# Characterization and Optimization of a Conical Corona Reactor for Seed Treatment of Rapeseed
## 3 Results and Discussion  
### Discharge Characterization  
To understand the discharge generation by the conical reactor, the plasma source was operated with a sinusoidal voltage signal with different amplitude values (without amplitude modulation). Pictures of the discharge operating with different voltage values are shown in Figure 2. At low voltage values (around 8 kV p-p), plasma generation started as a corona discharge at the tip of the high voltage pin electrode. Raising the applied voltage increased the region of the high electric field, expanding the discharge outward until it transited to a glowing corona at around 10.5 kV p-p. Eventually (at around 13.5 kV p-p), the charged particles drifted far enough to sense the grounded electrode and the formation of streamers reaching the Petri dish took place. Starting at 14.0 kV p-p on, the generation of streamers was stable, and the discharge was distributed throughout the entire funnel volume. The discharge pictures in Figure 2a are displayed together with the power density for different applied voltage values. For the power density calculation, the funnel internal volume (89.8 \(\pm\) 0.1 cm\({}^{3}\)), in addition to the Petri dish volume (50.0 \(\pm\) 0.1 cm\({}^{3}\)), was used. A linear trend of the power density increase with the applied voltage was observed for all tested ranges. However, once the discharge transited to streamer mode, the transferred charge fluctuated substantially. This explains the higher standard deviation for the calculated power densities starting from 14.2 kV p-p in Figure 2a. Under these operation conditions, streamers reached the bottom surface of the Petri dish increasing the volume of produced plasma. It has been demonstrated in previous works that operating a plasma source in burst mode allows a fine variation in the resulting mean discharge power [18]. This precise power control can be beneficial, especially for biological temperature-sensitive targets, such as seeds. Thus, the conical reactor was operated with different voltage duty cycle values for an applied voltage of 20.0 kV p-p (corresponding to around 105 mW/cm\({}^{3}\) without bursting the voltage signal), and the data are presented in Figure 2b. A linear increase in power density can be observed. Thus, the discharge power density could be lowered to around 10 mW/cm\({}^{3}\) using only 10% of the voltage duty cycle and keeping the amplitude voltage value. For some reactor configurations, such as plasma jets, power could also be regulated by the applied gas flow [34]. [PERSON] et al. (2018) [32], studied the influence of the gas flow rate in a reactor with a similar configuration. The discharge power was not much affected by the gas flow rate variation when using a conical nozzle. Therefore, in the present study, the argon flow rate was fixed to 4.0 slm. The discharge power adjustment by duty cycle variation led to a reduction in the discharge temperature [18]. Figure 3 presents the gas temperature for the voltage duty cycle values ranging from 10% to 90%. The temperature was monitored for 5 min of operation the time, including measurements prior to plasma ignition (around 20 \({}^{\circ}\)C). Within the measured time frame, the use of 90% duty cycles led to temperatures above 80 \({}^{\circ}\)C. For the same activity time (5 min), reducing the voltage duty cycle to 70%, 50%, 30%, and 10% decreased the gas temperature to around 68 \({}^{\circ}\)C, 54 \({}^{\circ}\)C, 41 \({}^{\circ}\)C, and 38 \({}^{\circ}\)C, respectively. Thus, controlling the discharge power can effectively lower the gas temperature preventing possible overheating when treating thermal sensitive targets. Optical emission spectroscopy (OES) was used to identify excited species in the plasma. The discharge produced by the funnel reactor generated a large variety of excited species, as can be seen in the OES spectra presented in Figure 4a,c. Even though the funnel was closed by the Petri dish, excited species from air molecules (O\({}_{2}\) and N\({}_{2}\)) were observed in the spectrum.