# Design of a Miniaturized Electron Cyclotron Resonance Ion Source for High-Voltage Proton Accelerator

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

## Featured Application

**We provide a microwave system design that is much smaller than the conventional design, and provide a method for judging whether the size of the discharge chamber is better under the condition of a limited permanent magnetic ring. This is of reference value for workers with similar design requirements.**

## Abstract

## 1. Introduction

## 2. Miniaturization of the Microwave System

#### 2.1. Conventional Microwave System

#### 2.2. Microwave Transmission System Miniaturization Design

_{11}wave in the classical electromagnetic wave theory [11]. The discharge chamber is so compact that electromagnetic waves cannot fit inside. The relative permittivity of the plasma ε

_{p}can be expressed as [14]:

_{ce}is the electron cyclotron frequency, ω

_{pe}is the plasma oscillation frequency given by ${\omega}_{\mathrm{pe}}=\sqrt{{n}_{\mathrm{e}}{e}^{2}/{m}_{\mathrm{e}}{\epsilon}_{0}}$, and ε

_{0}is the dielectric constant in vacuum. The electron cyclotron frequency is denoted by ${\omega}_{\mathrm{c}\mathrm{e}}=eB/{m}_{\mathrm{e}}$, where e is the elementary charge, B is the axial magnetic induction, and m

_{e}is the electron mass. Additionally, the microwave frequency is represented by $\omega =e{B}_{0}/{m}_{\mathrm{e}}$, with B

_{0}corresponding to a magnetic flux density of 875 G for a microwave frequency of 2.45 GHz. By substituting these values into Equation (1), we obtain:

_{e}represents the electron density. According to the above equation, when B > B

_{0,}the dielectric constant will be larger than the vacuum dielectric constant, and the minimum diameter D

_{p}that can transmit a 2.45 GHz microwave can be changed to ${D}_{\mathrm{p}}=\lambda /3.412\sqrt{{\epsilon}_{\mathrm{p}}}$. When the plasma dielectric constant is large enough, the 2.45 GHz electromagnetic wave can enter the small size discharge chamber.

^{17}m

^{−3}.

#### 2.2.1. Conventional Length Waveguide Plus Transition Waveguide

#### 2.2.2. Conventional Length Waveguide Plus Miniaturized Ridged Waveguide

_{n}), the number of ridge steps (N), the length of the ridge segment (L

_{n}), the gap between the ridge segments (d

_{n}), and the width of the ridge segment (s

_{n}). The commonly used ridge waveguide has 3–4 segments, and the length of each part is λ

_{g}/4, where λ

_{g}is the waveguide wavelength for transmitting microwaves. The relationship of each segment of the ridge waveguide is as follows [16,17]:

_{IV}is the input impedance, and Z

_{L}is the load impedance.

_{0}is the peak voltage across the ridge gap, and P is the power transmitted by the ridge waveguide.

_{PV}of the ridge waveguide is expressed as follows:

_{c}represents the cut-off wavelength of the relevant frequency, a represents the length of the rectangular surface of the ridge waveguide, and b represents its breadth.

_{g}of the waveguide. For 2.45 GHz, each section of the ridge waveguide is 30.6 mm, so the length of the conventional waveguide used should be around 91.8 mm or 122.4 mm. The dimensions of the waveguide do not suit our requirements. Therefore, based on the traditional ridge waveguide, referring to the design of the transition waveguide, the cavity part of the ridge waveguide is designed as a transition waveguide with three sections of openings gradually reduced. Three equal-width steps are arranged along the transmission direction at the center of both sides of the long side of the inner surface of the cavity, and the gaps between the three steps gradually decrease. By adjusting the width of the ridge and the relative size of the steps, the impedance-matching of the entire microwave system can be realized. The overall length of the miniaturized ridge waveguide is about half that of the conventional ridge waveguide.

#### 2.2.3. Reduced Length Waveguide Plus Miniaturized Ridge Waveguide

## 3. Optimization of Discharge Chamber Size and Magnetic Field

_{0}is vacuum permeability, k

_{0}is the vacuum wavenumber, ε

_{r}is relative permittivity, ε

_{0}is the vacuum permittivity, and σ is the plasma conductivity, which is a full tensor determined by the electron density collision frequency and the static magnetic flux density. Define the following parameters:

_{e}is the electron mass, and ν

_{e}is the collision frequency between electrons and neutral particles. The inverse matrix of plasma conductivity can be expressed as:

_{e}is the heating term, which comes from microwave heating and the motion of electrons in the bipolar field. R

_{e}is the electron source term, R

_{ε}is the energy loss due to inelastic collisions, D

_{e}is the electron diffusion coefficient, D

_{ε}is the electron energy diffusion coefficient, μ

_{ε}is the electron energy mobility, and μ

_{e}is the electron mobility, which are full tensors affected by the static magnetic flux density; the specific expressions are as follows:

#### 3.1. The Impact of the Discharge Chamber’s Dimensions on the Ionization Properties

#### 3.2. Effects of Three Classical Axial Magnetic Field Distributions on Ionization Characteristics

#### 3.3. Improvement of the Saddle Magnetic Field

## 4. Summary

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

- Kasatov, D.; Koshkarev, A.; Kuznetsov, A.; Makarov, A.; Ostreinov, Y.; Shchudlo, I.; Sorokin, I.; Sycheva, T.; Taskaev, S.; Zaidi, L. The accelerator neutron source for boron neutron capture therapy. J. Phys. Conf. Ser.
**2016**, 769, 012064. [Google Scholar] [CrossRef] - Kiyanagi, Y.; Sakurai, Y.; Kumada, H.; Tanaka, H. Status of accelerator-based BNCT projects worldwide. AIP Conf. Proc.
**2019**, 2160, 050012. [Google Scholar] [CrossRef] - Honda, S.; Yoshihashi, S.; Tomita, S.; Tsuchida, K.; Kiyanagi, Y.; Tsurita, Y.; Watanabe, K.; Yamazaki, A.; Uritani, A. Development of a Sealed Li target as an accelerator-driven neutron source for Boron neutron capture therapy at Nagoya University. Nucl. Instrum. Methods A
**2022**, 1040, 167140. [Google Scholar] [CrossRef] - Yoshioka, M. Review of accelerator-based boron neutron capture therapy machines. In Proceedings of the 7th International Particle Accelerator Conference (IPAC’16), Busan, Republic of Korea, 8–13 May 2016; pp. 3171–3175. [Google Scholar] [CrossRef]
- Porra, L.; Seppala, T.; Wendland, L.; Revitzer, H.; Joensuu, H.; Eide, P.; Koivunoro, H.; Smick, N.; Smick, T.; Tenhunen, M. Accelerator-based boron neutron capture therapy facility at the Helsinki University Hospital. Acta Oncol.
**2022**, 61, 269–273. [Google Scholar] [CrossRef] - Chen, G.; Xu, S.W.; Xie, Y.H.; Song, Y.T.; Chen, Y.Q.; Hu, C.D.; Zhao, Y.P.; Samsonov, E.V.; Karamysheva, G.; Shirkov, G.; et al. The construction of the inner ion source for SC200 compact superconducting cyclotron. AIP Conf. Proc.
**2018**, 2011, 090009. [Google Scholar] [CrossRef] - Ji, Q.; Sy, A.; Kwan, J.W. Radio frequency-driven proton source with a back-streaming electron dump. Rev. Sci. Instrum.
**2010**, 81, 02B312. [Google Scholar] [CrossRef] [Green Version] - Ren, H.T.; Peng, S.X.; Zhang, M.; Zhou, Q.F.; Song, Z.Z.; Yuan, Z.X.; Lu, P.N.; Xu, R.; Zhao, J.; Yu, J.X.; et al. The deuteron injector progress of the Peking University Neutron Imaging Facility project. Rev. Sci. Instrum.
**2010**, 81, 02B714. [Google Scholar] [CrossRef] - Gobin, R.; Beauvais, P.; Ismail, A.B.; Bogard, D.; Delferriere, O.; De Menezes, D.; Duperrier, R.; Gauthier, Y.; Harrault, F.; Leroy, P.; et al. ECR light ion sources at CEA/Saclay. Chin. Phys. C
**2007**, 31, 46–50. [Google Scholar] - Peng, S.X.; Zhang, A.L.; Wu, W.B.; Ma, T.H.; Jiang, Y.X.; Li, K.; Zhang, J.F.; Zhang, T.; Wen, J.M.; Xu, Y.; et al. Plasma simulation and optimization for a miniaturized antenna ECR ion source. Nucl. Instrum. Methods A
**2021**, 1011, 165586. [Google Scholar] [CrossRef] - Wen, J.M.; Peng, S.X.; Ren, H.T.; Zhang, T.; Zhang, J.F.; Wu, W.B.; Sun, J.; Guo, Z.Y.; Chen, J.E. A miniaturized 2.45 GHz ECR ion source at Peking University. Chin. Phys. B
**2018**, 27, 055204. [Google Scholar] [CrossRef] - Fatkullin, R.; Bogomolov, S.; Efremov, A.; Loginov, V.; Kostukhov, Y.; Bondarchenko, A. Current status of the compact 2.45 GHz ECR Ion Source at FLNR JINR. EPJ Web. Conf.
**2019**, 201, 08002. [Google Scholar] [CrossRef] [Green Version] - Delferrière, O.; Gobin, R.; Harrault, F.; Nyckees, S.; Sauce, Y.; Tuske, O. Advanced light ion source extraction system for a new electron cyclotron resonance ion source geometry at Saclay. Rev. Sci. Instrum.
**2012**, 83, 02A307. [Google Scholar] [CrossRef] - Shimada, M.; Watanabe, I.; Torii, Y. Compact electron cyclotron resonance ion source with high density plasma. J. Vac. Sci. Technol. A
**1991**, 9, 707–710. [Google Scholar] [CrossRef] - Qian, C.; Chen, Z.; Wu, Q.; Zhang, W.H.; Wang, Y.; Yang, Y.; Ma, H.Y.; Fang, X.; Sun, L.T.; Zhang, X.Z.; et al. Microwave impedance matching of A 2.45 GHz ECR ion source. Nucl. Phys. Rev.
**2013**, 30, 32–37. [Google Scholar] [CrossRef] - Swaroop, R.; Kumar, N.; Rodrigues, G.; Kanjilal, D.; Banerjee, I.; Mahapatra, S.K. Design and development of a compact ion implanter and plasma diagnosis facility based on a 2.45 GHz microwave ion source. Rev. Sci. Instrum.
**2021**, 92, 053306. [Google Scholar] [CrossRef] - Misra, A.; Pandit, V.S. Studies on the coupling transformer to improve the performance of microwave ion source. Rev. Sci. Instrum.
**2014**, 85, 063301. [Google Scholar] [CrossRef] - Wang, P. The Parameter Calculation for Ridge Waveguide. Fire Control Radar Technol.
**2004**, 3, 014. [Google Scholar] [CrossRef] - Sun, W.M.; Balanis, C.A. MFIE analysis and design of ridged waveguides. IEEE Trans. Microw. Theory
**1993**, 41, 1965–1971. [Google Scholar] [CrossRef] - Hagelaar, G.J.M.; Makasheva, K.; Garrigues, L.; Boeuf, J.P. Modelling of a dipolar microwave plasma sustained by electron cyclotron resonance. J. Phys. D Appl. Phys.
**2009**, 42, 194019. [Google Scholar] [CrossRef] - Mallick, C.; Bandyopadhyay, M.; Kumar, R. Spatio-temporal evolution of electric field inside a microwave discharge plasma during initial phase of ignition and its effect on power coupling. Phys. Plasmas
**2019**, 26, 043507. [Google Scholar] [CrossRef] [Green Version] - Mallick, C.; Bandyopadhyay, M.; Kumar, R. Plasma characterization of a microwave discharge ion source with mirror magnetic field configuration. Rev. Sci. Instrum.
**2018**, 89, 125112. [Google Scholar] [CrossRef] - Kinder, R.L.; Kushner, M.J. Consequences of mode structure on plasma properties in electron cyclotron resonance sources. J. Vac. Sci. Technol. A
**1999**, 17, 2421–2430. [Google Scholar] [CrossRef] - Gammino, S.; Ciavola, G. ECR ion sources and scaling laws. In Proceedings of the 14th International Conference, Cyclotrons and Their Applications, Cape Town, South Africa, 8–13 October 1995; pp. 377–380. [Google Scholar] [CrossRef]
- Cortazar, O.D.; Megia-Macias, A.; Vizcaino-de-Julian, A. Experimental Study of Breakdown Time in a Pulsed 2.45-GHz ECR Hydrogen Plasma Reactor. IEEE Trans. Plasma Sci.
**2012**, 40, 3409–3419. [Google Scholar] [CrossRef] - Song, Z.Z.; Peng, S.X.; Yu, J.X.; Ming, J.C.; Yuan, Z.Y.; Qian, F.; Guo, Z.Y. Minipermanent magnet high-current microwave ion source. Rev. Sci. Instrum.
**2006**, 77, 03A305. [Google Scholar] [CrossRef] - Wei, X.B.; Li, G.P.; Pan, X.D.; Wu, Q.; Liu, Y.G. Proton beam intensity and proton fraction measurement of the 2.45 GHz ECR ion source. J. Instrum.
**2020**, 15, T04005. [Google Scholar] [CrossRef] - Liu, Y.G.; Ke, J.L.; Zhao, G.Y.; Lou, B.C.; Hu, Y.H.; Liu, R. Effect of magnetic field distribution on ECR ion source discharge. Nucl. Sci. Tech.
**2018**, 29, 126. [Google Scholar] [CrossRef] - Ke, J.L.; Liu, Y.G.; Liu, B.L.; Hu, Y.H.; Liu, M.; Tang, J.; Zheng, P.; Li, Y.; Wu, C.L.; Lou, B.C. Development of a Compact Deuterium–Deuterium Neutron Generator for Prompt Gamma Neutron Activation Analysis. Instrum. Exp. Tech.
**2020**, 63, 616–620. [Google Scholar] [CrossRef] - Liu, Y.G.; Liu, J.L.; Wu, Q.; Sun, L.T. Ion beam production with an antenna type 2.45 GHz electron cyclotron resonance ion source. Rev. Sci. Instrum.
**2020**, 91, 023301. [Google Scholar] [CrossRef] - Jiang, Y.; Peng, S.; Wu, W.; Ma, T.; Zhang, J.; Ren, H.; Li, K.; Zhang, T.; Wen, J.; Xu, Y. A miniaturized ECR plasma flood gun for wafer charge neutralization. Rev. Sci. Instrum.
**2020**, 91, 033319. [Google Scholar] [CrossRef]

**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.

**Figure 2.**Structural components of the physical computational model: (

**a**) structural model for calculating the transmission of the miniature microwave system; (

**b**) structural model of the source body for calculating the discharge inside the discharge cavity.

**Figure 3.**Structural model of the ECR ion source experimental platform equipped with a conventional microwave transmission system and microwave transmission measurement results of the ion source: (

**a**) is a three-dimensional structural model of the ECR ion source experimental platform equipped with a conventional microwave transmission system; (

**b**) is a physical drawing of the experimental platform and microwave transmission measurement results of the ion source.

**Figure 4.**The electric field distribution at 2.45 GHz for the microwave transmission system of the three schemes is as follows: (

**a**) is a regular-length waveguide plus transition waveguide; (

**b**) is a regular-length waveguide plus ridge waveguide; (

**c**) is a reduced-length waveguide plus ridge waveguide.

**Figure 5.**The transmission performance of the three schemes: (

**a**) is the distribution of the scattering parameter S11 in the 2−3 GHz frequency band; (

**b**) is the 2−3 GHz frequency band distribution of transmission efficiency into the discharge chamber.

**Figure 6.**Spatial variation in electron density at different cavity sizes: (

**a**) is the electron density distribution variation with discharge cell radius; (

**b**) is the electron density variation with the length of the discharge cell.

**Figure 7.**The number of particles, the average electron density, and the maximum electron density varies with the cavity: (

**a**) is the number of particles, the average electron density, and the maximum electron density contrast with the radius of the discharge chamber. (

**b**) is the number of particles, the average electron density, and the maximum electron density compared with the discharge chamber’s length.

**Figure 8.**Three classic axial distributions and corresponding contour distributions: (

**a**) is three classical axial distributions; (

**b**–

**d**) are contour distributions of the flat magnetic field, Gaussian magnetic field, and saddle magnetic field, respectively (860 G–5 G–890 G).

**Figure 9.**The average electron density and average electron temperature change in the time domain: (

**a**) is the electron density; (

**b**) is the electron temperature.

**Figure 10.**Electron density under different magnetic field distributions: (

**a**) is a flat magnetic field; (

**b**) is a Gaussian-distributed magnetic field; (

**c**) is a saddle-shaped magnetic field.

**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.

Index Parameter | Design Indexes |
---|---|

Maximum energy | 2.5 MeV |

Maximum current intensity | 15 mA |

Minimum current intensity | 10 mA |

Energy stability | ≤0.5% |

Beam scanning range | 100 mm × 100 mm |

Index Parameter | Design Indexes |
---|---|

Beam intensity | ≥20 mA |

Extraction voltage | 40 kV |

Proton ratio | ≥85% |

Normalized emittance | ≤0.25 π mm. mrad |

External contour size | 600 mm(length) × 300 mm(width) × 300 mm(high) |

Maximum magnetic field | 1100 G |

Vacuum level | 10^{−5} Pa–10^{−3} Pa |

Microwave frequency | 2.45 GHz |

Microwave power | 300 W–500 W |

Organization | Ion Source Type | Ion Species | Extraction Voltage (kV) | Beam (mA) |
---|---|---|---|---|

Institute of Plasma Physics, CAS [6] | Penning ion source | H^{+} | 40 | 0.1 |

Lawrence Berkeley National Laboratory [7] | High frequency source | H^{+} | 15.5 | 16 |

Peking University [8] | ECR source | H^{+} | 50 | 100 |

CEA/Saclay [9] | ECR source | H^{+} | 85 | >100 |

Organization | Microwave Transmission Mode | Overall Size of the Ion Source System (mm) | Miniaturized Component | Power (W) | Magnetic Field Generation Mode | Extraction Voltage (kV) | Beam (mA) |
---|---|---|---|---|---|---|---|

Peking University [10,11] | waveguide + coaxial line | >600 (without coaxial line length) | microwave system + discharge chamber | 180 | permanent magnet ring | 40 | 21.7 |

antenna + coaxial line | <200 (without coaxial line length) | microwave system + discharge chamber | 200 | permanent magnet ring | 35 | 4.5 | |

Institute for Theoretical and Experimental Physics(ITEP) [12] | coaxial line | <200 (without coaxial line length) | microwave system + discharge chamber | 100 | permanent magnet ring | 15 | 0.5 |

CEA/Saclay [13] | waveguide | >1000 mm | extraction system | -- | focus coil | 50 | 48.5 |

Our work | waveguide | <350 mm | microwave system + discharge chamber | >300 | permanent magnet ring | 40 | >20 |

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content. |

© 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Yu, H.-F.; He, Z.-F.; Zhao, M.-H.; Wan, W.-S.; Liu, H.-L.; Wu, Y.; Lv, W.-Z.; Zhou, D.-Y.; Lu, H.-T.
Design of a Miniaturized Electron Cyclotron Resonance Ion Source for High-Voltage Proton Accelerator. *Appl. Sci.* **2023**, *13*, 8831.
https://doi.org/10.3390/app13158831

**AMA Style**

Yu H-F, He Z-F, Zhao M-H, Wan W-S, Liu H-L, Wu Y, Lv W-Z, Zhou D-Y, Lu H-T.
Design of a Miniaturized Electron Cyclotron Resonance Ion Source for High-Voltage Proton Accelerator. *Applied Sciences*. 2023; 13(15):8831.
https://doi.org/10.3390/app13158831

**Chicago/Turabian Style**

Yu, Hua-Fei, Zi-Feng He, Ming-Hua Zhao, Wei-Shi Wan, Huan-Ling Liu, Yue Wu, Wen-Zhuang Lv, Da-Yong Zhou, and Huan-Ting Lu.
2023. "Design of a Miniaturized Electron Cyclotron Resonance Ion Source for High-Voltage Proton Accelerator" *Applied Sciences* 13, no. 15: 8831.
https://doi.org/10.3390/app13158831