## 6 Conclusions  
The numerical modeling is employed to understand the operation of the MPJCVD system and optimize its geometry configuration and working conditions. The MPJCVD system with argon and hydrogen plasmas is modeled and simulated using FEM multi-physics modeling. It is found that, under optimal conditions, the plasmas are concentrated at the antenna tip after reconditioning, and the plasma generated in the MPJCVD system is consistent with the simulation results. Under the optimized operating conditions after reconditioning for fabrication, the SEM shows that the deposited diamond film has relatively high uniformity and continuity, and the size of the diamond particle is about 1 \(\upmu\)m. In addition, by increasing methane up to 32% during the fabrication, our research shows that the turn-on electric field of the diamond film grown on the graphite substrate can be as low as 4 V/\(\upmu\)m, and the corresponding field emission exhibits an extraordinarily high current density compared to other cases in this study. This might be attributed to the combined effects of a good electrical contact from the diamond film grown on the graphite substrate and the field enhancement of the CNPs sitting on the diamond film. The developed multiphysics model provides valuable physical insights into the MPJCVD system, which guided performance improvements. The diamond films developed by the MPJCVD can be beneficial for applications in surface hardening, and those grown on a graphite substrate might serve as bright field electron emission sources.  
Conceptualization, C.-R.L. and M.-C.L.; methodology, H.-Y.H. and M.-C.L.; software, C.-YL. and K.A.; validation, C.-YL. and C.-WL.; formal analysis, C.-YL. and C.-WL.; investigation, J.-S.Y. and C.-Y.L. and C.-W.L.; data curation, C.-YL., C.-W.L. and K.A.; writing--original draft preparation, [PERSON]-[PERSON] and [PERSON]-Y.H.; writing--review and editing, [PERSON]-Y.H. and [PERSON]-[PERSON]; Supervision, [PERSON]-R.L. and [PERSON]-[PERSON]; project administration, [PERSON]-[PERSON] and [PERSON]-[PERSON]; funding acquisition, [PERSON]-R.L. and [PERSON]-[PERSON]. All authors have read and agreed to the published version of the manuscript.  
Figure 14: Measurement of (**a**) the film thickness and (**b**) Raman spectroscopy of the diamond film grown on the graphite substrate with working gas containing 32% methane.  
Figure 13: SEM image with (**a**) 300 X and (**b**) 50,000 X of the diamond film grown on the graphite substrate with working gas containing 32% methane.  
**Funding:** This research was partially supported by the Ministry of Science and Technology (111-2221-E-027-075), the National Taipei University of Technology in Taiwan, Hanyang University (HY-20140000002393), and the National Research Foundation (2015R1D1A1A01061017) in the Republic of Korea.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Data will be made available upon reasonable request.  
**Acknowledgments:** The authors would like to acknowledge the financial support from the Ministry of Science and Technology (111-2221-E-027-075), the National Taipei University of Technology in Taiwan, Hanyang University (HY-20140000002393), and the National Research Foundation (2015R1D1A1A01061017) in the Republic of Korea.  
**Conflicts of Interest:** The authors have no conflicts to disclose.