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\title{An 85-133.5 MH\lowercase{z} Broadband RF Power Amplifier for Synchrocyclotrons}

\author{H. R. Fu, X. W. Cai, J. B. Luo, Z. G. Qiao, Y. S. Chen,B. Li, X. R. Wang,\\X. D. Wang, X. F. Li, H. Chen, H. N. Meng, Y. Z. Shi, Z. Y. Wang\thanks{wangzhiyu01@spic.com.cn}
\\SPIC Nuelectronic Company Limited, Wuxi, China}
		
	
\maketitle

\begin{abstract}
   This paper presents the design and implementation of a 15 kW broadband radio frequency (RF) power amplifier operating from 85 to 133.5 MHz for synchrocyclotron applications. The amplifier employs a modular architecture, utilizing eight parallel 2 kW power amplifier modules combined through a high-power quadrature hybrid network. A comprehensive monitoring and control system ensures stable and reliable operation. Measurement results demonstrate that the amplifier achieves a saturated output power exceeding 15 kW with excellent in-band flatness of better than ±0.75 dB, the power amplifier maintains stable operation even under severe load mismatch conditions with a reflection coefficient exceeding 25\% (corresponding to a VSWR of 3:1), fulfilling the critical requirements for driving particle accelerators.
\end{abstract}


\section{Introduction}
Synchrocyclotrons utilize RF frequency modulation to achieve synchronous particle acceleration[1], which imposes strict requirements on the RF power amplifier: it must deliver high power across a broad frequency band while maintaining stable operation under dynamic load conditions. To meet these requirements, solid-state RF power amplifiers (SSPAs) are progressively replacing conventional vacuum tube technology. Benefitting from rapid advancements in semiconductor technology, SSPAs demonstrate significant advantages in stability, power adjustment flexibility, service life, and maintenance costs [2, 3], thereby making them an ideal choice for modern synchrocyclotrons.

However, achieving high power across a broad bandwidth while maintaining stability under load mismatch presents a key challenge[4, 5]. This paper presents a 15 kW broadband RF power amplifier operating from 85 to 133.5 MHz. The amplifier employs a modular architecture, combining eight 2 kW power amplifier modules through a high-power quadrature hybrid network. A central design goal was to ensure robust operation under a load VSWR of up to 3:1, enhancing reliability in practical operational environments. The design and measured performance validate the amplifier as a capable RF power source for modern synchrocyclotrons.

\begin{figure}[!htb]
   \centering
   \includegraphics*[width=1\columnwidth]{Chain}
   \caption{Topology of a Broadband RF Power Amplifier}
   \label{fig:Topology of a Broadband RF Power Amplifier}
\end{figure}

\section{The  Broadband Power Amplifier RF Circuits Design}


The topology of the Broadband RF Power Amplifier are illustrated in Fig. 1. The system operates as follows: An external RF input signal is first conditioned by a bandpass filter and then boosted to 4 W by a pre-amplifier module. This amplified signal is routed to a one-to-eight power divider, which distributes 0.5 W signals to each of the eight identical power amplifier modules. Each power amplifier module amplifies its input to 2 kW. The outputs from all eight modules are then combined by a 15 kW eight-to-one power combiner, yielding a total output power of no less than 15 kW. To ensure in-band amplitude/phase balance, enhance inter-module isolation, and minimize combining loss, both the power divider and combiner are implemented using a three-stage quadrature hybrid coupler architecture. For thermal management, a cooling system that combines liquid cooling and vapor chamber plates is employed to maintain stable circuit operation.


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\subsection{Power Amplifier Module Design}

The power amplifier module serves as the core component of the entire power amplifier system. In practical operation, it is required to exhibit high stability, maintainability, and field-replaceable. The physical layout of the module is shown in Fig. 2. It integrates all RF, power supply, control, and water-cooling interfaces on the rear panel, thereby reducing the overall footprint, enhancing integration density, and facilitating replacement and maintenance.

\begin{figure}[!htb]
   \centering
   \includegraphics*[width=0.8\columnwidth]{Physical Overview of the Power Amplifier Module.png}
   \caption{Physical Overview of the Power Amplifier Module}
   \label{}
\end{figure}


Each power amplifier module delivers a total output power of 2 kW by combining four internal amplifier units. It begins with a 0.5 W input signal, which is first split by an internal one-to-four power divider and directed to the 4 W driver stage of each unit. The signals are then fed into their respective 600 W final-stage amplifiers, and the combined output of these four units achieves the total 2 kW.

The 600 W power amplifier unit serves as the core of each module. Its design is built around NXP Semiconductors' MRFE6VP61K25H microwave power transistor. A planar balun is integrated with this transistor to provide broadband impedance matching and excellent in-band flatness. This configuration achieves a stable output power exceeding 600 W across the entire operating band, with a gain greater than 23 dB and an efficiency of over 70\%. 

Each 600 W power amplifier is safeguarded by an output circulator and absorption load. This subsystem absorbs all reflected power at any phase angle, thus safeguarding the amplifier from damage under a complete load mismatch and ensuring the stable operation and enhanced reliability of the entire module. 

The outputs of the four amplifier units are combined using a four-to-one power combiner to generate a total power exceeding 2 kW. This combiner is based on transmission-line transformer technology. The transformers are fabricated with NXO-100 (5*13) magnetic cores wound with semi-flexible SFF-50-3 RF coaxial cable.

The overall architecture of the power amplifier module is depicted in Fig. 3. As shown, the module integrates four parallel amplification chains, each consisting of a 4 W driver amplifier unit, a 600 W power amplifier unit, and a protective circulator. The outputs of these four chains are combined by a four-to-one power combiner, enabling the module to deliver a total output power of 2 kW. 

\begin{figure}[!htb]
   \centering
   \includegraphics*[width=0.8\columnwidth]{Overall Architecture of the Power Amplifier Module.png}
   \caption{Overall Architecture of the Power Amplifier Module}
   \label{}
\end{figure}

The amplifier module was characterized using an Keysight E8257D signal generator as the RF source and an Keysight N9020A signal analyzer. The analyzer's sweep range was set from 60 to 150 MHz. During testing, the signal generator was operated in a multi-point sweep mode across 85 to 133.5 MHz, employing a pulsed signal with a \SI{700}{\micro\second} pulse width, a 1 ms repetition period, and an input level of 15 dBm. A 50-ohm high-power attenuator was used to protect the spectrum analyzer from the amplifier's output. The measured data was properly compensated for the combined losses from the attenuator and cables to ensure accuracy. Fig. 4 characterizes the measured output power and gain of the power amplifier module, where the blue and red curves correspond to the output power and gain, respectively. 

\begin{figure}[!htb]
   \centering
   \includegraphics*[width=1\columnwidth]{moduel test result.png}
   \caption{Amplifier Module Test Result}
   \label{}
\end{figure}

As shown in Fig. 4, under a 70\% duty cycle continuous-wave operation with a 15 dBm RF input drive, the power amplifier module achieves an output power exceeding 63 dBm (2 kW) across the entire 85-133.5 MHz band. The measured in-band amplitude variation is maintained within ±0.65 dB, indicating excellent RF performance. 


\subsection{Power Combiner Design}

To achieve a total output power exceeding 15 kW from eight 2 kW amplifier modules, a high-power eight-to-one combiner was designed. As shown in Fig. 5, the combiner employs a three-stage quadrature hybrid architecture. Its eight input ports (Ports 1–8) use L29-KFD32 connectors for direct compatibility with the amplifier modules, while the output port (Port 9) features a 3-1/8" coaxial line to ensure stable high-power transmission. A water-cooling system is integrated to maintain excellent thermal stability during operation. 

\begin{figure}[!htb]
   \centering
   \includegraphics*[width=1\columnwidth]{Photograph of the 15 kW Power Combiner.png}
   \caption{Photograph of the 15 kW Power Combiner}
   \label{}
\end{figure}

The insertion loss between input ports (Ports 1-8) and the output port (Port 9) of the power combiner was measured using a Rohde \& Schwarz ZNB4 vector network analyzer (VNA). The VNA was configured to sweep from 85 to 133.5 MHz. Prior to testing, the instrument and all test cables were calibrated with an electronic calibration module. As presented in Fig. 6, the measured insertion loss for all paths is approximately 9 dB, which closely matches the theoretical value for an eight-to-one combiner. The results also demonstrate minimal variation across the operating band and excellent amplitude consistency among the ports. 

\begin{figure}[!htb]
   \centering
   \includegraphics*[width=1\columnwidth]{8 to 1 result.png}
   \caption{Measured Insertion Loss of the Eight-to-One Power Combiner Ports}
   \label{}
\end{figure}

\section{Monitoring and Control Module and Monitoring Software Design}

The RF power source monitoring and control module is built around an analog processing board and a control board, which integrate key system functions including control, monitoring, fault detection, protection, and safety interlocking. A host monitoring system complements this unit by providing a real-time display of the overall operational status. The module's architectural framework is depicted in Fig. 7.

\begin{figure}[!htb]
   \centering
   \includegraphics*[width=1\columnwidth]{Architectural framework of the monitoring and control unit}
   \caption{Architectural framework of the monitoring and control unit}
   \label{}
\end{figure}

The monitoring software serves as the host computer for the RF power amplifier system, performing core functions including status monitoring, operational control, and parameter configuration. It tracks key parameters such as analog signals (power, temperature, interlocks), amplifier module status, and power supply readings. Operational controls include system power, RF drive enable/disable, and reset functions. The software interface is shown in Fig. 8. 
\begin{figure}[!htb]
   \centering
   \includegraphics*[width=1\columnwidth]{Monitoring Software Interface.png}
   \caption{Monitoring Software Interface (a) Main Interface (b) Amplifier Modules 1-4 (c) Amplifier Modules 5-8 (d) DC Power Monitoring Interface}
   \label{}
\end{figure}
\section{System Architecture and Testing of the Power Amplifier}
The 15 kW power amplifier is housed within a standard 19-inch rack cabinet. A photograph of the fully assembled unit is presented in Fig. 9. The RF input port utilizes an N-type connector, while the output employs a 3-1/8" EIA flange. 
\begin{figure}[!htb]
   \centering
   \includegraphics*[width=1\columnwidth]{Photographs of the Fully Assembled Unit.png}
   \caption{Photograph of the Power Amplifier (a) Front View (b) Rear View}
   \label{}
\end{figure}

The amplifier was driven by an Agilent E8257D signal generator, and its output was analyzed by an Agilent N9020A spectrum analyzer with its sweep range set to 70–150 MHz.  The test procedure was as follows: the signal generator was configured in a multi-point sweep mode across the 85-133.5 MHz frequency band. The test signal had a pulse width of \SI{500}{\micro\second}, a repetition period of 1 ms, and an input level of 4 dBm. A 50-ohm high-power load was connected to the amplifier output, which was then coupled to the spectrum analyzer. The test setup for the RF power amplifier, under both a matched 50-ohm load and a mismatched load with a 25\% reflection coefficient, are shown in Fig. 10(a) and Fig. 10(b), respectively. The resulting output spectrum obtained from the analyzer is shown in the Fig. 11.

\begin{figure}[!htb]
   \centering
   \includegraphics*[width=1\columnwidth]{test photo.png}
\caption{ Test Setup with (a)Matched Load (b) Mismatched Load}
   \label{}
\end{figure}

\begin{figure}[!htb]
   \centering
   \includegraphics*[width=1\columnwidth]{test result}
   \caption{Power Amplifier Test Results}
   \label{}
\end{figure}

As shown in the Fig. 11, the power amplifier achieves a saturated output power of 15 kW (71.76 dBm) over the frequency range of 85–133.5 MHz, with in-band flatness better than ±0.75 dB, demonstrating excellent RF performance. Moreover, under a 25\% load mismatch condition (corresponding to VSWR = 3:1), the amplifier maintains an output power above 15 kW throughout a continuous 30-minute stress test without any damage, highlighting its strong resilience to high reflected power. 


\section{CONCLUSION}

This paper has presented the comprehensive design, implementation, and performance evaluation of a high-power, broadband RF power amplifier specifically developed for synchrocyclotron applications. The amplifier operates over the 85–133.5 MHz frequency band and successfully delivers a saturated output power exceeding 15 kW, with excellent in-band flatness of better than ±0.75 dB. Measurement results confirm that the amplifier meets all critical design specifications, demonstrating its suitability for driving demanding loads such as synchrocyclotrons. The successful development of this amplifier provides a reliable and high-performance RF power solution that can facilitate advanced research in fields like particle physics and medical isotope production.

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	\bibitem{IEEE}
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	\bibitem{IEEE}
		R. Tong \textit{et al}., "Kilowatt Power Amplifier With Improved Power Back-Off Efficiency for Cyclotron Application," \textit{IEEE Trans. Microw. Theory Tech,} vol. 70, no. 2, pp. 1401-1409, 2021. doi:10.1109/TMTT.2021.3134957

    \bibitem{IEEE}
		P. Marchand, "Review and prospects of RF solid state power amplifiers for particle accelerators", in \textit{Particle Accelerator Conf. (IPAC2017),} Copenhagen, Denmark, May 2017, pp. 2537-2542.


    \bibitem{IEEE}
		A. Barthwal, K. Rawat and S. K. Koul, "Dual Input Digitally Controlled Broadband Three-Stage Doherty Power Amplifier With Back-Off Reconfigurability," \textit{IEEE Trans. Circuits Syst. I Regul. Pap.,} vol. 68, no. 4, pp. 1421-1431, 2021.\\doi:10.1109/TCSI.2021.3050543
        
    \bibitem{IEEE}
        H. Hao \textit{et al}., "A 3.4–3.5 GHz mode‐reconfigurable power amplifier with high efficiency with load mismatch and back‐off," \textit{Microw. Opt. Technol. Lett.,} vol. 66, no. 2, p. 34041, 2024.\\doi:10.1002/mop.34041




	

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