## 3 Results
### Plasma Imaging  
Typical sets of time-resolved images illustrating the evolution of the discharge plasma during a single duty cycle generated using the planar and cylindrical electrode reactors are shown in Figure 2. Each image in the set was captured at a progressively increasing time delay relative to the maximum of the voltage waveform. The specific time delay and corresponding voltage are given in each image. The evolution of the plasma morphology in both reactors follows a similar pattern, which can be divided into four characteristic phases. In the case of the planar electrode reactor (Figure 2A), the first phase (time delay 0 \(\upmu\)s) begins as the applied voltage approaches its maximum value, during which no discharge plasma is observed. This is followed by a second phase (time delay from 160 \(\upmu\)s to 340 \(\upmu\)s) in which a decrease in voltage initiates the formation of wide plasma plumes that appear at random locations at the edge of the electrode. This discharge mode, sometimes referred to as a pseudo-Trichel discharge (also known as a diffuse or glow microdischarge), corresponds to a series of negative current  
Figure 2: Typical time-resolved images of the SDBD plasma generated using the reactor with (**A**) planar electrode, (**B**) cylindrical electrode. The position of the planar electrode is indicated in the images with a dashed rectangle, while the cylindrical electrode is indicated with a dashed line (except for t = 850 \(\upmu\)s). The distance traveled by the plasma front is indicated by a vertical bar in the bottom right-hand corner of the images. pulses, and its morphology is similar to that of a negative DC corona discharge [45]. In the third phase (time delay 500 us), as the applied voltage approaches its minimum, the plasma decays. The final phase (time delay from 660 us to 850 us) of plasma evolution begins as the voltage increases again, resulting in the formation of the positive streamer discharge, which has the shape of long plasma filaments. In the cylindrical electrode configuration (Figure 2B), the plasma evolves similarly, with some notable differences. One of them is visible in the second phase (time delay from 170 us to 340 us) of the pseudo-Trichel discharge, where the plasma plumes are significantly smaller and more sparsely distributed along the electrode compared to the planar electrode. As the cylindrical electrode is positioned centrally with respect to the grounded electrode, plasma plumes are generated on both sides of the wire. The third phase (time delay 500 us) is characterized by plasma decay, followed by the fourth phase (time delay from 630 us to 850 us) of positive streamer formation. These streamers are shorter, thicker, and more branched than those produced by the planar electrode. They are also generated for a shorter duration than those in the planar electrode configuration. In the latter part of this phase, the streamer discharge transitions to a uniform glow from the wire electrode. The recorded images allowed us to characterize the propagation of the plasma front in both discharge modes (Figure 3). The pseudo-Trichel discharge lasted approximately 250 us for both electrode configurations, during which the plasma front advanced up to about 3.3 mm from the electrode. In contrast, the duration of the streamer phase varied: it lasted 350 us for the planar electrode reactor, allowing the plasma to travel about 4 mm in this time, while for the cylindrical electrode reactor, this phase lasted only 160 us, with the plasma front traveling only about 2 mm. Thus, while the cylindrical electrode partially suppresses positive streamer formation, it does not completely eliminate it. We found it interesting that the average plasma front velocity was approximately the same for both discharge modes and both reactor types, ranging from 12 to 15 m/s. This plasma front velocity differs from the results previously obtained by [PERSON] et al. [28], who reported approximately 85 m/s.