## 3 Results and Discussion
### Effect of Electrode and Working Gas Pressure on Plasma Density  
Both electrodes were applied to the low-pressure plasma elements, with argon as the working gas and a high-frequency switchable power supply as the driver. The devices with different electrodes were tested under the operating current at 100 mA, 150 mA and 200 mA. The relationship between plasma density and argon gas pressure is shown in Figure 5.  
Figure 4: (**a**) Spike surface structures on tungsten electrode surface under AFM. (**b**) Height measurement of spike surface structures.  
From Figure 5a,b, it can be observed that with the increase in the argon gas pressure in the tube under the high-frequency DC pulse drive of 100 mA and 150 mA, the plasma density under both electrodes exhibits a trend of increasing first, and then decreasing. For both the electrodes, the maximal plasma density appears at the gas pressure of 0.5 torr. In low-pressure environments below 0.5 torr, the plasma density increases as the pressure gradually rises. This occurs because at low pressures, electrons can accumulate sufficient kinetic energy under the influence of the electric field, leading to collisions and the ionization of gas molecules. As the pressure continues to rise, the frequency of collisions between gas molecules also increases, further boosting plasma density. However, when the pressure exceeds the critical point of 0.5 torr, the frequent collisions between gas molecules and the reduced mean free path of electrons due to higher pressure cause the electrons to no longer gain enough energy in the electric field to sustain the continuous ionization of gas molecules. This results in a decrease in plasma density.  
As shown in Figure 5, under 100 mA current, the Fe-Ni electrode element has the maximum values of \(4.14\times 10^{11}\)cm\({}^{-3}\), and the W electrode element has the maximum values of \(7.79\times 10^{11}\)cm\({}^{-3}\). The improvement is 88.2%. For 150 mA current, the maximum plasma density of the Fe-Ni electrode element is \(8.21\times 10^{11}\)cm\({}^{-3}\) and that of the W electrode element is \(9.47\times 10^{11}\)cm\({}^{-3}\). The improvement is 15.3%. It is worthwhile noticing that the plasma density increases with the increase in current, which is consistent with the plasma density calculation formula:  
\[n(x)=(\frac{j}{qD_{a}}x+n_{0})e^{-\frac{\delta E}{2T}x} \tag{3}\]  
where \(n\) is electron concentration, \(j\) is drift current, \(q\) is electronic power, \(D_{a}\) is bipolar diffusion coefficient, and \(E\) is electric field strength.  
In Figure 5c, under a high-frequency DC pulse drive of 200 mA, the Fe-Ni electrode only shows the plasma density of \(1.36\times 10^{11}\)cm\({}^{-3}\) at 0.5 torr, which is lower than the plasma density at 100 mA. In addition, the plasma density does not follow the same trend that changes with the gas pressure inside the element. This phenomenon can be attributed to the damage of the Fe-Ni electrode at 200 mA, which caused the plasma element to fail to work properly. Notably, for the W electrode, when the gas pressure inside the tube is 0.5 torr, the plasma density can reach a maximum value of 11.5 \(\times\) 10\({}^{12}\) cm\({}^{-3}\), which is more than an order of magnitude higher than those reported in similar cold plasma studies.  
Figure 5: The relationship between plasma density and argon pressure at different working currents: (**a**) 100 mA current; (**b**) 150 mA current; (**c**) 200 mA current.