# Enhancement of Radio Frequency Plasma Plume by Pulsed Plasma Bullet at Atmospheric Pressure
## 3 Results
### Cascade Discharge
The plasma bullet generated in the pulsed discharge region enters into the RF discharge region at the time instant of 1.2 us and leaves the RF electrode at the time instant of 1.7 us. With the propagation of the plasma bullet in the RF discharge region, the intensity of the RF discharge is enhanced from the ground electrode to the RF electrode and maintains a time delay. At the time instant of 2.0 us, the intensity of the plasma plume outside of the tube nozzle grows due to the enhancement of the RF discharge by the pulsed discharge. When increasing the duty cycle to 1.2% with the pulse voltage duration of 2.4 us, as shown in Figure 2(c1), the pulsed plasma bullet passes through the RF discharge region and moves out of the tube. The plasma plume is composed of the pulsed plasma bullet and RF discharge plume, which can be recognized by the discharge temporal evolution, as shown in Figure 2(c2) [14]. The RF discharge is enhanced with the passage of the pulsed plasma bullet, which is consistent with the result shown in Figure 2(b2), except for the interference by the pulsed discharge at the time instant of 2.6 us, corresponding to the falling phase of the pulse voltage. The intensity of the plasma bullet is reduced by 10 times in the plasma plume region outside of the tube, which shows the propagation of the plasma bullet with the length of 14 mm. The intensity of the RF discharge plume is elevated by two times to distinguish from the pulsed plasma bullet, which also shows the intensification of the RF discharge plume with the enhancement of the RF discharge. The length of the RF discharge plume is estimated to be 6.0 mm. The discharge maximum image intensities at each time instant in the regimes of pulsed discharge, RF discharge, and plasma plume are presented in Figure 3 as a function of the duration of the voltage pulse. In the regime of the pulsed discharge, the discharge image intensity grows from 58 to 860 when increasing the duty cycle from 0.4 us to 1.0 us, which corresponds to the growth of the pulsed plasma bullet from ignition in the vicinity of the pulsed electrode until reaching the ground electrode. As the duty cycle further increases to 2.4 us, the pulsed plasma bullet travels through the regime of pulsed discharge, and the discharge image intensity changes slowly to 1000. In the regime of RF discharge, before the duty cycle reaches below 1 us, the discharge image intensity maintains a low magnitude of around 23, which is elevated with the injection of the pulsed plasma bullet and reaches the magnitude of 47 at the duty cycle of 1.4 us. It is proposed that the enhancement in the RF discharge image intensity by two times can be attributed to the injection of the pulsed plasma bullet. With the duty cycle increasing to 2.4 us, the discharge image intensity reaches 65, which is more than one order of magnitude lower than that of the pulsed discharge. In the regime of the plasma plume, the discharge image intensity is as low as 4, which is much lower than that of the pulsed plasma bullet and can be distinguished from the pulsed plasma bullet. With the enhancement in the RF discharge, the discharge image intensity is elevated to 7 at the duty cycle of 1.4 us, which is further elevated to 23 at the duty cycle of 2.4 us with the assistance of the pulsed plasma bullet. This suggests that the intensified RF discharge and plasma plume can be achieved with the propagation of the pulsed plasma bullet through the RF discharge regime and plasma plume regime. Figure 4 shows the spatio-temporal evolution of the plasma plume with a pulse duty cycle of 1.2% and duration of 2.4 us at the nozzle. The exposure time was set to 20 ns for pulsed discharge and 500 ns for RF discharge during the experiment. The vertical dashed line in the figure indicates the position of the nozzle of the discharge tube. It is divided into three time domains for better demonstration: the stable RF discharge plume (\(-\)20-0 us), the pulsed plasma bullet passing through the RF plume (0-4 us), and the RF plume affected by the plasma bullet (4-60 us).