## 3 Results and Discussion
In addition, the density of negative charges is almost equal to the density of electrons in Figure 5b, which indicates that electrons are the dominant negative charges in CO\({}_{2}\) discharge under Martian conditions. While in atmospheric CO\({}_{2}\) plasma, the negative charges are mainly composed of CO\({}_{3}^{-}\) ions and electrons [23,24]. At the moment c when the negative current reach the peak value, a wide cathode fall region with a large gradient of positive charge density and a strong electric field is obtained in Figure 5c, and the narrow positive column is characterized by a large plasma density. When the negative discharge is extinguished, the charged particles in the cathode fall region quickly disappear, while the density of charged particles in the positive column decreases slowly due to the weak electric field and small charged particle density gradient. Therefore, the highest charged particle density can be obtained in the positive column. As shown in Figure 5d, electrons are completely dissipated and the charged particles in the discharge region are mainly heavy ions of CO\({}_{2}^{+}\) and CO\({}_{3}^{-}\). The rapid decrease in electron density can be attributed to the following: one is the large transport coefficients of electrons, and the other is the electronegativity of CO\({}_{2}\) gas, which causes many electrons to be attached by CO\({}_{2}\) molecules (\(\mathrm{e}+\mathrm{CO}_{2}\rightarrow\mathrm{CO}+\mathrm{O}^{-}\)). During the power-off phase, the positive and negative ions in the positive column are driven to the electrodes by the  
Figure 4: Temporal evolutions of current density, applied voltage, gas voltage, and memory voltage in P1 discharge with a gap width of 4.0 mm. electric field, and the positive column begins to dissipate at about 15 \(\upmu\)s after the voltage application. According to the simulation data, a lot of CO\({}_{2}^{+}\) ions stay in the cathode sheath on the left side of Figure 5d and many CO\({}_{3}^{-}\) ions could be observed in the anode sheath on the right side of Figure 5d. As shown in Figure 5e, although the mobilities of the CO\({}_{2}^{+}\) and CO\({}_{3}^{-}\) ions are nearly equal, the CO\({}_{3}^{-}\) ions will reach the electrode first because the positive column is closer to the anode. According to the simulation data, when the gap width is 4 mm, the electrons dissipate completely at 12.2 \(\upmu\)s, the CO\({}_{3}^{-}\) ions reach the anode at 17.8 \(\upmu\)s, and the CO\({}_{2}^{+}\) ions reach the cathode at 19.5 \(\upmu\)s. At the end of the voltage period, the spatial distributions of the charged particles in Figure 5f are the same as those in Figure 5a, which indicates that the ions produced by the discharge are completely dissipated during the power-off phase, without affecting the discharge of the next voltage period. As the gap width increases, the discharge evolves from P1 state into P2 state. Figure 6 shows the current density, applied voltage, gas voltage, and memory voltage in P2 discharge with a gap width of 4.2 mm. In the P2 discharge, the current density is repeated every two voltage periods, and the voltage period in which the largest current pulse is obtained in Figure 6 is regarded as the first voltage period of the P2 discharge. As shown in Figure 6, in the first voltage period, the gas is ignited when the gas voltage reaches 772 V, and a large current pulse with a maximum value of 10.6 mA/cm\({}^{2}\) is observed. However, in the second voltage period, the breakdown voltage is only 715 V, and the corresponding peak current density is only 6.7 mA/cm\({}^{2}\). It is obvious that the breakdown voltage and current density of the second discharge are smaller than those of the first discharge.