# Characterization and Optimization of a Conical Corona Reactor for Seed Treatment of Rapeseed
## 3 Results and Discussion
### Treatment of Rapeseed  
The effects of plasma treatment on seed properties and germination, as well as the influence of seeds on the discharge homogeneity, were investigated using seeds of rapeseed as a test model. First, the reactor was tested with different filling conditions to verify the effect on the discharge power. Figure 5 exhibits the power density values for different numbers of seeds treated (ranging from an empty reactor to a completely filled Petri dish). For this experiment, the voltage duty cycle was kept at 50% and new seeds were used for each batch of filling conditions. Complete filling of the Petri dish was achieved with approximately 1015 seeds (4.97 g). A small number of seeds (0.64 g, around 130 \(\pm\) 2 seeds) did not affect the discharge power, and thus, the power density value did not differ from the empty reactor condition (60 mW/cm\({}^{3}\)). Further increasing the seed number led to large fluctuations of power density values between 35 mW/cm\({}^{3}\) and 115 mW/cm\({}^{3}\). The large standard deviations for power values observed from around 300 (1.59 g, 330 \(\pm\) 2 seeds) to 1000 seeds (4.97 g, 1015 \(\pm\) 2 seeds) suggest an impact on the discharge caused by the filling condition. As a result, substantial inhomogeneity of the treatment with larger numbers of seeds should be expected. Therefore, to maintain the discharge uniformity, further treatments performed for this study comprised only 55 seeds of rapeseed for each batch. When dealing with direct treatments, it is important to consider that the substrate dielectric properties will affect the system leading to changes in the equivalent circuit model [37]. In the case of seeds, a capacitor and a resistor in parallel, whose values are excitation frequency-dependent, can represent the equivalent electrical circuit [37]. Therefore, the number of seeds placed inside a direct treatment reactor will impact the gap capacitance, resulting in variation in the discharge power. A large number of seeds inside the Petri dish resembles a packed-bed reactor. In such reactors, the discharge gap is completely filled with beads and the electric field at the voids (small space between the closely packed beads) is greatly enhanced [38; 39]. [PERSON] et al., 2017 [39] reported an influence of the beads' dielectric constant on the electron density in the voids. In addition, the electrons' density is lower for higher dielectric constant values (dielectric permittivity (\(\epsilon\)) \(>\) 25 depending on the applied voltage). [PERSON] et al. (2020) [40] investigated the electrical properties of rapeseed seeds and reported a frequency-dependent \(\epsilon\) ranging from 8.5 to 13 (for 1000 kHz and 1 kHz, respectively). Thus, the increase in power density observed in Figure 5 for larger numbers of seeds can be associated with an intensification of the electric field in the small regions (voids) between the seeds caused by surface charge  
Figure 4: Optical emission spectra of the discharge generated by the funnel reactor for different voltage duty cycles. (**a**) Spectra for the wavelength range from 200 to 450 nm (mainly UV region) and (**b**) detailed spectra highlighting the emission bands of NO and OH from 225 nm to 325 nm. The visible region of the spectra is shown in (**c**) from 650 nm to 950 nm. (**d**) Overview of the OES peaks for different values of voltage duty cycle. accumulation. Since the space in the conical reactor is not fully packed with seeds as in a packed bed reactor, the seeds can drift during treatment. Therefore, the voids between the rapeseed will have variable sizes leading to inconsistent discharge power with large measurement uncertainty. To avoid operating the reactor in the heterogeneous plasma density regions, for further seed treatments, the number of seeds placed in the reactor was kept with 55 seeds per batch. Another important aspect that should be considered when treating any kind of plant material is the temperature. As discussed in Section 3.1, raising the duty cycle leads to an increase in produced ROS.