## 2 Materials and Methods
For iron-nickel alloy electrodes, since Cs\({}_{2}\)O electron powder is added to lower the work function, a relatively uniform barrel-shaped structure is required, making it unsuitable for forked or filament structures. Moreover, iron-nickel has relatively low fatigue resistance, necessitating the use of a barrel-shaped structure to more evenly distribute stress and reduce local stress concentration. Figure 2a,b show the perspective views of the element structure. Figure 2c shows a tungsten electrode plasma element in operation. Ultimately, these elements can be arranged in arrays to form a large-area, ultra-high-density, low-pressure cold plasma. Tungsten wire is known as a pyrocondensation metal material, because it becomes thinner as the temperature rises, which limits the increase in the current and enables long-term operation. On the other hand, other thermal expansion metals expand when exposed to high temperatures, resulting in a reduction in resistance and an increase in current, which can lead to burnout. Additionally, the electron emission coefficient of tungsten wire increases significantly at high temperatures, leading to a higher rate of thermionic emission. As the temperature rises, more electrons acquire enough energy to overcome the surface potential barrier and escape, creating a large amount of electron emission [29]. Figure 2: (**a**) Perspective view of plasma chamber and barrel-shaped Fe-Ni electrode. (**b**) Perspective view of plasma chamber and Tungsten electrode. (**c**) Large-area high-density cold plasma formed by an array of plasma elements. Thermonic emission occurs when the kinetic energy of the electrons within the solid is increased by heating. Meanwhile, the surface of the tungsten electrode underwent hydrothermal treatment, resulting in the creation of irregular, rough surface features. This process involved immersing the tungsten electrode in a solution containing tungsten ions, which reacted hydrothermally with hydrazine hydrate, forming irregular spike-like structures on the electrode. This treatment enhances the field emission characteristics of the electrode by leveraging the surface inhomogeneity to strengthen the partial electric field. Consequently, this improves the electrode's breakdown capability and reduces the turn-on field. The surface morphology of the electrode after hydrothermal treatment will be presented in Section 3.1. Plasma diagnosis refers to the use of various experimental techniques to determine plasma parameters. Commonly used plasma diagnostic techniques include the probe method, microwave method, and spectroscopic method. Among these, the probe method is considered one of the most effective diagnostic methods. The Langmuir dual-probe system used in this paper has several advantages, including:  
1. The dual-probe system collects current without exceeding the ion saturation current, thus preventing the probe from burning while collecting electron current in high-density plasma. 2. The dual-probe structure is simple, and the probe system is easy to operate. Since the dumet wire and the glass tube have similar thermal expansion coefficients, a 0.2 mm diameter dumet wire was selected as the probe for the Langmuir dual-probe system. In this way, the chamber will not be damaged when the diagnostic system is matched to the plasma chamber in a heat-treated manner, and a stable plasma can be maintained. Figure 3 shows a sketch of the Langmuir dual-probe system structure.