## 1 Introduction  
Because of their high beam quality, stability, dependability, high beam power and low cost, high-voltage accelerators are widely used in electron irradiation, nuclear physics investigations, and analysis. High-voltage proton accelerators have become popular in recent years for proton irradiation, boron neutron capture treatment (BNCT), and as injectors for proton medical accelerators. Budker Institute of Nuclear Physics in Russia, IBA in the United States, Neutron Therapeutics in Finland, and Nagoya University in Japan are all working on high-power, high-voltage proton accelerators capable of producing proton beams of several MeV and tens of milliameperes [1, 2]. These institutions have achieved different results in the development of high-voltage accelerators. Among them, the high-current, high-voltage proton accelerator constructed by Nagoya University can extract a 2.8 MeV/15 mA DC proton beam, which has been utilized in the development of the BNCT neutron source [3, 4]. The 2.6 MeV/30 mA commercial proton high-voltage accelerator developed by Neutron Therapeutics has been used in the BNCT project at the University of Helsinki and completed its first clinical trial. The University of Birmingham also purchased the company's high-voltage proton accelerator to develop high-flux neutron sources for producing nuclear energy materials [5]. Shanghai Institute of Applied Physics (SINAP) is currently developing such a kind of proton accelerator for BNCT. Figure 1a shows the main structure of the accelerator, which is designed to hit the target and generate the neutron beam. The design parameters of the high-voltage proton accelerator are shown in Table 1:  
The ion source is typically set to the high potential of the high-voltage accelerator, necessitating a small size, high microwave transmission efficiency, and good heat dissipation. The proton accelerator developed by SINAP has two requirements for the ion source: 1. The ion source needs to be positioned in a high-voltage spherical shell at the high-voltage end, as shown in Figure 1b. 2. The extracted beam must meet the injection intensity requirements of the accelerator and ensure that most ions can enter the accelerating tube and be consistently accelerated. The design parameters of the ion source are thus displayed in Table 2. \begin{table}
\begin{tabular}{c c} \hline \hline
**Index Parameter** & **Design Indexes** \\ \hline Maximum energy & 2.5 MeV \\ Maximum current intensity & 15 mA \\ Minimum current intensity & 10 mA \\ Energy stability & \(\leq\)0.5\% \\ Beam scanning range & 100 mm \(\times\) 100 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Design index of the high-voltage accelerator. Figure 1: The main structure of the high-voltage accelerator and the internal distribution of the high-voltage spherical shell: (**a**) is the main body of the accelerator; (**b**) is the distribution inside the spherical shell. Currently, various institutions employ ion sources such as the Penning source, the radiofrequency (RF) source, and the ECR ion source for proton accelerators, as illustrated in Table 3. The ECR ion source is a cathode-free source. Compared to other ion sources, it is stable and reliable, has high repeatability, can generate multiple types of highly charged ions and extract ion beams with low energy divergence and low emittance, and theoretically has no lifetime limit. Nonetheless, the typical ECR ion source is usually equipped with a longish microwave transmission system, preventing it from being well placed in the high-voltage spherical shell of the high-voltage accelerator, as shown in Figure 1b. The miniaturization of ECR ion sources can be classified into three categories: 1. Miniaturization of the microwave system. 2. Miniaturization of the source body. 3. Miniaturization of the extraction system. The miniaturization of the microwave system primarily involves the use of coaxial lines instead of waveguides for microwave energy input.