Design of 83.2 MHz RF cavity for SKKUCY-10 cyclotron

Nuclear Inst. and Methods in Physics Research, A 939 (2019) 66–73

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Design of 83.2 MHz RF cavity for SKKUCY-10 cyclotron Jongchul Lee a , Mitra Ghergherehchi b , Seung-wook Shin b , Huisu Kim a , Donghyup Ha b , Ho Namgoong b , Ho Seung Song c , Jong-Seo Chai b ,∗ a

Sungkyunkwan University, WCU Department of Energy Science, 2066, Seobu-ro, Jangan-gu, Suwon-si, Republic of Korea Sungkyunkwan University, College of Information & Communication Engineering, 2066, Seobu-ro, Jangan-gu, Suwon-si, Republic of Korea c Advanced Research Technology Institute, Korea Atomic Energy Research Institute, Jeongeup-si, Republic of Korea b

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Keywords: RF cavity Half-wavelength Cyclotron Resonance frequency Coupling coefficient

ABSTRACT A vertical half-wavelength RF cavity was designed for SKKUCY-10 cyclotron. The design of the RF cavity was implemented in three stages as follows. Firstly, the structure of the cavity was modeled as an HWR without Dee geometry. Secondly, the Dee configuration including geometry was modeled, and finally, the power coupler, fine tuner, and the central region including ion source and puller were designed. For the first structure, the length of the resonator was selected as 560 mm to minimize the resonance frequency. For the second structure, the resonance frequency, unloaded quality factor (𝑄0 ) and electric field strength were simulated to be 83.2 MHz, 5830 and 7.8 MV/m, respectively. A coupling coefficient of (𝛽) 0.98 and characteristic impedance (𝑍0 ) of 50 𝛺 were obtained for the final structure. The RF cavity was fabricated based on design and resonance frequency, coupling coefficient were measured 83.2 MHz, 1.09, respectively. The reflection ratio of 5 ± 0.1% and cavity pick-up power of 0–0.12 μW were tested by increasing the input RF power from 0 to 9 mW. The specifications of the RF cavity showed that there is a good agreement between the simulation and experimental results.

1. Introduction In particle accelerators, an RF cavity can generate a sinusoidal wave of electric field with a specified resonance frequency. RF accelerators are divided into linear accelerators and circular accelerators and are classified into electron, proton, and heavy ion accelerators. For cyclotron RF cavity the resonance frequency and acceleration voltage can be up to a few tens of MHz and kV, respectively [1]. The RF cavities in a cyclotron can be divided two main categories: Half-Wavelength Resonator (HWR) and Quarter-Wavelength Resonator (QWR) [2]. The resonance frequency of HWR (half-wavelength 𝜆∕2) and QWR (quarterwavelength 𝜆∕4) depend on the resonator length. For the same resonance frequency, the size of a QWR is more compact compared to HWR, but the electric and magnetic fields of an HWR for the beam acceleration are symmetrical in the vertical direction, so we use HWR. The design of the RF cavity depends on various parameters such as the resonance frequency, number of Dee, accelerating harmonics, and operation mode of the cyclotron. The RF cavity of a classical cyclotron are used quarter-wavelength RF cavity, which consisted of a Dee, an external inductor, and coaxial line [2–4]. Azimuthally Varying Field (AVF) cyclotrons with a deep-valley electromagnet were developed in order to obtain a strong focusing force using 𝜆∕2 RF cavities [5–11]. At Ion Beam Applications (IBA, Belgium), the vertical RF cavity, which ∗

was perpendicular to the acceleration plane, was developed with specifications of 65.5 MHz resonance frequency and 50 kV Dee voltage for the Cyclone-30 that was developed in 1988 [5]. At TRIUMF in Canada, the horizontal RF cavities were developed with resonance frequency and Dee voltage of 73.3 MHz and 50 kV for the TR-30 in 1990 [6], and 73 MHz and 50 kV for TR-13, which was a prototype of TR-18, in 1993 [7]. Meanwhile in 1993, horizontal RF cavities with resonance frequencies of 27.2 MHz and 101 MHz were developed with 35 kV Dee voltage for two cyclotrons named PETtrace and MINItrace, respectively, at GE Healthcare in USA [12]. Also in 2002, the Korea Institute of Radiological and Medical Sciences (KIRAMS) developed a horizontal RF cavity for the cyclotron, named KIRAMS-13, with resonance frequency and Dee voltage of 77.3 MHz and 45 kV, respectively [8]; moreover, a 63.96 MHz, 50 kV vertical RF cavity was developed for KIRAMS-30 in 2007 [9]. In 2016, a new version of Cyclone-18 was launched by IBA with 42 MHz vertical RF cavity by increasing the Dee voltage up to 40 kV, which was called Cyclone Kiube [10]. A compact 9 MeV cyclotron was developed at Sungkyunkwan University (Korea), and it was called SKKUCY-9 [13]. The RF cavity of SKKUCY-9 was developed with specifications of 83.2 MHz resonance frequency, 40 kV Dee voltage, and 20 kW RF power [14–16]. The cavity structure used a vertical 𝜆∕2 resonator connected to two Dees with an resonator length of 680 mm, and the unloaded quality factor (𝑄0 ) was

Corresponding author. E-mail address: [email protected] (J.-S. Chai).

https://doi.org/10.1016/j.nima.2019.05.072 Received 21 March 2019; Received in revised form 13 May 2019; Accepted 23 May 2019 Available online 24 May 2019 0168-9002/© 2019 Published by Elsevier B.V.

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Nuclear Inst. and Methods in Physics Research, A 939 (2019) 66–73

Table 1 Specifications of the 10 MeV cyclotron. Design parameter Particle Beam energy Dimension of cyclotron Number of sectors Resonance frequency Harmonics Number of Dee Dee voltage Dee angle Type of ion source

Negative hydrogen 10 MeV Diameter 1500 mm, Height 815 mm 4 83.2 MHz 4th 2 40 kV 35◦ Internal PIG

optimized for 4500 of SKKUCY-9 [14]. Finally, in order to improve the RF stability, our team decided to design a new 10 MeV cyclotron with lower power consumption and lower surface electric field for the RF cavity. In this study, our design was implemented using 𝑄0 value analysis for HWR to increase the RF power efficiency. A new Dee geometry was applied to optimize the electric field and the resonance frequency and 𝑄0 values. Finally, the RF cavity was fabricated, and its experimental results were compared with the simulation data.

Fig. 1. Schemes of the conceptual design of RF cavity.

Eigenvalue analysis based on Maxwell’s equation was applied for the eigenmode simulation in the Cartesian vector plane. Maxwell’s vector field equation is shown in Eq. (2) [18].

2. Methods and materials

𝜎 (2) curl𝜇 −1 curl𝐸⃗ = 𝜔′2 (𝜀 − 𝑖 )𝐸⃗ 𝜔 where 𝜀 is the permittivity, 𝜇 is the permeability, and 𝜎 is the conductivity. The eigenvalue of 𝜔2 is calculated using Eq. (2), and the 1 ), where complex angular frequency (𝜔′ ) is estimated by 𝜔(1 + i 2𝑄 𝜔 is real value of angular frequency and 𝑄 is a quality factor. In Computer Simulation Technology Microwave Studio (CST-MWS) code, the eigenvalue is calculated by using several solver methods [19].

2.1. Conceptual design of RF cavity The design objectives of the 10 MeV cyclotron are shown in Table 1. The resonance frequency and Dee voltage were 83.2 MHz and 40 kV, respectively. The Dee angle was selected as 35◦ based on the beam dynamics analysis. Improvement of RF power efficiency can be achieved by increasing the 𝑄0 value. The design of the RF cavity was performed in three stages as follows. Firstly, the structure of the cavity was modeled as an HWR without Dee geometry. Secondly, the Dee configuration was modeled, and finally, the power coupler, fine tuner, and the central region including the ion source and puller were designed. In the first stage, the theorical RF characteristics were analyzed through conceptual design of the RF cavity with a half-wavelength 𝜆∕2 structure, which was compared with an ideal half-wavelength resonator. In the ideal HWR, the resonance frequency was calculated by using factor frequency: f. The factor f is equal pc∕2𝓁, where c is the speed of light, 𝓁 is the height of the HWR, and p = 1 for the dominant mode of the transverse electromagnetic (TEM) mode. The dominant mode was selected because the maximum electric field is generated at the middle of the HWR in the vertical direction. The stored energy in the cavity is proportional to the square of the electric field [17]. The 𝑄0 value depends on the Dee voltage under the same condition of cavity dissipation power. For the ideal HWR, 𝑄0 was calculated using Eq. (1). Q0 = 𝜔U∕Pc =

p𝜋 Rs

√

ln( ba ) 𝜇0 , p = 1, 2, 3 ⋯ ( ) 𝜀0 𝓁 1 + 1 + 4 ln( b ) a b a

2.2. Dee configuration of RF cavity The Dee configuration shown in Fig. 2 was positioned at the center of the HWR, and an accelerating electric field was generated between the Dee and the Liner. The energy gain (𝛥E) per turn for cyclotron depends on the Dee angle and harmonic number in Eq. (3) [20]. h𝜃 ) (3) 2 Where 𝑁𝑔 is number of acceleration gap, q is charge of particle (C), 𝑉𝐷𝑒𝑒 is Dee voltage (kV), h is harmonic number, 𝜃 is Dee angle (◦ ). The Dee angle with 4th harmonics was chosen as 35◦ and the electric field was analyzed based on the synchronization of phase slip between the magnetic field and the RF phase. In the ideal design of a 10 MeV cyclotron, the Dee angle was selected as 35◦ using a beam dynamics analysis. The magnetic field was designed to increase according to the cyclotron radius to satisfy the isochronous condition. Because of the coil power efficiency, the electromagnet was designed with a 60◦ hill angle to secure the magnetic field at a value of radius ranging from 290–345 mm; consequently, the Dee angle was decreased from 35◦ to 30◦ at this radius. The energy gains were calculated to be 150 keV for Dee angle of 35◦ and 139 keV for Dee angle of 30◦ by using Eq. (3). The phase slip caused by the reduction of the energy gain at the cyclotron radius ranging from 290–345 mm was optimized by the shimming bar of the electromagnet. The radius of Dee was selected as 332 mm to cover the collimated extraction beam for 10 MeV negative hydrogen beam by r = 𝑚0 𝛽𝑐∕𝑞𝐵0 , where 𝑚0 is the rest mass, 𝛽𝑐 is the velocity of the particle, and 𝐵0 (T) is the average magnetic field. 1 is The details of the RF cavity are shown in Fig. 3. In Fig. 3(a), ⃝ 2 is Dee, which is designed with the vertical offset of the the Liner, ⃝ 3 is the puller in the central region where the Dees acceleration region, ⃝ are connected with each other, which pulls and accelerates particles 4 is the Stem, which connects the Dee and Liner. from the ion source. ⃝ 𝛥E = 𝑁𝑔 q𝑉𝐷𝑒𝑒 sin(

(1)

where 𝜔 = 2𝜋f, U is the stored energy, Pc is the cavity dissipation power, and Rs is the √ surface resistance [17]. The surface resistance is expressed by Rs = 𝜇0 𝜔∕2𝜎, 𝜎 is the conductivity of the material due to the skin effect, and 𝜀0 and 𝜇0 are the permittivity and permeability in vacuum, respectively. The variables elements a and b are the inner radius and outer radius, respectively. The conceptual structure of the RF cavity is shown in Fig. 1. The conceptual design consists of two vertical HWRs, which are away from the center because of the acceleration region, and the maximum distance between the two HWRs was limited by the optimum cyclotron radius. Two horizontal inner conductors for Dee are connected to the middle of the vertical inner conductor to generate an electric field. 67

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Fig. 2. Dee geometry inside the electromagnet (cross-section view).

1 Liner; ⃝ 2 Dee; ⃝ 3 Puller; ⃝ 4 Stem. Fig. 3. Details of RF cavity ⃝

through beam dynamic analysis of the incident negative hydrogen beam.

By considering the Dee geometry inside the conceptual design, the accelerating electric field is generated at the space between the Dee and the Liner; this electric field cause the extra capacitance in the RF cavity. The √ additional capacitance reduces the resonance frequency by f = 1∕(2𝜋 LC), where L is the inductance and C is the capacitance. The capacitance between the Dee and Liner is expressed as C = 𝜀0 𝜀𝑟 𝐴∕𝑑, where 𝐴 is the Dee thickness and 𝑑 is the distance gap between Dee and Liner. The maximum electric field was considered in the Dee geometry by applying the Kilpatrick criterion [21]. The Kilpatrick electric field 𝐸𝐾 was calculated to be 83.2 MHz using Eq. (4), and if the simulated electric field (𝐸) by CST-MWS code is bigger than 𝐸𝐾 , then an RF spark is caused practically. 2 −8.5∕𝐸𝐾 f = 1.64𝐸𝐾 𝑒

The power coupler for RF power transmission from RF amplifier was located at the right side of the Dee, and it had a characteristic impedance of 50 Ω and a rigid coaxial line with a standard size of 79 mm. The RF window was designed under high vacuum condition (10−7 mbar), and it was selected using Alumina (Al2 O3 ) with a relative permittivity of 9.4. The pick-up probe was designed to measure RF signals. The power coupler was optimized with critical coupling (𝛽 = 1) to minimize the reflection power in Eq. (5) [22]. √ 1 ± 𝑃𝑟 ∕𝑃𝑓 𝑄 𝛽= 0 = (5) √ 𝑄𝑒 1 ∓ 𝑃𝑟 ∕𝑃𝑓

(4)

where 𝑄0 is unloaded quality factor, 𝑄𝑒 is external quality factor, 𝑃𝑟 ∕𝑃𝑓 is ratio between the reflected and forward RF powers. The over-coupled, under-coupled state are used upper and lower sign, respectively in Eq. (5). The position of pick-up probe was considered the amount of transmitted power from the cavity dissipation power. The power ratio of reflection and transmission for the power coupler and pick-up probe are calculated using the scattering parameters, namely reflection coefficient (𝑆11 ) and transmission coefficient (𝑆21 ), by using 𝑃 ,𝑃 the expression 𝑆11,21 = 10 log10 1𝑃 2 .

The electric field and magnetic field regions are shown in Fig. 3(b). For 𝜆∕2 resonance mode the electric field is generated at middle of vertical inner conductor and the magnetic field is induced around Stem with opposite phase of the electric field. The Dee and Stem geometries were optimized for 83.2 MHz based on the conceptual design of RF cavity. 2.3. Design of power coupler, pick-up probe, fine tuner, and central region

1

Fig. 4 shows the schematic of the power coupler, pick-up probe, fine tuner, and central region in the RF cavity system. The central region, power coupler, and fine tuner generate an additional electric field by adjusting the Dee position, and the resonance frequency is decreased due to an additional electric field. Therefore, the RF cavity geometry was optimized to compensate the reduction in resonance frequency. The position of the ion source and the puller structure were designed

The capacitive type fine tuner produces a constant resonance frequency during cyclotron operation, and the tuner is electrically grounded. The fine tuner was focused on the frequency tuning range with a coupling coefficient. The process of optimization of the power coupler and fine tuner was performed during modification of the cavity geometry. 68

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Fig. 4. Scheme of power coupler, pick-up probe, fine tuner, and central region.

Fig. 5. 𝑄0 and resonance frequency trends for the conceptual design in terms of (a) a and (b) 𝓁 values.

Fig. 6. Resonance frequency and 𝑄0 trends according to the Dee structure.

3. Simulation result of RF cavity

Desired resonance frequency (83.2 MHz) was happened at 𝓁 = 1400 mm, and at the 𝑎 = 0.1 mm in Fig. 5(a) and (b), respectively. But in the conceptual design highest 𝑄0 was main parameter for the consideration of our RF cavity. In Fig. 5(a), the constant 𝓁 value was selected as 560 mm due to the limitation of maximum length of valley space and the minimum inner radius a was chosen as 15 mm, because of low 𝑄0 value. From Fig. 5(a), maximum 𝑄0 was achieved when the inner radius was 30 mm for ideal HWR and the conceptual design of RF cavity. The resonance frequency trend was constant for the ideal design; however, the resonance frequency trend of the conceptual design increased from 195 to 244 MHz when the inner radius was increased from 15 to 50 mm. In Fig. 5(b), the inner radius was selected as 30 mm based on the highest 𝑄0 , and the minimum 𝓁 value was chosen as 250 mm in order to keep the vertical acceleration region. Fig. 5(b) shows the results of 𝑄0 and resonance frequency according to

3.1. Result of conceptual design of RF cavity The conceptual design of two HWR cavities is more complicated than that of a single HWR, so the 𝑄0 and resonance frequency values were analyzed using CST-MWS eigenmode solver. Fig. 5(a) and (b) show the theoretical (ideal HWR) and simulation (conceptual design) values of 𝑄0 and resonance frequency in terms of the inner radius (a) and length of the resonator (𝓁). In Fig. 5, because of the limitation of valley geometry, we chose the outer radius (b value) to be constant (100 mm) for both ideal HWR and the conceptual design of RF cavity, and the surface resistance (Rs ) was calculated for the copper conductivity (𝜎) of an ideal HWR. 69

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Fig. 7. Resonance frequency and 𝑄0 trends according to the Stem structure.

Fig. 8. Field distribution result by Eigenmode solver (a) Dee voltage (b) Angle of acceleration gap and FWHM of electric field.

the variable length of the resonator 𝓁. The 𝑄0 and resonance frequency values decreased when the 𝓁 value was increased in both the ideal design and the conceptual design. For the conceptual design minimum resonance frequency was obtained 198.6 MHz at the maximum length (𝓁) 560 mm to minimize the resonance frequency amount.

According to Kilpatrick’s limitation, the Kilpatrick electric field (𝐸𝐾 ) was calculated to be 10 MV/m at 83.2 MHz. A low electric field was required for avoiding of RF spark by 40 kV of Dee voltage with a continuous wave operation mode. Therefore, the electric field was simulated at 7.8 MV/m on the Dee surface with a Dee–Liner gap distance of 5 mm. The Dee thickness were selected for as 22 mm with Stem radius for resonance frequency. Fig. 7 shows the resonance frequency result according to the Stem radius and position. The resonance frequency and 𝑄0 trends increased linearly with increase in the Stem radius. The Stem radius was selected as 16 mm for 83.2 MHz in order to achieve optimization. Fig. 7(b) shows the resonance frequency and 𝑄0 characteristics according to the Stem position from the central region. The Stem position was selected as 248 mm for the highest 𝑄0 5830.

3.2. Result of dee configuration of RF cavity Fig. 6 shows the resonance frequency and 𝑄0 results according to the Dee geometry, including (a) Dee thickness and (b) Dee–Liner gap distance. The initial Dee thickness and Dee–Liner gap distance were selected 20, 6 mm, The resonance frequency varies √ respectively. √ according to f ∝ 1∕ 𝐶 ∝ 𝑑∕𝐴, and the resonance frequency was reduced from 198.6 MHz (without Dee geometry) to around 83.2 MHz. 70

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Fig. 9. Power coupling result by Frequency domain solver (a) 𝑆11 , 𝑆21 parameter and (b) Smith chart.

probe, fine tuner, and central region. Fig. 9 shows the results of (a) the scattering parameters (𝑆11 , 𝑆21 ) through the power coupler and pick-up probe and (b) the characteristic impedance (𝑍0 ) in the Smith chart. 𝑆11 and 𝑆21 were obtained as −37.9 dB and −45.5 dB, respectively by using the CST-MWS code, and the ratio of the reflected power to the input power was obtained as 0.01%. The critical coupling (𝛽) was achieved by characteristic impedance (𝑍0 ) 50 Ω at 83.2 MHz with Smith chart. Fig. 10 shows the simulation results of the resonance frequency and 𝛽 behaviors according to the tuner gap distance. The resonance frequency and 𝛽 values were increased by increasing the tuning gap distance, and the 𝛽 value was slightly increased by reducing 𝑄𝑒 . The initial tuning gap distance, which ranges between 4–9 mm at 83.2 ± 0.5 MHz, was selected as 5.5 mm and the 𝛽 value was optimized as 0.98. The values of the resonance frequency and 𝑄0 in the simulation were achieved at 83.2 MHz and 5830, respectively, using the primary design values of the resonator length (𝓁) of 560 mm, Dee angles ranging from 35–30◦ , gap distance between Dee and Liner of 5 mm, Dee thickness of 22 mm, Stem radius of 16 mm, and Stem position of 248 mm. The cavity dissipation power was calculated to be 12.4 kW at the Dee voltage of 40 kV, and the surface electric field was 7.8 MV/m at the side surface of the Dee. The Dee voltage was distributed to ±0.5 kV at the cyclotron radius, and the average angle of the acceleration gap and the FWHM of the electric field were calculated at 39◦ in the range of 50–32◦ and 18 mm in the range of 16–20 mm, respectively. The power coupler was designed based on a standard size of the rigid coaxial line (79 mm) the coupling coefficient (0.98) and characteristic impedance (50 Ω). The variable resonance frequency was covered ±0.5 MHz by a tuning gap distance ranging from 4–9 mm, and the pick-up probe with a power attenuation of −45.5 dB. The results of the RF specifications for the cavity are listed in Table 2.

The electric field, Dee voltage, and angle of acceleration gap are shown in Fig. 8. Fig. 8(a) shows the Dee voltage distribution according to the cyclotron radius. If the Dee voltage is less than 40 kV, the beam will not get enough energy to accelerate the particles. The Dee voltage was decreased when the cyclotron radius was increased because of the Stem position. The cavity dissipation power was calculated to be 12.4 kW at 40 ± 0.5 kV Dee voltage by using the CST-MWS code. Fig. 8(b) shows the angle of acceleration gap and the Full Width Half Maximum (FWHM) of electric field. The angle of acceleration gap was obtained for the cyclotron radius range (50∼350 mm), and the average angle was estimated to be 39◦ . Because the Dee angle was decreased at the values of the radius ranging from 290 mm to 345 mm, the angle of acceleration gap was decreased from 36◦ to 32◦ . The energy gain of the beam per turn for electric field was calculated as 157 keV. The FWHM was distributed between 16 to 20 mm depending on the cyclotron radius, and the average FWHM was selected 18 mm. 3.3. Results of power coupler, pick-up probe, fine tuner, and central region The power coupler and fine tuner were optimized using the resonance frequency and coupling coefficient (𝛽) values by frequency domain solver. The resonance frequency of RF cavity was matched as a 83.2 MHz including the geometries of power coupler, pick-up

Table 2 Main RF specifications for cavity design.

Fig. 10. Resonance frequency and 𝛽 according to the fine tuner by simulation code.

71

Design parameter

Simulation result

Resonance frequency [MHz] Unloaded quality factor Cavity dissipation power [kW] Dee voltage [kV] Surface electric field at Dee [MV/m] Angle of acceleration gap [◦ ] FWHM of electric field [mm] Frequency tuning range [MHz] Coupling coefficient [arb.u.] Characteristic impedance [Ω] Attenuation of pick-up probe [dB]

83.2 5830 12.4 40 7.8 39 18 ±0.5 0.98 50 −45.5

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Fig. 11. Fabricated RF cavity and RF measurement system.

Fig. 12. Experimental result of RF cavity by using network analyzer (a) 𝑆11 parameter (b) Smith chart.

Fig. 13. Experimental result of RF signal (a) pick-up signal obtained using spectrum analyzer (b) transmission rate and pick-up power.

4. Fabrication and experimental result of rf cavity

and the experimental process was continued without magnetic field and vacuum implementation. The simulation was performed with ideal vacuum condition, for overcome the problem of the vacuum condition in the experimental process the position of coupler slightly was changed to get the same condition as simulation. Fig. 12 shows the results of the scattering parameter and Smith chart using the network analyzer. The minimum 𝑆11 parameter and the characteristic impedance were measured to be −27.1 dB and 50 Ω, respectively, with 0.2% reflection ratio and 1.09 coupling coefficient. Fig. 13(a) shows the result of the pick-up probe as a spectrum, which was measured using the spectrum analyzer. Peak amplitude at 83.2 MHz was measured −56.3 dBm, but the signal amplitude was calculated to be −48.1 dB because of the power loss in the RF cable. The

Fig. 11 shows (a) the fabricated RF cavity and (b) the details of the measurement system. Oxygen-free copper, C10200 [23], was used for the RF cavity, which was divided to the brazed Dee and Liner. The skin depth of the Dee’s surface was calculated to be 7.25 μm √ with 𝛿 = 1∕𝜎𝜇𝜋𝑓 , where the electrical conductivity 𝜎 of copper is 58 MS/m [24]. Therefore, the surface roughness of the Dee should be less than the skin depth, which is achieved through polishing. The Dee, Stem, and power coupler were cooled by water-cooling system. The RF cavity was installed inside the magnet including the Dee, Liner, ion source, power coupler, pick-up probe, and fine tuner. The RF specification was achieved using a network and spectrum analyzers 72

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the simulation and experimental results. In the next step, the 𝑄0 will be measured by using a set-up vacuum condition and RF conditioning can achieve by correction vacuum condition and magnetic field. Finally, vacuum tube amplifier will be replaced by a solid-state amplifier. Acknowledgments This work was supported by the Radiation Technology R&D program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2017M2A2A4A02020347). References [1] [2] [3] [4] [5] [6] Fig. 14. Experimental result of frequency tuning by fine tuner. [7] [8]

reflection ratio 5 ± 0.1% and the cavity pick-up power 0∼0.12 μW were measured by increasing the RF input power from 0 to 9 mW, which is illustrated in Fig. 13(b). The tuning gap by network analyzer for the resonance frequency between 83–84 MHz was shown in Fig. 14. The error discrepancy of the resonance frequency for the simulation data and experimental result was ±0.1 MHz, which indicates that there is good agreement between the experimental data and simulation result.

[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

5. Conclusion and discussion A vertical half-wavelength (𝜆∕2) RF cavity was designed and fabricated for the 10 MeV cyclotron. The conceptual design based on the vertical HWR was simulated for 83.2 MHz in the 10 MeV cyclotron, and our design was found to reduce the consumption power and electric field strength. The specifications of the power coupler, pick-up probe, fine tuner, and central region were optimized by using CST-MWS code. The resonance frequency, 𝑄0 , and the electric field strength were simulated to be 83.2 MHz, 5830, and 7.8 MV/m, respectively. The cavity dissipation power was 12.4 kW for the 40 kV Dee voltage. The RF cavity was fabricated and found to have good agreement between

[24]

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