## 2 Experimental Setup  
The pressure chamber, gray box in Figure 1, is designed for pressures up to 517 kPa. The pressure chamber is pumped down via a roughing pump to below 40 mtorr before being filled with the gas under test.  
Inside the chamber is the spark gap with a ground shield across the gap to maintain the coaxial transmission line nature of the cable spools on either side, see Figure 1. The ground shield permits nanosecond temporal resolution across the gap. The cable spools are 2121 high voltage, HV, coaxial transmission line cables and are electrically long to temporally isolate the spark gap from source and load reflections. The one-way electrical delay of the cable spools is 85 ns. The measured impedance of the 2121 HV cable is 59 \(\Omega\). The HV pulse is dissipated using a 50 \(\Omega\) resistive load. The temporal region of interest in  
Figure 1: A pictographic representation of the spark gap test setup. The cable spools consist of 14.9 m of 2121 coaxial transmission line cable. The cables temporally isolate the spark gap from voltage reflections at the [PERSON] generator or load. The Rogowski coils and capacitive voltage dividers, CVD, provide current and voltage diagnostics, respectively. The Marx generator is electrically triggered using a trigatron. The 2121 cable is sourced from US manufacture dielectric sciences (Chelmsford, MA, USA). The [PERSON] generator, Rogowski coils, and CVDs are designed and fabricated the Texas Tech University Center for Pulsed Power and Power Electronics.  
the measured gap voltage signal is \(\approx\)30 ns. Thus, reflections from the impedance mismatch of the 2121 HV cable and the load arrives well after the spark has finished breaking down.  
On either side of the chamber are Rogowski and capacitive voltage divider, CVD, probes to measure transient current and voltage waveforms. The probes are mounted on the high and load side of the chamber; the high side is connected to the [PERSON] generator, and the load side is connected to the load. These probes measure the nanosecond pulses across the gap if breakdown occurs. Voltage and current measurements are denoted with H or L subscripts depending on which side of the chamber they are mounted, see Figure 1. The high voltage pulse is produced using a pressurized Marx generator, which is electrically triggered. The output of the Marx generator is measured with its own CVD probe. The Marx generator is fired in 90 s intervals. To mitigate any effects of material compatibility, all insulating gases were evacuated from the system after 90 min. After the data were taken, the chamber was pumped down to below 30 mtor and back-filled with lab air or \(N_{2}\).  
The gas mixtures are created by filling the chamber with partial pressures of \(C_{4}F_{7}N\) and the buffer gas. The gas mixtures tested are 50/50 \(C_{4}F_{7}N\) with \(N_{2}\) or \(CO_{2}\) as the buffer gas or a 20/80 \(C_{4}F_{7}N\) mixture with \(N_{2}\). To create a homogeneous gas mixture, a fan at the base of the chamber is spun via magnetic coupling for ten minutes prior to testing. During this time, the mixing fan is spun at 2000 rpm or greater with an approximate gas flow rate of 337 L/min or greater.