## 3 Result and Discussion
### The Effects of the Helium Flow Rate on FEGLD  
Since the gas flow rate influences the time for gas stay, it further affects temperature transition and discharge characteristics. The effect of helium flow rate on the emission intensity of N\({}_{2}\) (C\({}^{3}\Pi_{\rm u}\rightarrow\rm B^{3}\Pi_{\rm g}\)), N\({}_{2}^{+}\) (B\({}^{2}\Sigma\rightarrow\rm X^{2}\Pi\)), OH (A\({}^{2}\Sigma\rightarrow\rm X^{2}\Pi\)), O (3p\({}^{5}\)P \(\rightarrow\) 3s\({}^{5}\)S\({}^{0}\)), and He (3s\({}^{3}\)S \(\rightarrow\) 2p\({}^{3}\)P\({}^{0}\)) are obtained and shown in Figure 11, under the experimental conditions of 12 mm discharge gap and 30 kV pulse peak voltage. It can be seen that as the  
Figure 10: Effects of the discharge gap on gas temperature and electron density.  
gas flow rate is turned up, the emission intensities of N\({}_{2}^{+}\) (B\({}^{2}\Sigma\rightarrow\) X\({}^{2}\Pi\)), He (3s\({}^{3}\)S \(\rightarrow\) 2p\({}^{3}\)P\({}^{0}\)), and O (3p\({}^{5}\)P \(\rightarrow\) 3s\({}^{5}\)S\({}^{0}\)) increased gradually. Meanwhile, the reduction in emission intensity of N\({}_{2}\) (C\({}^{3}\Pi\)\({}_{\rm u}\rightarrow\) B\({}^{3}\Pi\)\({}_{\rm g}\)) and OH (A\({}^{2}\Sigma\rightarrow\) X\({}^{2}\Pi\)) can be observed. In the study by [PERSON] et al. [62], OH distribution with different gas flow rates is reported. As the gas flow rate is increased, the OH distribution trends to uniform; meanwhile, the peak intensity value of OH-LIF is getting weaker. The influence of gas flow rate on O-LIF is also reported; it is due to the O (3p\({}^{5}\)P) having generated a relatively long lifetime of up to milliseconds [63] that the distribution of atoms could be influenced by gas flow. Similarly, as discussed before, the emission intensity of N\({}_{2}^{+}\) (B\({}^{2}\Sigma\rightarrow\) X\({}^{2}\Pi\)) is dominated by the Penning process, and the metastable helium with long lifetime is driven by gas flow in the FEGLD, which leads to emission intensities of N\({}_{2}^{+}\) (B\({}^{2}\Sigma\rightarrow\) X\({}^{2}\Pi\)) increasing.  
The effects of gas flow rate on gas temperature and electron density are illustrated in Figure 12, where the pulse peak voltage and discharge gap are kept at 30 kV and 12 mm, respectively. It clearly shows that the gas temperature of FEGLD almost remains at 330 \(\pm\) 10 K when the gas flow rate varied from 0.5 L/min to 3.5 L/min, suggesting that the FEGLD has good stability. The electron density increases obviously from 2.3 \(\times\) 10\({}^{15}\) cm\({}^{-3}\) to 2.7 \(\times\) 10\({}^{15}\) cm\({}^{-3}\), while the helium gas flow rate increased from 0.5 L/min to 3.5 L/min, which is the most obvious consistent improvement in this work. Since gas flow could promote collision processes [34], the reactions that could generate electrons are highly promoted by the increase in the gas flow rate. Meanwhile, the increase in high energy metastable helium amount is beneficial to generating the Penning process, which provides seed electrons for discharge. As the results show, increasing the gas flow can avoid gas heating and at the same time improve electron density and the capacity that generates reactive species such as O, which is one of the most important active species in plasma medicine applications [52]. This shows the potential for meeting the needs of such temperature-sensitive applications.