## 2 The MPJCVD System and Its Reconditioning  
The experimental setup of our homemade MPJCVD system for fabricating diamond films is shown in Figure 1, and the corresponding schematic design is shown in Figure 2. The equipment had been developed for several years, but the fabrication of diamond films was not successful. Sometimes a diamond film could be fabricated; however, with the same conditions, the experimental results could not be reproduced. In other words, the MPJCVD system is not reliable, and its configuration has to be reconditioned.  
Figure 1: Experimental setup for diamond film fabrication using the microwave plasma jet chemical vapor deposition (MPJCVD) system.  
Compared with the MPECVD system, MPJCVD can efficiently generate a uniform distribution of plasma [7]. The MPJCVD system can stably generate a firm and uniform plasma with relatively low energy consumption to fabricate smooth diamond films. At the tip of the antenna, localized plasma can be obtained. The conical structure of the antenna is essential for localizing the input microwave energy to enhance the electric field in a certain region and stabilize the energy distribution in plasma. The activity of reaction species and dissociation rate can be greatly increased through the microwave-coupled plasma jet. Compared with the CVD process, MPJCVD reduces the required power for the operating pressure using CH\({}_{4}\) and H\({}_{2}\) gases.  
Numerical simulations of an MPJCVD system are essential to gain a deeper understanding of fabrication processes. These simulations enable the data collection and optimization of fabrication conditions, replacing difficult experimentation. However, modeling microwave plasmas at atmospheric pressure also has its difficulties, as it is a numerically stiff problem due to their high nonlinearity. In addition, plasma behavior spans wide ranges in both time and length scales. Therefore, the numerical study of microwave plasma is computationally expensive [31; 32; 33; 34; 35; 36; 37; 38]. Figure 3 shows the full research flow for the optimization of the MPJCVD chamber configuration with FEM modeling. In this study, a complete three-dimensional MPJCVD model has been constructed to investigate various parameters such as reactor geometry, especially the sample holder, microwave power, and gas pressure [28; 33]. The motivation is to gain more physical insight and a better understanding of the composition, spatial distributions, and plasma parameters under different operating conditions and a variety of gas pressures. Using the optimized simulation result, the growth of diamond film on different substrates has been successfully conducted. This experiment is based on the optimization simulation analysis of the chamber cavity with the sample holder. The scanning electron microscope (SEM) images have been taken to characterize the thin films to see the presence of a diamond film structure and the uniformity. The field emission properties of the MPJCVD fabrication diamond film grown on several substrates have been measured and compared in order to find a good cold cathode for field electron emission. The experimental results also serve as feedback to validate the numerical simulation model of the MPJCVD system. Both the numerical simulation and experimental studies of reconditioning the MPJCVD system play an essential role in successfully fabricating diamond films on a variety of substrates.  
Figure 2: Schematics of the microwave plasma jet chemical vapor deposition (MPJCVD) system, in which the sample holder can be adjusted or modified.