## 3 Results and Discussion
### Morphological Features of LS-APPJ  
Figure 3 shows the discharge images of the BPPJ, PPPJ, and NPPJ. The pulse peak voltage, pulse repetition rate, and gas flow rate are kept at 24 kV, 150 Hz, and 3.5 L/min, respectively. Since a large-diameter plasma jet tube, with a length of up to 35 cm, is employed and no electrode is placed inside, the gas flow remains very smooth, and the turbulence is effectively controlled. Therefore, the length of the plasma jet mainly depends  
Figure 2: Waveforms of bipolar voltage and unipolar voltage: (**a**) positive pulse in BPPJ and PPPJ; (**b**) negative pulse in BPPJ and NPPJ. on the electrical field density at the plasma jet head [17]. When the positive pulse voltage is employed, the PPPJ has a length of about 10 cm with a pulse peak voltage of 24 kV, which already reaches the maximum jet length at 7 L/min reported in ref. [22]. Compared with the PPPJ, the NPPJ has a length of about 7 cm because of the high applied voltage. When a bipolar pulse voltage is employed as an excitation voltage, the plasma jet with a diameter of about 1.2 cm can propagate over 14 cm (excluding the streamer filament in the trail of the plasma jet, as shown in Figure 3). It exhibits a higher discharge intensity and a longer jet length compared with the plasma jet excited by positive pulse voltage and negative pulse voltage. The BPPJ presents a stable and uniform profile, and no turbulence is formed at the trail of the plasma jet, while the jet tube's length significantly influences the length of the plasma jet. The corresponding comparison images are shown in Figure 4, where the BPPJs generated with the jet tube lengths of 35 cm and 10 cm are displayed on the left side and right side, respectively. When a shorter quartz tube (10 cm in length) is employed, the length of the BPPJ is only about 6.5 cm, since a disturbance still exists in the tube nozzle, and the position for the formation of turbulence is closer to the tube nozzle. The length of the plasma jet reflects the propagation distance of the \"plasma bullet\". The propagation is inhibited before the ignition of the secondary discharge, which is caused by the quenching process of the plasma by molecules of nitrogen or oxygen in the air diffused into the ionization channel. This limitation is called the transition point, which separates the plasma bullet from the inhibition zones [17]. Since the duration of the nanosecond pulse in our study is only about 50 ns, the plasma bullet can stop propagating immediately after the secondary discharge ignition [17]. Therefore, the propagation of secondary discharge ignition is directly influenced by the initial electric field intensity when the primary plasma bullet is formed. In the experiment, benefitting from the short voltage rising time and pulse matching with the spiral electrode, the applied pulse voltage could reach up to 24 kV without the formation of an arc or spark. In this case, the plasma bullet ignited by the secondary discharge, with a high local electric field intensity, can propagate over a long distance. This is the reason that the length of the plasma jet in this study is longer than that reported in previous studies. Figure 3: Discharge images of BPPJ (**a**), PPPJ (**b**), and NPPJ (**c**) with pulse voltage of 24 kV, pulse repetition rate of 150 Hz, and gas flow rate of 3.5 L/min. The polarity of the pulse voltage is also a crucial factor influencing the plasma jet length [19,20]. When a unipolar positive pulse voltage (either positive or negative) is employed, the reversed built-in electric field formed by the space charges accumulated on the tube surface and in the plasma jet channel can limit the propagation of the plasma bullet. Therefore, even if a high pulse voltage is applied, the length of the plasma jet cannot propagate further when the pulse voltage is increased. In contrast, a bipolar pulse voltage, with a positive directional pulse following by a negative directional pulse alternately, is more beneficial for the excitation of an LS-APPJ [23].