## 3 Negative Streamer and Leader Formation in CO2 and CF3I-CO2
It also appears that in CF3I-CO2 gas mixtures the formation of stepped leaders from the streamers are present whereas this is not the case for CO2 where only streamers are present. In other electronegative gases, such as pure SF6, a similar stepped leader process can be observed in short point-plane 20-50 mm gaps [12] and at pressures of 0.15 MPa typical of gas insulated switchgear (GIS). In GIS it has been found that the breakdown voltage under fast-fronted surges in non-uniform gaps was determined by direct leader inception and propagation in the absence of pre-existing corona space charge [12]. In fast-fronted impulse conditions, the voltage passes very quickly through the theoretical streamer onset level and the initial intense streamers can lead to the formation of a highly ionised leader channel before there is time for space charge stabilisation of the field at the tip of the electrode [12]. As shown in Figure 4c,d and Figure 5c,d, the first stage streamer progression was further from the electrode when the gas density was lower i.e., the pressure was 0.1 MPa rather than when the pressure was raised to 0.15 MPa. It can also be shown that when the applied negative impulse voltage was increased from Figure 4c,d to Figure 5c,d that the leader formation in CF3I-CO2 progressed further across the gas gap due to an increase in the electric field intensity at the electrode tip and the energy imparted on the initiatory electron avalanche. In Figures 6 and 7, the gas gap distance was increased to 50 mm. As with the 40 mm gap, it can be shown that, at 50 mm, a 30:70% CF3I-CO2 gas mixture forms stepped leader channels across the electrodes and during no-breakdown events forms first stage and subsequent stages of streamer propagation, in particular this is shown in Figure 6c,d. In SF6, the length of the initiatory  
Figure 5: 0.15 MPa. Schlieren images taken of no-breakdown events in a rod-plane electrode configuration with a 40 mm gas gap subjected to a negative standard lightning impulse voltage waveform. (**a**) and (**c**) Uncited image with contrast. (**b**) and (**d**) Subtracted image highlighting areas of difference. (**a**) and (**b**) CO2 negative 93 kV \(-\)0.15 MPa. (**c**) and (**d**) 30:70% CF3I-CO2 negative 119 kV. and subsequent stages of streamers determines the length of that step of the leader channel which propagates into the gap in typical steps of a few millimetres until streamer activity is too weak to form a further channel step [24]. In CF3I-CO2 it seems from Figure 6c,d and Figure 7c,d that stepped leaders at voltages between 125-132 kV can form a total leader channel approx. 30 mm long across the 50 mm gap and that the probability of bifurcation (two leader channels) seems to increase with the applied voltage level/electric field intensity. For CO2 in Figure 6a,b and Figure 7a,b, the pressure was increased from 0.1 to 0.15 MPa, in order to achieve the impulse voltage required to cause an ionisation streamer event the applied voltage was increased from 90 to 98 kV. It also appears that, as the pressure was increased, the length of the first stage streamers was decreased. This confirms the impression that the probability of ionisation from a single collision will depend on the given energy of the electron (i.e., applied impulse voltage) and the number density of the gas (i.e., pressure) [12]. In Figure 4a,b for CO2, it can be shown that although the pressure is low there was a short streamer length which was due to the applied impulse voltage level being relatively low at 70 kV compared to all other cases presented in this paper. However, when the CO2 pressure was increased to 0.15 MPa, as shown in Figure 5a,b, the streamer length appears to increase but this was actually due to the much larger applied impulse voltage of 93 kV which seems to have a significant impact on the gases ability to form longer streamer channels than in Figure 4a,b. Figure 6: 0.1 MPa.