## 4 Discussion
### Luminescence of Leader Channel During Relaxation Process  
Based on the results of previous long-spark studies, during the relaxation process of the leader channel before the occurrence of restrike, there is no obvious ionization activity at the head of the channel, nor does the channel show obvious luminous phenomena [3]. However, the high-speed photographs shown in Figures 3 and 5 and the corresponding current waveforms indicate that during the relaxation process before the occurrence of restrike, although the discharge current measured at the high-voltage electrode is zero, the leader channel exhibits a certain degree of weak luminescence, that is, \"the dark period is not actually dark\", which indicates that, during the relaxation process before the occurrence of restrike, ionization activities may take place in the leader channel, and a large number of photons are emitted simultaneously. Moreover, as the relaxation process progresses, the number of photons gradually increases toward the moment of restrike occurrence. However, due to the restrictions of the limited time resolution of the high-speed camera, within a few microseconds before the leader restrike occurred, it was impossible to conduct a more detailed observation of this discharge development process, even though there might still have been other forms of ionization in the channel. The widely recognized critical temperature for streamer-to-leader transition is 1500-2000 K [3; 23], which is the critical temperature for the dissociation of a large number of negative ions inside the streamer stem. During the continuous development of the leader channel, its axial temperature can reach more than several thousand Kelvins [3; 24; 25], which is much higher than the critical temperature of 1500-2000 K for negative ion dissociation. Under the effect of the high temperature in this channel, the negative ions within the leader channel are dissociated into neutral particles and free electrons, making the density of free electrons in the leader channel far greater than that of negative ions [4; 24; 25]. In light of the above results, we suggest that during the continuous development of the leader, the charged particle components inside the leader channel are primarily free electrons and positive ions, and the density of negative ions can ultimately be ignored; this finding differs considerably from that presented by the authors of Ref. [8], who contend that in addition to positive ions, there are also negative ions and electrons inside the channel. A schematic diagram of the charged particle components of the channel during this stage is shown in Figure 7a. Under certain external conditions, when the streamer region at the leader head stops ionizing and the discharge enters the relaxation stage, the number of electrons injected into the channel decreases sharply, causing the conductivity of the channel to drop rapidly. In addition, the channel voltage drops and the electric field inside the channel may increase rapidly. The schematic diagram of this instantaneous process is shown in Figure 7b. Subsequently, under the action of the increasing electric field within the channel, the residual electrons migrate into the anode rapidly, resulting in the charged particle components within the channel primarily being positive ions. Therefore, the leader channel at this time may not be a true plasma channel. Under the effect of the externally applied electric field in the gap, the positive ions within the leader channel migrate toward the channel head along the direction of the electric field and collide with the high-temperature neutral molecules inside the channel, as illustrated in Figure 7c. Through the momentum exchange during collisions, the high-temperature neutral molecules in the channel gradually move away from the root of the channel along with the positive ions, resulting in a continuous reduction in the number of high-temperature gas molecules in the discharge channel originally connected to the electrode tip.