## 4 Summary  
The design of an ECR ion source system for high-voltage accelerators is present in this paper from two perspectives: the miniaturization of microwave systems, and the optimization of the discharge chamber size and magnetic field. In terms of microwave transmission, a solution was proposed by analyzing the existing compact ion source and the microwave system: the removal of matching devices and the overall miniaturization design of the microwave transmission line from head to the source load. The microwave system and source matching are optimized using microwave circuit integrity simulation so that the devices used for impedance mismatch can be removed in the actual use process and the size of other waveguide devices is optimized, resulting in a 234 mm microwave system. The distance between the magnetron and the bottom of the discharge chamber is only 306 mm, which meets the requirement for size reduction while achieving an S11 of \(-\)23.752 dB. The transmission efficiency exceeds 90%.  
By simulating the plasma, the effects of the discharge chamber's size and the distribution of the static magnetic field on the ionization characteristics were studied. It was possible to obtain a trend for the effect of a small discharge chamber size on ionization characteristics. The results showed that selecting a discharge chamber with \(\Phi\)30 mm \(\times\) L40 mm can produce the most particles. The saddle-shaped magnetic field distribution is more able to produce dense plasma than the other two field patterns among the three distinct magnetic field distributions often employed in various ECR ion sources. At the same time, the saddle-shaped magnetic field was optimized, and the valley point of the saddle-shaped magnetic field was adjusted. It could be found that the ionization effect of altering the saddle-shaped valley point on the plasma is not as strong as the change in field type. The impact of the saddle-shaped magnetic field on the plasma can be somewhat enhanced by altering the valley point to within a narrow range of 875 G or setting it to 875 G.  
Conceptualization, M.-H.Z. and H.-F.Y.; methodology, Z.-F.H. and W.-S.W.; software, W.-Z.L. and H.-T.L.; validation, M.-H.Z.; formal analysis, H.-F.Y., M.-H.Z., W.-S.W. and Y.W.; investigation, H.-L.L., W.-Z.L. and M.-H.Z.; resources, M.-H.Z.; data curation, H.-F.Y., W.-Z.L., Y.W., D.-Y.Z. and H.-L.L.; writing--original draft preparation, H.-F.Y.; writing--review and editing, Z.-F.H.; visualization, H.-F.Y., D.-Y.Z. and H.-T.L.; supervision, M.-H.Z. and Z.-F.H.; project administration, Z.-F.H.; funding acquisition, Z.-F.H. and H.-L.L. All authors have read and agreed to the published version of the manuscript.  
Figure 11: The influence of particle number and average electron density on the magnetic fields of seven different valley points: (**a**) is the axial distribution of saddle-shaped magnetic fields at seven kinds of valley points; (**b**) the number of particles, the average electron density, and the maximum electron density varies with the valley point.  
**Funding:** This work was supported by the Major Development Project of the Shanghai Institute of Applied Physics, Chinese Academy of Sciences [Y951011031], the National Key Research and Development Plan of China [2016 YFC0105400], and the Shanghai Sailing Program [22 YF1457900].  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data that support the findings of this study are available from the corresponding author upon reasonable request.  
**Acknowledgments:** The authors would like to thank colleagues [PERSON] and [PERSON] of the Shanghai Institute of Applied Physics for their help with the article. The authors would also like to thank the anonymous reviewers for their constructive comments.  
**Conflicts of Interest:** The authors declare no conflict of interest.