Optical diagnosis of the N2–O2 ratio effect on mode transition and asymmetric suppression in helicon plasma

The effect of N2–O2 ratio on mode transition and asymmetric suppression behavior in helicon plasma discharge was systematically investigated by using optical emission spectroscopy. N2–O2 ratio ranging from 1:4 to 4:1 was used to simulate the atmospheric composition of ultra-low Earth orbit for air-breathing electric propulsion applications. The results reveal that mode transition power exhibits notable reduction at N2:O2 = 1:4. It was found that N2(A3Σu+) metastable molecules promote O2 dissociation through synergistic energy transfer and lower transition power compared to pure O2 discharge. The suppression effect of N2 on O2 is significantly stronger than that of O2 on N2. The O I 777.4 nm intensity decreases by approximately 75% at N2:O2 = 4:1. In contrast, the N I 746.83 nm band intensity shows a smaller reduction of approximately 55% at N2:O2 = 1:4. This asymmetric suppression is quantitatively linked to the dominant role of N2 vibrational energy loss and competitive chemical consumption of oxygen species. Simplified collision frequency estimations indicate that N2 vibrational losses are likely the dominant electron energy loss channel across the tested ratios. N2:O2 ≈ 2:3 represents an approximate transition region where discharge characteristics shift from O2-dominated to N2-dominated behavior. This is evidenced by inflections in rotational temperature (Tr), vibrational temperature (Tv), and temperature difference (ΔT). These findings provide experimental insights into the orbital adaptability design of air-breathing helicon plasma thrusters.