Preamble

Development of a Compact X-Band Dual-Mode Medical Linac Based on a Pulse Compressor

Haipeng Zhi,¹,² Jiaru Shi,¹,²,* Hao Zha,¹,² Jian Gao,¹,² Qiang Gao,¹,² and Huaibi Chen¹,²
¹Department of Engineering Physics, Tsinghua University, Beijing, 100084, China
²Key Laboratory of Particle and Radiation Imaging (Tsinghua University), Ministry of Education, Beijing 100084, China

In this study, we developed a compact X-band dual-mode electron linear accelerator (linac) for medical radiotherapy capable of generating both 100 mA / 6 MeV beams for X-ray radiotherapy (low-energy mode) and 10 mA / 13.5 MeV beams for electron radiotherapy (high-energy mode). The dual-mode linac employs a 3 MW multi-beam X-band klystron as the power source, with a total length of 1.5 m. A novel spherical SLED pulse compressor was specially designed for the high-energy mode to achieve flat-top output, with a gain factor of 2.2. Mode switching between the two operating configurations is realized through dynamic tuning of the pulse compressor's resonant frequency. Furthermore, a standing-wave (SW) bi-periodic accelerating structure operating in π/2 mode was developed, which enables effective bunching and acceleration of electron beams under both energy modes. A comprehensive experimental platform has been established for the linac, and high-power RF tests have been conducted. The experimental results validate the feasibility of the dual-mode linac, demonstrating its potential for versatile radiotherapy applications with optimized spatial compactness.

Keywords: Dual-mode linac, SLED pulse compressor, Standing-wave accelerating structure, High-power RF experiment.

Introduction

Cancer remains a pressing global challenge. According to the 2024 statistical report by the International Agency for Research on Cancer (IARC), 2022 witnessed over 20 million new cancer cases and 9.5 million cancer-related deaths globally \cite{1}, highlighting the urgent demand for advanced therapeutic modalities. Radiotherapy, employed as primary treatment or in combination with chemotherapy, surgery, or other modalities for over 60% of cancer patients, contributes to nearly 50% of curative outcomes \cite{2}, underscoring its pivotal role in oncology. Innovations in radiotherapy technologies not only hold profound scientific significance but also generate substantial economic benefits, thereby making critical contributions to the advancement of global health.

The 6 MeV low-energy medical electron linac, designed for X-ray radiotherapy, features a compact configuration and low treatment costs, enabling its widespread adoption in public cancer care \cite{3-7}. However, for superficial tumors amenable to electron therapy, such as breast, skin, and thyroid cancers, low-energy medical linacs are constrained by their RF power sources, failing to produce electron beams with higher energy \cite{8,9}. Furthermore, most prior medical linacs engineered to produce higher-energy electron beams (10–20 MeV) have relied on high-energy RF power sources, resulting in elevated costs, complex system configurations, and stringent maintenance requirements \cite{10}. Thus, there is an urgent demand for medical electron linacs that combine the compactness and cost-effectiveness of low-energy systems with the capability of accelerating higher-energy electron beams, especially for the development of cancer treatment capabilities in less developed regions.

Since its inception in the 1970s, the pulse compressor has been widely adopted in diverse accelerators \cite{11-14}. By compressing pulse width to enhance pulse power, it overcomes the power limitations of RF power sources, enabling higher-energy electron acceleration \cite{15-17}. Specifically, integrating pulse compressors with low-energy medical electron linacs holds promise for generating electron beams exceeding 10 MeV. Moreover, by dynamically tuning the resonant frequency of the pulse compressor, switching between low-energy and high-energy modes can be achieved. Meanwhile, X-band high-gradient technology has progressively matured in recent years \cite{18-20}. Compared with conventional S-band and C-band systems, X-band accelerating structures exhibit higher shunt impedance \cite{21,22}. This facilitates the compact design of medical electron accelerators with substantially lower spatial requirements. Although extensive research has been conducted on the development of X-band medical accelerating structures, existing efforts are largely confined to the design of accelerating structures themselves \cite{4-7}, with no reported studies on integrating pulse compressors with X-band accelerating structures to develop X-band dual-mode medical electron linacs.

Based on the above analysis, we have developed a compact X-band dual-mode medical electron linac integrated with a SLED pulse compressor, enabling seamless switching between X-ray and electron therapy. The linac employs a 3 MW multi-beam X-band klystron to accelerate electrons to 6 MeV for X-ray radiotherapy, while the SLED pulse compressor enhances the RF power to generate 13.5 MeV beams for electron radiotherapy. An X-band SW bi-periodic accelerating structure operating in π/2 mode was developed to accommodate the bunching and acceleration of electrons with distinct pulse currents and energies required for the two operating modes. Precise regulation of beam current between the two operating modes was achieved via a grid-controlled thermal DC gun, with the emission current adjustable in the range of tens to hundreds of milliampere.

This paper presents a comprehensive overview of the dual-mode medical linac. Section II and Section III detail the development of the X-band SLED pulse compressor and the X-band SW accelerating structure. The high-power RF experiment setup and the experimental results are described in Section IV, with conclusions provided in Section V.

II. X-Band Spherical Pulse Compressor

The pulse compressor is a core component to achieve 13.5 MeV electron acceleration. Compared with active pulse compressors, passive ones not only exhibit stronger operational robustness and superior stability \cite{23} but also feature the advantage of compactness in their SLED-type structure, making them better suited to the application requirements of the dual-mode linac. Consequently, we have developed a compact spherical 9.3 GHz SLED pulse compressor based on a dual-mode RF polarizer \cite{14,15}.

A. RF Design and Analysis

Theoretical determination of physical parameters for the pulse compressor was first conducted based on accelerator theory \cite{23}. In a linear accelerator, the kinetic energy gained by electrons can be expressed as:

$$W = \sqrt{Z_{eff} P L}$$

where $Z_{eff}$ is the longitudinal effective shunt impedance, $P$ is the input RF power and $L$ denotes the longitudinal length of the cavity chain. For a 0.3 m X-band SW accelerating structure with a typical shunt impedance of 120 MΩ m⁻¹ \cite{19}, Eq. (1) demonstrates that an input RF power of 5.1 MW is required for the accelerating structure to achieve an electron energy gain of 13.5 MeV. Estimates based on the insertion loss of each microwave component indicate a power transmission loss of approximately -1 dB. Based on this, the output power of the pulse compressor was ultimately specified as 6.5 MW, corresponding to a gain factor of 2.2.

Given its higher intrinsic quality factor ($Q_0$) compared to cylindrical resonant cavities \cite{15}, a spherical resonant cavity was designed for the pulse compressor, which operates in TE₁₁₄ mode with an approximate $Q_0$ of 10⁵. The SLED pulse compressor functions primarily through the energy storage and release of the resonant cavity, whose time-domain equation can be expressed as:

$$\frac{dE_e}{dt} + \frac{E_e}{\tau_c} = \alpha E_{in}$$

where $E_{in}$ denotes the amplitude of the input field, $E_e$ represents the emitted field from the cavity, $\tau_c = 2Q_0/\omega(1 + \beta)$ is the filling time constant of the cavity, $\beta$ is the coupling coefficient and $\alpha = 2\beta/(1 + \beta)$ characterizes the steady-state ratio coefficient of $E_e$ to $E_{in}$. For a square-wave input $E_{in}$, $E_e$ can be expressed as:

$$E_e(t) = \alpha E_{in}(1 - e^{-t/\tau_c})$$

indicating that $E_e$ tends to the steady-state value $\alpha E_{in}$. As $P \propto E^2$, the gain factor can be specified as $\alpha^2$, so a coupling factor of $\beta = 3.2$ is required to achieve the target power gain of $\alpha^2 = 2.2$.

The amplitude of the total output field $E_{out}$ before phase inversion is the superposition of reflected field $\Gamma E_{in}$ and emitted field $E_e$:

$$E_{out} = E_{in}[\alpha(1 - e^{-t/\tau_c}) + \Gamma]$$

where the reflection coefficient $\Gamma$ can be derived from the coupling factor $\beta$:

$$\Gamma = \frac{1 - \beta}{1 + \beta}$$

When the coupling coefficient $\beta > 1$, $\Gamma$ is negative, indicating that $E_e$ and $E_{in}$ cancel each other. According to Eq. (4) and Eq. (5), the relative relationship between the input and output power of the resonant cavity before phase inversion can be calculated, as demonstrated by Fig. 1 [FIGURE:1] (a). Initiating phase reversal at $t = 4$ µs optimizes the compression ratio of the pulse compressor by leveraging the fully established stored energy.

Subsequently, output pulse width of the pulse compressor was determined. Amplitude of the electric field in the SW accelerating structure increases exponentially with time, which can be expressed as follows:

$$E_{steady} = 1 - e^{-\tau t}$$

where $\tau = 2Q_L/\omega$ is the field buildup time and $Q_L$ is the loaded quality factor. For the high-energy mode with a small pulse current of 10 mA, $Q_L \approx Q_0/2$. Drawing on the experiences of developing X-band 9.3 GHz SW accelerating structures at Tsinghua University \cite{18,21}, $Q_0$ of the accelerating structure is approximately 7000. Thus, $Q_L \approx 3500$ and the field buildup time $\tau \approx 120$ ns. The electric field reaches more than 90% of the steady-state field after 300 ns and the output pulse width was determined to be 500 ns to fully meet the requirements of high-energy mode operation. Consequently, a compression ratio of 8 was calculated based on the 4 µs input pulse and the 0.5 µs output pulse.

In summary, the physical parameters of the pulse compressor are listed in table 1 [TABLE:1], while the input field $E_{in}$, emitted field $E_e$ and total output field $E_{out}$ are shown in Fig. 1 (b). The 4 µs-duration flat-top input pulse charges the pulse compressor and the dip at $t = 4$ µs corresponds to the phase reversal-trigger, initiating the energy discharge from the resonant cavity. Unlike most pulse compressors, the inverted RF power in this pulse compressor does not further enhance output power. Instead, an amplitude-modulating phase inverter compensates for power decay during the discharge process, achieving flat-top output.

RF simulations were conducted using Ansys HFSS. For efficient RF power transmission, a compact dual-mode RF polarizer was developed for the pulse compressor \cite{14,15}. Its RF characteristics emulate a 3dB coupler, with two orthogonal TE₁₁ modes in cylindrical port replacing the traditional two ports of the 3dB coupler, reducing the number of SLED cavities from two to one. Additionally, the butterfly-shaped structure eliminates the symmetric cylindrical waveguide, achieving 40% vertical dimension reduction compared to conventional designs. The coupler optimization followed the CERN-proposed $F_{max}$ function \cite{15}, which has been optimized to -95 dB, ensuring minimal power leakage. Separate tuning of the two polarization modes was critical to ensuring operational consistency. A single-mode RF polarizer dedicated to independently measuring the S-parameters of the two polarization modes has been developed. By coupling the single-mode RF polarizer to the spherical resonant cavity at orthogonal orientations, the S-parameters of the two orthogonal TE₁₁₄ modes could be characterized.

The vacuum model and electric field magnitude of the complete pulse compressor are presented in Fig. 1 (c), with all simulated RF parameters matching the design specifications. In particular, at an input power of 3 MW, the maximum surface electric field occurring at the coupling aperture was only 19 MV m⁻¹. This low-field design significantly reduces the risk of vacuum breakdown and improves the operational robustness of the pulse compressor \cite{25}.

Table 1. Physical parameters of X-band SLED pulse compressor.

Parameter Value Resonant Frequency 9.3 GHz Input RF Power 3.0 MW Intrinsic Quality Factor 1.0 × 10⁵ Coupling Factor 3.2 Input Pulse Width 4.0 µs Output Pulse Width 0.5 µs Compression Ratio 8 Power Gain 2.2

B. Lower-Power RF Experiment

Cold test of the pulse compressor was performed using a Rohde & Schwarz Vector Network Analyzer (VNA). Because of the cylindrical waveguide, direct measurement of the dual-mode RF polarizer was infeasible. Thus, the cylindrical waveguide was terminated with a metallic short, converting the RF polarizer into a two-port network. Within the frequency range of 9.25-9.35 GHz, both $S_{11}$ and $S_{22}$ parameters remained below -30 dB, while $S_{12}$ and $S_{21}$ exceeded -0.15 dB, demonstrating satisfactory performance of the dual-mode RF polarizer. The single-mode RF polarizer was employed to measure the frequency separation of two orthogonal TE₁₁₄ polarization modes in the spherical resonant cavity. After tuning, the frequency separation between the two modes was finally reduced to less than 100 kHz. The comprehensive cold test results following pulse compressor welding confirmed excellent consistency between the measurements and simulations.

Leveraging the linear response characteristics of the pulse compressor, low-power RF experiment was conducted to evaluate its performance, with the experimental setup illustrated in Fig. 2 [FIGURE:2] (a) and Fig. 2 (b). As the power source for the low-power RF experiment, the low-level RF (LLRF) system includes an excitation source, a solid-state amplifier (SSA), a phase inverter, and a trigger source. The excitation source, serving as the primary generator of microwave signals, provides initial RF excitation for the system. The SSA is integrated with the excitation source to amplify the microwave signal, serving as the klystron driving power. It was not activated during low-power RF experiment. The phase inverter performs dual functions: phase inversion and amplitude modulation of RF signals. And the trigger signal source synchronizes the entire LLRF system, ensuring precise timing coordination among components for coherent operation. To keep consistency with high-power test conditions, the pulse compressor operated under low-vacuum conditions, maintained by a mechanical pump, during the low-power RF experiment. A Rohde & Schwarz NRP-Z81 power meter was employed for measurement of the compressed pulse.

The experimental results are shown in Fig. 2 (c), with the input pulse consisting of a 4 µs flat-top signal and an 500 ns inverted compensation signal. With an input pulse power of 290 µW, an output pulse power of 660 µW was successfully measured, achieving the predetermined power gain. However, the phase inverter demonstrated suboptimal performance, with the modulation of the inverted signal exhibiting abrupt transitions. The imperfect performance of the phase inverter directly impacted output waveforms of the pulse compressor, causing the output waveforms to exhibit ripples. Additionally, at the theoretical phase inversion time of 4 µs, a spike with a pulse width of approximately 100 ns was observed. This was likely attributed to the slow response of the phase inverter, which failed to immediately invert the input field, leading to the abnormal output field \cite{26}. Because of the spike, the measured usable pulse width was only 400 ns, narrower than the designed 500 ns, but still satisfied the high-energy mode operation requirements of the linac. Further optimization of the output pulse necessitates improving the performance of the phase inverter. Nevertheless, from the perspective of principle feasibility verification, the current experimental results already meet the requirements for high-power RF experiments.

III. X-Band SW Accelerating Structure

In this section, based on a grid-controlled thermal DC gun, we developed an X-band 9.3 GHz SW accelerating structure operating in π/2 mode, which is capable of generating both 100 mA / 6 MeV and 10 mA / 13.5 MeV electron beams. The pulse current is adjusted by changing the voltage of the DC gun. The capture ratio and acceleration efficiency of the structure were systematically investigated in different operating modes to optimize its performance.

A. RF Design and Analysis

Structural compactness represents the pivotal advantage of the dual-mode medical linac. The accelerator adopted an X-band 9.3 GHz π/2 mode bi-periodic SW accelerating structure, which maximizes the frequency separation between adjacent resonant modes while improving the acceleration efficiency \cite{24}. In addition, nose-cone structures were incorporated to improve the longitudinal shunt impedance, and magnetic coupling holes were integrated into the cavity chains to enable efficient power transmission. These designs collectively contribute to improving the compactness of the accelerating structure.

The bunching section, consisting of four bunching cavities, was designed through iterative calculations of field distribution and beam dynamics. To maintain structural compactness, the accelerating structure employs a single-feed SW design. However, this introduces an inherent limitation: under the two operating modes with different input powers, the bunching section maintains consistent normalized field distributions while the amplitude scales with the $P \propto E^2$ relationship, leading to significant disparities in bunching efficiency between the two modes. To mitigate this discrepancy, we adjusted the gun voltage and emission current, aiming to identify an appropriate bunching section field distribution that achieves acceptable capture ratios for both modes. Concurrently, the iris radius was increased from 1.5 mm to 2.5 mm to reduce beam transport losses. Distinct from conventional low-energy electron linacs, the high-energy mode represents the key innovation of the new linac, so the bunching section prioritized the optimization for 13.5 MeV electron beams, achieving electron capture ratios of 22% in low-energy mode and 35% in high-energy mode. The DC gun parameters were ultimately determined as follows: 8 kV gun voltage and 500 mA emission pulse current for the low-energy mode; and 10 kV gun voltage and 30 mA emission pulse current for the high-energy mode.

Furthermore, an external waveguide coupler was integrated into the cavity chain to facilitate efficient RF power coupling. The coupler cavity adopts a single rectangular waveguide port feeding structure, with the external coupling coefficient $\beta$ set to 1.1 to minimize reflected power. This design prioritizes power matching for high-energy mode operation, which features a small pulse current of 10 mA but demands higher RF power. Although it increases reflected power during low-energy mode operation, which features a large pulse current of 100 mA, the power losses are still acceptable due to the significantly lower power requirement of the low-energy mode. Following completion of the waveguide coupler, a compensating structure was added at its symmetric position for dipole field cancellation. The comprehensive vacuum model of the accelerating structure was constructed in Ansys HFSS, with an overall length of 0.33 m, comprising 4 bunching cavities and 18 standard cavities, as shown in Fig. 3 [FIGURE:3] (a). The longitudinal effective shunt impedance of the standard cavities is 115 MΩ m⁻¹. The simulation results, including the $S_{11}$ curve, smith plot and normalized on-axis electric field distribution are presented in Figs. 3 (b), 3 (c), 3 (d), respectively.

Beam dynamics simulation was conducted using ASTRA. The kinetic energy and pulse beam current of the two operating modes are shown in Figs. 4 (a) and 4 (b). According to our design, at the input RF power of 2.0 MW, electrons gain 6.0 MeV energy with the 100 mA pulse current in low-energy mode operation. When the input power reaches 5.2 MW, electrons gain 13.5 MeV energy with 10 mA pulse current in high-energy mode. The transverse phase space distributions at the exit of the accelerating structure are shown in Figs. 4 (c) and 4 (d). And the transverse emittance curves are shown in Figs. 4 (e). Because of the higher longitudinal velocity of electrons in the high-energy mode, the transverse emittance at the exit of the accelerating structure is superior to the low-energy mode beams, with more concentrated position distribution and smaller angle divergence. Energy spectrum of the two modes are shown in Fig. 4 [FIGURE:4] (f). In the high-energy mode, the rms energy is 13.53 MeV with an rms energy spread of 2.4%. By contrast, the low-energy mode yields an rms energy of 6.11 MeV and an rms energy spread of 8.3%. This elevated energy spread in the low-energy mode arises because the optimization of the SW accelerating structure prioritizes the high-energy mode, exceeding the typical 3%–5% energy spread range for medical linacs. To further improve the capture ratio and rms energy spread of the low-energy mode, consideration can be given to a scheme where RF power is fed separately to the bunching section and the light-speed section. This approach maintains a constant field distribution in the bunching section across different modes while enabling acceleration to distinct energies.

B. Cold Test

After fabrication of the accelerating structure, the resonant frequency, quality factor and coupling factor of each cell were examined using the plunger method. Due to the thin thickness, all copper disks were not processed with tuning holes or tuning pins during manufacturing. Thus, tuning of each cell was achieved by reprocessing unsatisfactory copper disks with high-precision machine tools. After two reprocessing cycles, all cavities were tuned into an appropriate range around the operating frequency of 9300 ± 3 MHz, as shown in Fig. 4 (a). Deviation of the inter-cavity coupling coefficient from the design values was within 5%, and no further adjustment was required.

The assembled copper disks and waveguide coupler were brazed with silver-copper alloy braze. The complete accelerating structure is shown in Fig. 4 (b). Then, cold test of the complete accelerating structure was conducted using the VNA. The $S_{11}$ parameter, sampled at 9.3 GHz, exhibited a value of -23.7 dB. And the on-axial electric field distribution of the accelerating structure was measured by bead-pull method showing good agreement with RF simulation results, as illustrated in Fig. 4 (c).

IV. High-Power RF Experiment

The pulse compressor has been validated via low-power RF experiments and cold test of the accelerating structure has been conducted. By integrating the SW accelerating structure as the load terminal, configuring a high-voltage power supply for the grid-controlled thermal DC gun, and expanding synchronous trigger settings, we successfully established a high-power RF experimental platform for the dual-mode X-band linac. This section details the construction and configuration of the high-power RF experimental platform, along with the sampling, processing, and analysis of experimental data.

A. High-Power RF Experiment Setup

The photograph of the dual-mode experimental platform is shown in Fig. 6 [FIGURE:6] (a). Benefiting from the compact X-band pulse compressor and SW accelerating structure, the main body of the experimental platform has an overall length of only 1.5 m, advancing the way for the linac toward commercialization.

For the RF power section, a 3 MW X-band 9.3 GHz multi-beam klystron served as the primary power source, delivering full RF output during high-energy mode operation. A phase inverter capable of amplitude modulation was interposed between the LLRF excitation source and the solid-state amplifier (SSA), enabling phase inversion and amplitude modulation of the RF power. The klystron modulator had a maximum high-voltage pulse width of 6 µs, imposing a strict constraint on the RF power pulse width. To safeguard the klystron from reflected waves, a four-port circulator connected with water loads was installed between the klystron and pulse compressor. The resonant frequency of the pulse compressor was tuned via a mechanical tuning pin positioned axially above its cavity. When the pulse compressor was detuned from the operating frequency, it functioned only as a lossy waveguide with measurable power attenuation of approximately -0.2 dB. The pulse compressor maintains a constant cavity temperature of 35 °C through four internal water cooling channels, each configured with a flow rate of 10 L/min. Notably, we specifically reinforced the cooling structure near the coupling holes with high surface fields to prevent significant changes in the coupling coefficient due to thermal deformation, thereby ensuring stable operation of the pulse compression system.

In the accelerating section, a directional coupler was installed between the pulse compressor and the SW accelerating structure to enable real-time sampling of incident and reflected RF power, thereby detecting vacuum breakdown in the structure. The grid-controlled thermal DC gun, serving as the electron source, allowed precise adjustment of emission current by regulating the gun voltage, accommodating the beam current demands for both operation modes. The DC gun had a maximum pulse width of 10 µs, enabling full coverage of RF power pulse. Electron beams were expected to be bunched and accelerated to 6 MeV and 13.5 MeV by the SW structure in the low-energy mode and high-energy mode, respectively. The system was cooled by water through ten 6 mm-diameter internal water channels on copper disks with a flow rate of 3 L/min.

Titanium window or tungsten target was installed at the exit of the SW accelerating structure to extract electron beams or generate X-rays through bremsstrahlung. The linac was sealed using RF windows, and a 4 kV titanium pump maintained high-vacuum environment with air pressure below 10⁻⁷ Pa in the accelerator, preventing arcing and ensuring beam stability. Corresponding detectors were installed downstream for beam measurements, and their operational principles were detailed in the measurement results section.

The pulse compressor and SW accelerating structure were conditioned in advance to reduce the breakdown rate (BDR) \cite{27,28}. Breakdown events were identified by abnormal reflected wave signals and transient increases in vacuum pressure. During the conditioning process, RF power injection into the accelerator was gradually increased to avoid any irreversible damage to the structure. After the accelerating structure was fully conditioned with 10⁷ RF macro pulses under each of the two operation modes, the BDR was calculated to be less than 10⁻⁴/pulse for both.

B. Low-Energy Mode Measurement Results

When the pulse compressor is detuned, RF power is coupled into the accelerator without pulse compression, thereby enabling low-energy mode operation under a heavy beam pulse current of 100 mA, as depicted in Fig. 6 (b). Measurements in the low-energy mode specifically include the beam energy, pulse current and X-ray dose rate generated by bremsstrahlung.

An integrated current loop was employed for pulse current measurement. The loop sensitively detects magnetic field variations induced by electron beams and converts such changes into electrical signals that are routed to an oscilloscope via a BNC interface for observation. As shown in Fig. 6 (c), measurement result indicates a pulse current of 116 mA with a pulse width of 5.6 µs.

The electron energy in the low-energy mode was measured via current attenuation method, where 0.1 mm steel sheets and 50 Ω terminated aluminum collector were used to characterize the beam attenuation. By measuring current signals with different number of steel sheets and fitting the data to GB/T 25306-2010, the electron energy was determined to be 6.05 MeV, consistent with the designed value.

A water-cooled tungsten target was mounted at accelerator exit, facilitating X-ray generation through bremsstrahlung. The X-ray dose rate was measured using a Farmer-type ionization chamber positioned at 1 m from the tungsten target. At a repetition frequency of 150 Hz and duty factor of 8.4 × 10⁻⁴, the measured dose rate was 720 cGy/min. The experimental results of the low-energy mode are listed in table 2 [TABLE:2].

Table 2. Experimental results of the low-energy mode.

Parameter Value Repetition Rate 150 Hz Pulse Width 5.6 µs Duty Factor 8.4 × 10⁻⁴ Pulse Current 116 mA Kinetic Energy 6.05 MeV Dose Rate 720 cGy/min

C. High-Energy Mode Measurement Results

When the pulse compressor is tuned to the 9.3 GHz operating frequency, the RF power undergoes pulse compression and amplitude modulation, generating a 400 ns flat-top pulse with the gain factor of 2.2, as depicted in Fig. 6 (d). This enables the linac to operate in high-energy mode with the pulse current of 10 mA.

Pulse current measurements in the high-energy mode reused the same integrated current loop as in the low-energy mode experiment, as shown in Fig. 6 (e). Before activating the phase inverter, the RF power from klystron failed to undergo effective pulse compression, leading to reduction in field magnitude and phase mismatch. This resulted in ineffective electron capture and acceleration, with no noticeable current observed, as indicated by the blue curve. The results confirm that the accelerator fails to produce electron beams without inverse signal excitation, aligning with theoretical predictions.

Following phase inverter activation, an obvious pulse current signal was recorded, with an average pulse current of 10.3 mA, as indicated by the red curve. The rising edge of the pulse current characterizes the process by which the SW electromagnetic field gradually stabilizes within the accelerator and the electron capture ratio increases to the stable value. Additionally, the non-zero baseline and crosstalk were noted, though they did not affect the identification of the pulse current. The non-zero baseline is likely caused by imperfect grounding, while crosstalk may arise from electromagnetic interference in the experimental platform or high-voltage line coupling, which requires resolution through shielding optimization or equipment calibration.

The spectrum of electron energy was measured using a magnetic analyzer, as shown in Fig. 7 (a). The measurement setup was mainly composed of three parts: the beam transport device, the magnetic analyzer, and the imaging system. The vacuum pump was connected to the magnetic analyzer via a tee joint and a bellows. To improve the energy spectrum resolution, a slit was installed at the entrance of the magnetic analyzer to filter out low-energy electrons and restrict the beam spot size. By adjusting the excitation current, the deflection magnetic field strength can be precisely regulated, thereby changing the deflection radius of the electron beam. The electron energy formula considering relativistic effects can be expressed as:

$$E = [\sqrt{1 + (eBR_e/m_0c)^2} - 1]m_0c^2$$

where $B$ is the deflection magnetic field, and $R_e$ is the deflection radius.

The imaging system utilized optical imaging method. When the electrons bombarded the YAG screen, fluorescent material was excited, generating visible light reflected by a 45° mirror and then captured by a CCD camera. By integrating the pre-calibrated correlation between excitation current and magnetic field, the electron energy spectrum was accurately derived through analyzing the spatial distribution and intensity variation of the optical signals, as shown in Fig. 7 (c). The measured energy spectrum peak is at 13.9 MeV, with a FWHM of 0.43 MeV. This result slightly exceeds the high-energy mode design target of 13.5 MeV, which is speculated to originate from the actual power attenuation in the high-energy experiment being less than the expected value of -1 dB. The actual power attenuation calculated from the measurement results is -0.73 dB. Experimental results of the high-energy mode are listed in table 3 [TABLE:3].

Table 3. Experimental results of the high-energy mode.

Parameter Value Repetition Rate 20 Hz Pulse Width 400 ns Duty Factor 8 × 10⁻⁶ Pulse Current 10.3 mA Central Energy 13.9 MeV FWHM 0.43 MeV

Considering the -0.73 dB power attenuation in the transmission, corresponding to an 85% power retention ratio, a correction factor of $1/\sqrt{0.85}$ is induced, yielding an energy of 15.5 MeV under lossless conditions. Further minimizing the power attenuation of RF components would enhance electron energy in the high-energy mode.

V. Conclusion

A compact X-band 9.3 GHz dual-mode medical linac dedicated to radiotherapy applications was first developed in the Accelerator Laboratory of Tsinghua University. The accelerator employed a 3 MW X-band klystron as the power source, coupled with a SLED pulse compressor, enabling dual-mode operation: low-energy mode delivering 100 mA pulse current at 6 MeV for X-ray radiotherapy and high-energy mode delivering 10 mA pulse current at 13.5 MeV for electron radiotherapy. With lower treatment costs and spatial requirements, the linac can significantly facilitate the popularization of cancer therapy.

Design, fabrication and cold test of the pulse compressor and accelerating structure have been conducted, alongside the establishment of the high power RF experimental platform. Low-energy mode experiments generated 5.6 µs electron beams with 116 mA pulse current and 6.05 MeV energy at 300 Hz repetition rate, demonstrating a dose rate of 2435 cGy/min at 1 m from the tungsten target. High-energy mode experiments yielded electron beams featuring 0.47 µs pulse width, 10.3 mA pulse current, 13.9 MeV central energy and 0.43 MeV FWHM. The results validated the feasibility of the linac, demonstrating its potential for versatile radiotherapy applications with optimized spatial compactness.

The linac is currently undergoing system integration design, and preparations are underway for subsequent biological experiments in collaboration with hospitals. This compact X-band dual-beam linac provides valuable references for the development of future compact medical linacs.

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