# Enhancement of Radio Frequency Plasma Plume by Pulsed Plasma Bullet at Atmospheric Pressure
## 3 Results
### Carbon Fiber Treatment  
The enhancement of the RF discharge plume can also be demonstrated by the optical emission intensities at the nozzle in Figure 4. The behaviors of the optical emission intensities of 706 and 777 nm at the nozzle are further shown in Figure 5. The optical emission line at 706 nm is attributed to the transition of He (3\({}^{3}\)S\({}_{1}\)\(\rightarrow\)2\({}^{3}\)P\({}_{0,1,2}\)); it is agreed that the intensity of the optical emission line at 706 nm is an indication of the presence of electrons with energies above 2.9 eV. The optical emission line at 777 nm is attributed to the transition of O (3s\({}^{5}\)S\({}^{0}\)\(\rightarrow\)3p\({}^{5}\)P). It can be found that when the pulse duration is below 1.2 \(\upmu\)s, the intensity of high-energy electrons at the nozzle is proposed to be extremely low. When the pulse duration is greater than 1.2 \(\upmu\)s, the pulsed plasma bullet reaches the outside of the tube and the optical emission intensity at 706 nm increases from 15 to 85. Meanwhile, the optical emission intensity at 777 nm increases from 42 to 57, which indicates that the injection of the pulsed plasma bullet results in the enhancement in the RF discharge plume. Figure 4: Temporal evolution of discharge spatial profile with the pulse duty cycle of 1.2% at the nozzle. Microsecond images of RF discharge are shown in the insert. Figure 3: Time-dependent discharge image intensity of pulsed discharge, RF discharge, and plasma plume with duration of voltage pulse. Atmospheric pressure plasmas have been used for the etching and modification of carbon fibers (CFs) in the ambient air condition, and it is agreed that the surface modification is caused by the chemical reactions of oxygen-containing reactive species with the CFs. It has been proven that the amount of active oxygen in the RF discharge plume with the injection of the pulsed plasma bullet is elevated by 36%, as shown in Figure 5. These reactive O atoms are believed to play an important role in the surface etching and modification of CFs [15]. XPS was used to study the chemical composition of CFs before and after cascade plasma plume treatment. The surfaces of CFs mainly contain three types of atoms: C, O, and N. The relative content of each atom is shown in Figure 6a. The C, O, and N atoms of the untreated CFs are 85.18%, 11.92%, and 2.90%, respectively. After plasma treatment under different pulse durations for 24 s, the oxygen concentration on the surface increased, while the carbon concentration decreased. As the pulse duration increases from 0.6 us to 2.4 us, the carbon concentration on the carbon fiber surface decreases from 84.03% to 78.80%, the oxygen concentration increases from 12.98% to 17.92%, and the O1s/C1s ratio grows from 13.99% to 22.74%, respectively. Both the increments in the oxygen concentration and the O1s/C1s ratio indicate that plasma treatment introduces extra oxygen-containing polar groups on the surfaces of CFs. For comparison, a pulsed discharge jet with the pulse duration of 2.4 us was used to treat the CFs. Although the pulse discharge can also elevate the O1s/C1s ratio of the CF surface, the effect is not as strong as that of the cascade plasma plume. To further investigate the chemical changes of the CF surfaces induced by the cascade plasma plume, deconvolution analyses of the C1s peaks were performed. According to previous reports [16], four peaks (C-C (284.6 eV), C-O-C (285.9 eV), C=O (287.1 eV), and COOH or COOR (288.5 eV)) can be deconvoluted from the C1s spectra. Figure 6b gives the curve fitting results of the C1s envelope. It shows that the peak area of COOH or COOR components increased significantly with cascade plasma plume treatment at the pulse duration of 0.6 us and 1.2 us. The peak area of C-O and COOH or COOR components is dominated at the pulse duration of 2.4 us. On the other hand, it is found that the component peak area of the C-O increased significantly when treated by the pulsed discharge jet, which contains travelling plasma bullets.