## 3 Result and Discussion
### The Effects of Discharge Gap on FEGLD  
Considering the space compatibility during actual operation, the effect of the discharge gap on active species, the gas temperature, and the electron density are discussed. Figure 9 shows the effects of the discharge gap on the emission intensities of N\({}_{2}\) (C\({}^{3}\Pi_{\rm u}\)\(\rightarrow\) B\({}^{3}\Pi\)\({}_{\rm g}\)), N\({}_{2}^{+}\) (B\({}^{2}\Sigma\)\(\rightarrow\) X\({}^{2}\Pi\)), OH (A\({}^{2}\Sigma\)\(\rightarrow\) X\({}^{2}\Pi\)), OH (A\({}^{2}\Sigma\)\(\rightarrow\) X\({}^{2}\Pi\)), O (3p\({}^{5}\)P \(\rightarrow\) 3s\({}^{5}\)S\({}^{0}\)), and He (3s\({}^{3}\)S \(\rightarrow\) 2p\({}^{3}\)P\({}^{0}\)) of the FEGLD under the measurement condition of 30 kV plus peak voltage and 3.5 L/min gas flow rate. It clearly shows that the emission intensities of N\({}_{2}\) (C\({}^{3}\Pi_{\rm u}\)\(\rightarrow\) B\({}^{3}\Pi\)\({}_{\rm g}\)), N\({}_{2}^{+}\) (B\({}^{2}\Sigma\)\(\rightarrow\) X\({}^{2}\Pi\)), OH (A\({}^{2}\Sigma\)\(\rightarrow\) X\({}^{2}\Pi\)), and O (3p\({}^{5}\)P \(\rightarrow\) 3s\({}^{5}\)S\({}^{0}\)) remain almost constant as the discharge gap is enlarged from 10 mm to 15 mm. But in the same conditions, the emission intensity of He (3s\({}^{3}\)S \(\rightarrow\) 2p\({}^{3}\)P\({}^{0}\)) decreases while the discharge gap changes from 10 mm to 13 mm. Since the emission spectrum of He (3s\({}^{3}\)S \(\rightarrow\) 2p\({}^{3}\)P\({}^{0}\)) is an indicator of energetic electrons with relatively high threshold energy of 22.7 eV [59], the decrease of emission intensity of He (3s\({}^{3}\)S \(\rightarrow\) 2p\({}^{3}\)P\({}^{0}\)) reflects the change in electron energy distribution, which is controlled by the applied reduced field \(E/N\)[60]. As the discharge gap is enlarged, the increase in the value of pressure multiplied by electrode gap distance led to the fall in the reduced electric field, which induces the energy of electrons to decrease [61], and then the emission intensity of He (3s\({}^{3}\)S \(\rightarrow\) 2p\({}^{3}\)P\({}^{0}\)) is obviously decreased here.  
Figure 8: Gas temperature and electron density distribution in the axial direction.  
The effects of the discharge gap on gas temperature and electron density of FEGLD are illustrated in Figure 10 under the measurement conditions of 30 kV plus peak voltage and 3.5 L/min gas flow rate. As shown in the figure, the electron density shows almost no change as the discharge gap increases. However, the gas temperature increased from 280 K to 350 K with the discharge gap elevated from 10 mm to 14 mm. Since the optical fiber is aimed at the medium area of FEGLD, it means this increase in gas temperature happened in the middle of the discharge plasma. Considering Figure 2, which shows clearly that the diameter of the discharge area near the liquid surface decreased with the discharge gap enlarging, the mushroom-like shape discharge, which is around the mean middle discharge, is not obvious. Therefore, the cooling effect is limited, which is expressly significant for the helium gas with high thermal conductivity at 1.6 \(\times\) 10\({}^{3}\) J cm\({}^{-1}\) s\({}^{-1}\) K\({}^{-1}\). Hence, in this work, the weakness of the cooling effect around the middle discharge area leads to the increase in gas temperature.