# Enhancement of Radio Frequency Plasma Plume by Pulsed Plasma Bullet at Atmospheric Pressure
## 3 Results  
### Cascade Discharge  
It is shown in Figure 1a that the pulsed discharge is generated between the pulse electrode and ground electrode in terms of a travelling plasma bullet and the RF discharge is ignited between the RF electrode and ground electrode. At the exit of the tube, the plasma plume can be attributed to both the RF discharge and pulsed discharge, which will be characterized by the spatio-temporal evolution of the discharge. The waveform of the pulse voltage, as shown in Figure 1(b1), has the amplitude of 5.2 kV and duration of 1.4 us with the duty cycle of 0.7%. The waveform of the RF voltage with the amplitude of 1.0 kV is shown in Figure 1(b2). In the experiments, the duty cycle of the pulse voltage was changed to manipulate the length of the pulsed discharge while keeping other discharge parameters unchanged. Figure 1(b3) gives the waveform of the total discharge current collected from the ground electrode, which includes the pulsed discharge current and RF discharge current. There are two current peaks with amplitudes of 1.5 A and 1.6 A at the time instants of 0.24 us and 1.64 us, respectively, which correspond to the rising phase and falling phase of the pulse voltage, as shown in Figure 1(b1). By subtracting the  
Figure 1: (**a**) Schematic of experimental setup and waveforms of (**b1**) pulsed voltage, (**b2**) RF voltage, (**b3**) total discharge current, and (**b4**) pulse discharge current. stable operation of the RF discharge current with the amplitude of 0.5 A from the total discharge current, the pulsed discharge current can be obtained, with two current peaks with amplitudes of 1.1 A and 1.2 A, as shown in Figure 1(b4), which is consistent with that in the dielectric barrier pulsed discharge [8]. To demonstrate the enhancement of the pulsed discharge on the RF discharge and plasma plume, the pulsed discharge was manipulated to propagate into the pulsed discharge region, RF discharge region, and plasma plume region, by changing the duty cycle of the pulse voltage, as shown in Figure 2(a1-c1), respectively. With a duty cycle of 0.3%, corresponding to the pulse voltage duration of 0.6 \(\upmu\)s, as shown in Figure 2(a1), the pulse discharge does not reach the ground electrode, and the plasma plume with the length of 2.0 mm can be attributed mostly to the RF discharge. The temporal evolution of discharges is presented in Figure 2(a2) with the time resolution of 10 ns, which is the exposure time of the ICCD camera and the time interval between two consecutive discharge images. The dashed lines in the figure indicate the position of the electrodes and the nozzle of the tube. The pulsed discharge can be recognized as a travelling plasma bullet, which is ignited near the pulsed electrode and reaches the position of 20 mm with the time interval of 0.6 \(\upmu\)s [13]. The RF discharge and plasma plume keep their spatial profile and intensity within 4.0 \(\upmu\)s of the ignition of the pulsed discharge, which suggests that the operation of the RF discharge is independent of the pulsed discharge. It is worth noting that the image intensities of the pulsed discharge and plasma plume are reduced and elevated by 10 times, respectively, for better demonstration, as indicated in Figure 2(a2). With the duty cycle of 0.7%, corresponding to the pulse voltage duration of 1.4 \(\upmu\)s, as shown in Figure 2(b1), the pulsed discharge reaches the ground electrode. It also shows that the intensity of the plasma plume is enhanced in terms of plume length growing to 4.0 mm. Figure 2: Discharge appearance of pulsed discharge, RF discharge, and plasma plume with the pulsed voltage duration of (**a1**) 0.6 \(\upmu\)s, (**b1**) 1.4 \(\upmu\)s, and (**c1**) 2.4 \(\upmu\)s, and corresponding temporal evolution of discharge spatial profiles as shown in (**a2–c2**). Figure 2(b2) gives the temporal evolution of discharge, which demonstrates the interaction between the pulsed plasma bullet with RF discharge.