# A Coaxial Dielectric Barrier Discharge Reactor for Treatment of Winter Wheat Seeds
## 3 Results and Discussion  
### Discharge Characterization of the DBD Reactor Operating with Different Feed Gases  
The coaxial DBD reactor was fed with two different gases: argon and helium. Figure 2 presents the electrical signals of both discharges operating with a duty cycle of 30% and amplitude voltage of 6 kV. In both graphs, current peaks can be observed during the rising and falling parts of the voltage. Argon plasmas are generally filamentary; i.e., they are constituted by several short-lived discharge filaments randomly distributed over the entire electrode's surface [46]. This filamentary characteristic of argon plasmas is evidenced by the multiple current peaks in both the positive and negative half-cycle of the voltage in Figure 2a. It can also be observed in the discharge picture presented along with the current waveform in Figure 2c. On the other hand, helium plasmas can operate in glow mode at atmospheric pressure, and their current waveform is characterized by one single peak in the positive and one in the negative half-cycle of the voltage [64]. Figure 2b exhibits the current and voltage waveforms for the helium discharge using the shaking DBD reactor. Here, several current peaks can be observed even though the discharge is clearly homogeneous, as to be seen in Figure 2d. The electrical signals presented in Figure 2a,b were acquired with seeds placed inside the reactor, while the formation of several current peaks in the helium DBD can be associated to a possible charge density enhancement caused by the accumulation of charge at the seeds surface [65]. The formation of several current peaks in a helium discharge can also be related to the presence of N\({}_{2}\) impurities, as investigated by [PERSON] and coauthors [66]. Thus, the formation of the current peaks observed in Figure 2b can also be attributed to the high level of impurities inside the reactor caused by the vortex mixer device leading to an admixture of ambient air into the discharge region. The formation of several short-lived current peaks can complicate the assessment of the dissipated power in the discharge by current integration. Therefore, the discharge power was evaluated by the QV Lissajous figure method. Figure 3 presents the power values for different applied voltages for both discharges (argon and helium). It can be observed for both cases that the power values tend to rise almost linearly with the increase of applied voltage, where the argon plasma (black curve) exhibits higher power values than the helium (red curve). For a voltage range between 10 and 14.5 kV peak to peak (p-p), the power values can vary from 1 to 10 W, which also leads to a temperature rise that could harm the seeds. In order to compare both discharges, different parameters were chosen for both gases operating with around 6.5 W of mean power (73.9 mW/cm\({}^{3}\)), resulting in an applied voltage of 11.5 kV p-p for argon and 13 kV p-p for helium. Varying the feed gas can modify the produced plasma excited species, and thus, optical emission spectroscopy (OES) was used to analyze both discharges during wheat seed treatment. Figure 4 presents the optical emission spectra of argon (a) and helium (b). One important common feature between the two spectra is the presence of weak NO(\(\gamma\)) emission below 280 nm with similar intensity. Emission bands of OH (308 nm) and an N\({}_{2}\) second positive system (between 300 and 500 nm) can also be observed for both spectra due to the slots at the reactor cap allowing an admixture of ambient air. The argon discharge emission spectrum (Figure 4a) presents strong Ar atomic lines between 650 and 900 nm and an intensive OH peak when compared to the weak nitrogen emission bands. It is important to notice the preferred energy transfer to oxygen species rather than N\({}_{2}\) in the argon plasma evidenced by the hydroxyl radical peak and weak atomic oxygen line in 777 nm and less intense N\({}_{2}\) peaks.