## 2 Materials and Methods  
The electrode plays a crucial role in the performance of the plasma element. To generate high-density plasma, the electrode must be able to withstand a large current (\(\geq\)100 mA). Traditional closed-structure plasma elements usually employ barrel-shaped iron-nickel alloy as the electrode. However, the electron emission performance of iron-nickel alloy is poor. Therefore, it is necessary to add Cs\({}_{2}\)O electron powder to the electrode to reduce its work function. The iron-nickel alloy material, which consists of 40% iron and 60% nickel, is bent into a straight barrel to produce a barrel-shaped electrode. The inner side of the electrode is coated with Cs\({}_{2}\)O electron powder and then welded to a metal wire that is embedded in the glass tube. Dumet wire is used at the intersection of the metal wire and glass tube to ensure the tightness and stability of the plasma chamber. To import the argon gas with different gas pressures, the gas delivery tube of the closed plasma chamber is also made of glass. After the gas delivery is completed, the gas transmission pipeline is sealed by high-temperature melting. Cs\({}_{2}\)O is an n-type semiconductor material with a smaller work function (W\({}_{2}\)) than that of iron-nickel alloy (W\({}_{1}\)), as shown in Figure 1a. In the figure, E\({}_{0}\) is the vacuum energy level, E\({}_{\rm f1}\) is the Fermi energy level of the iron-nickel alloy, E\({}_{\rm f2}\) is the Fermi energy level of Cs\({}_{2}\)O, E\({}_{\rm c}\) is the conduction band of Cs\({}_{2}\)O, E\({}_{\rm v}\) is the valence band of Cs\({}_{2}\)O, and E\({}_{\rm f3}\) is the Fermi energy level of tungsten. Figure 1: (**a**) Energy band diagram of iron–nickel alloy and Cs\({}_{2}\)O before contact. (**b**) Energy band diagram of iron–nickel alloy and Cs\({}_{2}\)O after contact; energy band diagram of tungsten electrode. When Cs\({}_{2}\)O is in contact with the metal electrode, they compose a Schottky contact, and the Fermi energy levels of metal E\({}_{\text{f1}}\) and Cs\({}_{2}\)O E\({}_{\text{f2}}\) remain the same due to the pinning effect, as shown in Figure 1b. Thus, the work function of the iron-nickel electrode with Cs\({}_{2}\)O decreases from W\({}_{1}\) to W\({}_{2}\), making it easier to generate plasma. For the tungsten electrode, its work function (W\({}_{3}\)) is smaller than W\({}_{2}\), making it more suitable for use as a plasma electrode material. However, when the applied current is high, the plasma chamber generates a lot of heat, which lowers the plasma density and can cause damage to the electrode or the chamber. Moreover, Cs\({}_{2}\)O is prone to volatilization under high-temperature conditions, leading to a low working life and the poor continuity of conventional iron-nickel electrodes. Therefore, we developed tungsten electrode plasma elements based on the fabricated iron-nickel electrode elements. Using NHWY10K-0.5 high-frequency switching power supply as the working power source and high-purity argon gas (99.99% by volume) as the working gas, we compared the plasma densities under different currents with a barrel-shaped Fe-Ni electrode and a tungsten electrode. The structure of the tungsten wire electrode is similar to that of the cartridge iron-nickel electrode, but the iron-nickel part is replaced with a forked structure with a tungsten wire attached. The forked structure and the tungsten wire electrode can increase the electrode's surface area. Combined with the high curvature radius spiked structure on the tungsten electrode surface, it further enhances field emission characteristics, improving electron emission efficiency. Additionally, the forked structure facilitates better heat dissipation and adapts to thermal expansion or contraction, reducing the likelihood of mechanical failure or degradation and extending the electrode's lifespan.