## 6 Conclusions  
In this paper, it has been shown that CO2 negative streamers form more multi-path streamer filaments than in CF3I-CO2, giving the impression that the initial first stage electron can progress further into the gas gap as part of the avalanche along the field gradient. It also appears that as the pressure is increased the length of the initiatory streamers is 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). In all gases and gas mixtures used, the applied voltage was increased as the pressure was increased in order to increase the electric field strength and excite streamer formation. In 30:70% gas mixtures of CF3I-CO2, the addition of CF3I to CO2 to form a mixture causes streamer growth around the electrode to be considerably stunted compared to pure CO2 and there is formation of stepped leaders. In CF3I-CO2, the first stage negative streamer progression reaches further into the gas gap between the electrodes when the gas density is lower i.e., the pressure is 0.1 MPa than when the pressure is raised to 0.15 MPa. In CF3I-CO2, when the applied negative impulse voltage is increased the leader formation progresses further across the gas gap due to the increase in electric field at the electrode tip and the energy imparted on the first stage electron avalanche. Under an applied negative lightning impulse voltage CF3I-CO2 exhibits streamer and stepped leader formation across all 20-50 mm gas gaps at a pressure of 0.15 MPa, which is not the case in CO2 where first stage streamers are likely too long to record. In the first frame of the recorded images, the streamers depicted show the schlieren effect but also show the emitted visible light radiation from electron attachment so in most examples the second frame of just the schlieren effect is used. This work has provided valuable insight into the use of pure CO2 and the use of electronegative gas mixtures, such as CF3I-CO2, as a potential replacement for SF6. This work demonstrates that under lightning conditions on the electrical network electronegative gas mixtures of CF3I-CO2 when used in gas insulated switchgear would behave similarly to SF6 by producing stepped leader channels. Future designs of switchgear insulated with electronegative gas mixtures, such as CF3I-CO2, can be adapted by following similar electric field and electrode geometry design principles as those currently undertaken with SF6. **Supplementary Materials:** The following are available online at [[http://www.mdpi.com/2076-3417/10/22/8006/s1](http://www.mdpi.com/2076-3417/10/22/8006/s1)]([http://www.mdpi.com/2076-3417/10/22/8006/s1](http://www.mdpi.com/2076-3417/10/22/8006/s1)), Figure S1: CO2 0.1 MPa Schlieren images taken of no-breakdown events in a rod-plane electrode configuration subjected to a standard lightning impulse voltage waveform; Figure S2: CO2 0.15 MPa Schlieren images taken of no-breakdown events in a rod-plane electrode configuration subjected to a standard lightning impulse voltage waveform; Figure S3: 30-70% CF3I-CO2 0.1 MPa Schlieren images taken of no-breakdown events in a rod-plane electrode configuration subjected to a standard lightning impulse voltage waveform; Figure S4: 30-70% CF3I-CO2 0.15 MPa Schlieren images taken of no-breakdown events in a rod-plane electrode configuration subjected to a standard lightning impulse voltage waveform; Figure S5: Technical Air 0.1 MPa Schlieren images taken of no-breakdown events in a rod-plane electrode configuration subjected to a standard lightning impulse voltage waveform. **Author Contributions:** Conceptualization, P.W. and D.M. ; methodology, P.W. and D.M. ; software, P.W. and D.M. ; validation, P.W., and D.M. ; formal analysis, P.W. ; investigation, P.W. and M.H. ; resources, P.W. ; data curation, P.W. and M.H. ; writing--original draft preparation, P.W. ; writing--review and editing, P.W. and D.M. ; visualization, P.W.