## 4 Discussion
### Luminescence of Leader Channel During Relaxation Process
With the passage of time, the gas temperature near the electrode gradually decreases to match that of the environment, and the gas density of the channel also increases to match that of the surrounding gas, resulting in the original leader channel appearing \"disconnected\" from the high-voltage electrode [16], as shown in Figure 7d, where the blackdashed line represents the bottom edge of the leader channel and the blue dashed line represents the edge of the channel after being \"disconnected\" from the electrode. Due to the high gas temperature and low density within the channel, the mobility of positive ions in the channel is greater than that at ambient temperature; therefore, these ions can migrate toward and accumulate in the channel head more quickly under the external electric field. According to the schlieren observation results and the simulation analysis of the leader channel during the relaxation process presented in Ref. [26], it can be concluded that as the positive ions migrate toward and accumulate in the leader head, the leader head additionally extends forward, and the radius range of the head region also increases further, as shown by the blue dashed line at the leader head in Figure 7d; this finding also differs from the schematic diagram shown in Figure 6 of Reference [8]. Subsequently, the Poisson field formed by the aggregated positive ions superimposes with the externally applied electric field in the gap, causing the distortion and enhancement of the local electric field in the area near the head of the original discharge channel. While this superimposed electric field reaches the electric field threshold of 26-30 kV/m in air [2; 3], the streamer region near the channel tip reactivates and emits photons. The schematic diagram is shown  
Figure 7: A schematic diagram of the dynamic process for the luminescence of the leader channel during the relaxation process. (**a**) Leader propagation; (**b**) The moment of the leader just entering the relaxation phase; (**c**) Migration of positive ions and high-temperature neutral particles; (**d**) Disconnection of the leader and electrode; (**e**) Luminescence at the channel tip; (**f**) New corona inception at the electrode; (**g**) Occurrence of leader restricke. in Figure 7e. Under the action of the electric fields both inside and outside the channel, the electrons generated in the streamer region at the channel head directly inject into the channel and then quickly recombine with the positive ions in the channel and release a large number of photons, causing the channel head to exhibit the channel luminescence phenomenon during the relaxation process before restrike, as shown in Figures 3 and 5. Because of the high density of positive ions inside the channel, the electrons entering the channel through streamer discharge neutralize directly with the positive ions or attach to neutral particles to form negative ions, and then recombine with the positive ions during their migration toward the high-voltage electrode. Ultimately, they cannot directly enter the high-voltage electrode from the inside of the channel. Consequently, as shown in Figures 3 and 5, before the occurrence of restrike, although there was discharge and luminescence at the channel head, the current measured at the high-voltage electrode was ultimately zero. As the externally applied electric field increases further, the local electric field in the head region of the leader channel increases accordingly, and the streamer discharges generated thereby become increasingly intense. It can therefore be seen from Figure 3 that compared with F4, F5 shows a larger luminous range and more obvious luminous intensity formed by the streamer discharge at the channel head, and the same finding is true for F11 compared with F10 or F9. Similarly, as shown in Figure 5, the discharge and luminescence at the channel head are also the same for F12\({}^{\prime}\) compared with F11\({}^{\prime}\).