In one of our previous papers we presented a fast axial flow CO2 laser excited by 150 kHz resonant silent discharge. The optimized gas mixing ratio of this setup was CO2:N2:He = 1:22:5. This ratio was very peculiar but the laser efficiency thus achieved was optimized and comparable to that of other fast axial flow CO2 lasers. We have already put forth some preliminary postulations explaining the reason for this particular result; nevertheless in order to obtain a clearer insight into this laser we designed and carried out a series of experiments. The data collected were helpful for understanding why N2 is more important than He, and what roles they play in such a laser system.