## 3 Negative Streamer and Leader Formation in CO2 and CF3I-CO2  
Following experimentation, a comparison between an applied negative lightning impulse in CO2 (Figures 4a,b and 5a,b) and a 30:70% CF3I-CO2 gas mixture (Figures 4c,d and 5c,d) was examined for a 40 mm rod-plane gas gap. It was found that at these short gap lengths it was very difficult to obtain schlieren videography without a breakdown event occurring when a positive lightning impulse was applied to the rod-electrode and that videography could only be obtained under negative voltage conditions. It is likely that, because the initiatory electron in a positive streamer formation is progressing as part of an avalanche towards the rod electrode and towards a high field region, streamers and eventual breakdown is rapid and far reaching once the critical reduced field strength has been exceeded. However, in negative polarity, the initiatory electrons are moving outwards from the rod electrode towards an electric field region of decreasing intensity, so progression is more difficult. Under lightning impulse conditions, where the applied voltage is often much higher than under AC or DC stress, the field intensity around the electrode is higher and occurs very rapidly for the test conditions used in the events shown. For a positive streamer, the initiatory electron must exist in the gas between the electrodes to commence streamer formation but in electronegative gases electrons are strongly attached as negative ions and so it is harder to form free electrons without the chance of cosmic rays, background radiation or one of many theorised mechanisms. In gases such as air and SF6 it has been shown that the positive impulse conditions are a valuable reference for high-voltage testing because it has a lower breakdown voltage than with negative polarity which increases linearly with gap length, however, in CF3I gas mixtures it has been shown that under some electrode conditions the breakdown voltage can be lower in negative polarity rather than positive [23]. Figure 4: 0.1 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**) Undeited image with contrast. (**b**) and (**d**) Subtracted image highlighting areas of difference. (**a**) and (**b**) CO2 negative 70 kV \(-\)0.1 MPa. (**c**) and (**d**) 30:70% CF3I-CO2 negative 110 kV. In CO2 (Figure 4a,b and Figure 5a,b), streamers form distinct multi-channels for negative streamer progression away from the rod caused by electrons moving outwards into the electric field of decreasing intensity. These negative streamer formation filaments in pure CO2 are not the same as in CF3I-CO2 gas mixtures (Figure 4c,d and Figure 5c,d) where there appears to be fewer streamers. This gives the impression that the first stage streamer length can spread further into the CO2 gas gap as part of the avalanche along the field gradient. This is likely caused by the critical reduced electric field strength being lower in CO2 than in CF3I-CO2, allowing for net ionisation and streamer development to occur at a lower applied voltage. It is also important to note that the molecular size and mass of a CO2 molecule is less than CF3I, this means that a CO2 molecule has a higher molecular diffusion rate than CF3I but has less mass, therefore exerting the same pressure as a heavier, slow moving but larger sized CF3I molecule. This means that a CO2 and CF3I-CO2 mixture have the same number of molecules in the same volume when filled at exactly the same pressure and temperature. The frequency of an electron collision with a neutral molecule during streamer formation is dependent on the temperature, pressure, mass, size of the molecule and its molecular diffusion rate. In 30:70% gas mixtures of CF3I-CO2, as shown in Figure 4c,d and Figure 5c,d, the addition of CF3I to CO2 as a mixture causes streamer growth around the electrode to be considerable stunted compared to pure CO2.