwave plasma jet and compares discharge behaviors in N 2 , N 2 H 2 and Ar systems. Time-resolved optical emission spectroscopy is combined with incident and reflected microwave power measurements and phase diagrams to diagnose the ignition and decay processes. The results show that a stable localized enhanced electric field forms at the needle tip, enabling reliable discharge. Power signals and phase diagrams indicate stable microwave-plasma coupling, while the power-balance analysis at 60% duty cycle shows only minor differences in absorbed power and coupling efficiency among the three gas systems. Time-resolved spectroscopy reveals distinct temporal evolution among the three gases. At 25 kHz and 80% duty cycle, the N 2 and N 2 /H 2 emissions reach their observed maxima within the first 0.2 μ s acquisition interval and relax toward quasi-steady levels within 2.6 and 2.2 μ s, respectively. Their detectable afterglow durations are both 0.4 μ s. The Ar emission reaches its first transient maximum after 1.2 μ s and remains detectable for 3.2 μ s after microwave termination. A simplified CR framework relates these characteristic intervals to the competition between effective excitation and loss processes. The slightly shorter N 2 /H 2 relaxation is consistent with an additional H 2 -induced collisional-loss contribution, whereas the prolonged Ar afterglow supports the involvement of long-lived metastable species and residual stepwise excitation. The results indicate that gas-dependent kinetic processes contribute substantially to the observed emission dynamics under matched pulse-modulation and incident-power settings.

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