ENN's Xuanlong-50U (EXL-50U) is the world's first fully privately funded and operated MA-level magnetic confinement fusion experimental facility. EXL-50U is also the world's first spherical torus device (ST) to achieve a magnetic field exceeding 1 Tesla and a pulse width reaching the seconds level in actual operation. The major radius of EXL-50U ranges from 0.6 m to 0.8 m, with an aspect ratio of 1.4-1.8. The key physics issues of EXL-50U's experiments related to the future ST reactors are: non-inductive current start-up and drive, development of scenarios for stable high density hot ion mode for ST (Ti0 = 3 keV similar to 10 keV), and investigation of energy confinement scaling through wide range scans of aspect ratio (1.4 similar to 1.8) and BT (0.5 T similar to 1.2 T). Another key research focus of EXL-50U is the physical properties of hydrogen-boron (p-B) plasmas and hydrogen-boron fusion, which is closely aligned with ENN's p-B fusion roadmap. In the 2025 experiment, EXL-50U achieved a p-B plasma discharge at 1 MA (with boron ions exceeding 10% in the ion fraction), marking the world's first MA-level discharge of high-concentration p-B plasma. A recording 270 kA fully non-inductive current by 380 kW electron cyclotron resonance heating (ECRH) is also achieved. The initial real-time boron-power injection experiments with metal wall in EXL-50U demonstrates the positive effective of boron on plasma performance. Through neutral beam injection (NBI) heating, a stable proton-boron plasma H-mode has also been achieved for the first time on EXL-50U. On the other hand, experimental results also indicate that simultaneous injection of ECRH and NBI is detrimental to ion temperature enhancement. AI technology was also applied in EXL-50U experiments, achieving stable 500 kA plasma current and displacement control for over 500 ms. Supersonic molecular beam injection was applied for density feedback control for the first time in ST device. ENN researchers plan to achieve all of EXL-50U's milestone objectives within a three-year timeframe, contributing their efforts toward the rapid commercialization of magnetic confinement fusion.

Scaling Fixed Field Alternating Gradient Accelerators (FFAs) are promising candidates for high-intensity hadron drivers due to their large momentum acceptance. However, simulating the full-cycle acceleration in Scaling FFAs requires modeling the interplay between strong magnetic non-linearities and the variable-frequency RF acceleration necessary to maintain synchronicity. This paper presents the development of a general variable-frequency RF module for the tracking code FIXFIELD, enabling self-consistent 6D coupled dynamics studies within its high-order magnetic field solver. Utilizing a cubic-spline-based interpolation architecture synchronized with absolute time of flight, the module allows for precise tracking under arbitrary frequency modulation profiles. Benchmarking against the Zgoubi code, performed on a CSNS-II spiral FFA lattice, demonstrates consistency in longitudinal phase space evolution, including adiabatic damping and nonlinear frequency detuning. Furthermore, investigation into large-emittance beams reveals that high-order magnetic field terms are essential for the realistic assessment of resonance crossing topologies and error tolerances. As low-order approximations can lead to an overestimation of machine stability, the upgraded FIXFIELD framework provides a consistent tool for 6D coupled dynamics studies, assisting in the physical design and error tolerance analysis of Scaling FFAs.

Nanocrystalline soft magnetic alloy (MA) cores are essential magnetic loading components in proton/heavy-ion synchrotron rf cavities. While conventional MA cores fabricated from 16 μm ribbons offer reliable performance, further enhancing their shunt impedance and thermal stability to meet the demands of next-generation high-power synchrotrons presents significant challenges. Although the use of thinner ribbons (e.g., 13 μm) can improve high-frequency magnetic properties, their widespread application is hindered by manufacturing difficulties and reduced packing factor consistency. To address this, we propose an embedded MA core design that strategically integrates 13 μm ribbons in specific radial regions alongside normal 16 μm ribbons. This approach not only increases shunt impedance but also optimizes the electromagnetic field distribution, thereby reducing peak surface temperature and improving thermal stability. This paper first elucidates the physical mechanism through which the embedded structure improves temperature uniformity. Additionally, a two-dimensional heat transfer numerical model is developed and experimentally validated to assess the thermal performance of the embedded MA core under actual operating conditions. Experiments are conducted and the results demonstrate the embedded core’s peak surface temperature is reduced by 2.5 °C compared to a normal MA core. Finally, parameter optimization of the embedded structure is conducted, demonstrating that optimizing the radial position, volume ratio, and thickness of thinner ribbons can achieve a temperature reduction of approximately 6 °C and a 12% impedance increase. The findings of this study provide a theoretical foundation for the performance optimization of MA cores in high-power synchrotrons.

In many engineering applications, multi-objective optimization problems can be reformulated as single-objective problems with multiple constraints to improve computational efficiency. This paper discusses the characteristics and challenges of RF accelerating structure optimizations and proposes an enhanced single-objective optimization strategy based on progressive exploration method to find the global optimal solution within a large solution space characterized by a continuous and confined distribution of feasible solutions. It begins from an arbitrary feasible solution and progressively slides and expands the solution space fragment along the distribution path of feasible solutions to rapidly explore the entire space. By incorporating a re-initialization mechanism to enhance swarm diversity and introducing penalty factors in place of constraints to increase the number of feasible solutions, the algorithm significantly improves its ability to escape local optima traps. The proposed algorithm is applied to optimize a DAA structure, yielding satisfactory results and convergence speed. These results highlight the method’s effectiveness and its potential applicability to other complex constrained optimization problems.

In a rapid cycling synchrotron (RCS), the magnetic field is synchronized with the beam energy, creating a highly dynamic magnetic environment. A ceramic chamber with a shielding layer (RF shield), composed of a series of copper strips connected to a capacitor at either end, is typically employed as a vacuum chamber to mitigate eddy current effects and beam coupling impedance. Consequently, the ceramic chamber exhibits a thin-walled multilayered complex structure. Previous theoretical studies have suggested that the impedance of such a structure has a negligible impact on the beam. However, recent impedance measurements of the ceramic chamber in the China Spallation Neutron Source (CSNS) RCS revealed a resonance in the low-frequency range, which was confirmed by further theoretical analysis as a source of beam instability in the RCS. Currently, the magnitude of this impedance cannot be accurately assessed using theoretical calculations. In this study, we used the CST Microwave Studio to confirm the impedance of the ceramic chamber. Further simulations covering six different types of ceramic chambers were conducted to develop an impedance model in the RCS. Additionally, this study investigates the resonant characteristics of the ceramic chamber impedance, finding that the resonant frequency is closely related to the capacitance of the capacitors. This finding provides clear directions for further impedance optimization and is crucial for achieving a beam power of 500 kW for the CSNS Phase-II project (CSNS-II). However, careful attention must be paid to the voltage across the capacitors.

Developing multifrequency acceleration systems is crucial for optimizing beam dynamics in high-brightness particle beams, particularly for minimizing energy spread, manipulating bunch length, and facilitating beam bunching. This paper introduces a novel bimodal radio frequency (rf) cavity design that integrates both fundamental and harmonic modes within a single structure, offering enhanced spatial efficiency and improved operational flexibility for multifrequency rf systems. Unlike traditional designs, our cavity enables independent and simultaneous operation of each mode, along with effective suppression of higher-order modes (HOMs), which is critical for high-current beam stability. This breakthrough addresses significant challenges in bimodal cavity technology, such as intermode tuning coupling, rf-driving coupling, and effective HOM damping, providing essential solutions for high-current beam applications. Our work opens new research avenues in advanced rf cavity design, with significant implications for the future of high-brightness beam technologies, including advanced accelerator designs and improved beam quality.

Radiofrequency cavities are crucial components in synchrotron light sources, providing beam energy replenishment and accelerating voltage. TM020 mode cavities offer superior performance compared to conventional TM010 cavities, exhibiting a higher quality factor and accelerating gradient. Their unique electromagnetic field distribution enables effective suppression of both higher and lower order modes via strategically placed dampers at the radial magnetic or electric field wave nodes. However, the sensitivity to misalignment is heightened near these nodes, where the magnetic or electric field gradient is maximized. Consequently, even minor deviations in coupling slot or wave node positioning can induce substantial electromagnetic field leakage, limiting the achievable accelerating voltage due to finite damper power handling. We present a novel leakage suppression scheme employing a Sector waveguide for precise control of coupling slot placement and a waveguide-to-coaxial input coupler to minimize azimuthal perturbations, thus stabilizing the field wave node. This approach drastically reduces accelerating field leakage, enabling stable operation of TM020 damped cavities at high accelerating gradients. Simulations of a 500 MHz TM020 cavity demonstrate leakage reduction to below 1% under operational conditions.

A dual-harmonic acceleration system is utilized to mitigate the space-charge effect in the rapid-cycling synchrotron of the China Spallation Neutron Source upgrade project (CSNS-II). A magnetic alloy (MA)-loaded cavity with a high accelerating gradient is developed to satisfy the requirements of dual-harmonic acceleration and provide the necessary second-harmonic cavity voltage. However, the MA-loaded cavity exhibits a wideband frequency response, resulting in numerous higher harmonics in the radio-frequency (RF) voltage. These higher harmonics are caused by both the beam-loading effect and distorted amplifier current, which distort the RF bucket, increase the power dissipation in the cavity, and lower the gradient. To address these issues, a multiharmonic independent feedback-control approach is implemented to compensate for higher harmonics. The effectiveness of this control strategy is validated experimentally. This study provides details regarding the feedback-control design and presents the commissioning results.

The development of high-intensity muon sources is of great importance for numerous applications in fundamental and applied research. This paper proposes a novel High rEpetition rate Muon Source (HEMS) at the China Spallation Neutron Source (CSNS) to meet this demand. The primary objective of the HEMS project is to design and establish a facility capable of delivering high-intensity, pulsed muon beams with a repetition rate of 100 Hz for various user experiments. The conceptual design centers on a rapid cycling synchrotron (RCS), which accelerates proton beams from the existing CSNS Linac to 500 MeV. A key innovation involves reusing each proton bunch: After acceleration, a bunch is extracted to strike a thin graphite target for muon production, then recaptured and re-accelerated by the RCS. This process is repeated 20 times per bunch. With a 5 Hz injection rate from the Linac, a final muon production rate of 100 Hz is achieved. Comprehensive simulations were conducted to analyze proton beam energy loss and emittance growth during target interactions, as well as the overall beam stability within the RCS. The design of both surface muon and decay muon beamlines is also presented. Simulation results demonstrate that the HEMS facility can generate pulsed surface and decay muon beams with intensities exceeding 106 μ/s, utilizing a proton beam power of 1.125 kW. The design ensures beam stability after multiple target passages. The proposed HEMS concept offers a promising approach to creating a high-repetition-rate, high-intensity muon source. This paper details the overall design, target station, and muon beamlines, establishing the feasibility of the project for future implementation.

Analyzing tetrode tube operation is crucial for the design and optimization of RF systems in proton/heavy ion synchrotrons, especially for high beam circulating current operations. This task presents significant challenges due to the complexity of the system. In this article, we introduce a novel approach to analyze tube operation using a circuit simulator. By modeling the entire RF system within the circuit simulator, we enable real-time simulation of the system's behavior, which allows for accurate determination of tube operation. This method proves to be effective in multiharmonic scenarios where multiple variables need to be resolved. We detail the methods used to model the RF system of the China Spallation Neutron Source (CSNS) in the circuit simulator, which includes the wideband cavity, vacuum tube amplifier, and low-level RF (LLRF) control system. The simulation results are presented and discussed, showing reasonable agreement with experimental measurements.

The upgrade of the China Spallation Neutron Source (CSNS-II) encompasses the development of a dual harmonic RF system for the Rapid Cycling Synchrotron (RCS). The objective of this system is to achieve a maximum second harmonic voltage of 100 kV. To meet this requirement, a high gradient cavity is being used in place of the traditional ferrite loaded cavity. Magnetic alloy (MA) loaded cavities, which can attain very high field gradients, have demonstrated their suitability for high -intensity proton synchrotrons. As a result, designing an RF system with MA -loaded cavities has emerged as a primary focus. Over the past decade, substantial advancements have been made in the development of MA -loaded cavities at CSNS. This paper provides an overview of the RF system that incorporates the MA -loaded cavity and presents the high -power test and beam test results of the system.

The rf system of China Spallation Neutron Source (CSNS) Rapid Cycling Synchrotron (RCS) utilizes vacuum tubes to drive the ferrite-loaded cavities. Ensuring stable tube operation in the rf system is crucial for the acceleration of high-intensity beams, as any instability may impede further increases in beam power. Therefore, the operation of the vacuum tube under heavy beam loading has attracted significant attention, particularly in the case of CSNS-II RCS, where the circulating beam current is expected to reach up to 15.25 A, imposing a substantial burden on the vacuum tube for beam loading compensation. Thus, a comprehensive analysis of tube operation is imperative. However, current methods for the tube operation analysis are developed based on wideband rf cavities, which are inadequate when the load of the tube is a narrowband ferrite-loaded cavity equipped with a tuning system. The presence of a tuning system renders the modeling method for wideband rf cavities inapplicable for ferrite-loaded cavities. Therefore, the development of a new method is necessary. This paper introduces a novel method for analyzing operation of vacuum tube driving a tuned cavity. The effectiveness of the method is validated by comparing the analysis results with measurements under beam loading conditions. Furthermore, an estimation of tube operation under 500 kW beam loading for CSNS-II is provided.

Radiofrequency cavities play a crucial role in synchrotron light sources by replenishing beam energy and providing sufficient accelerating voltage. Compared to traditional TM010 cavities, TM020 cavities offer advantages such as higher quality factor, higher accelerating voltage, and lower characteristic impedance. Additionally, their unique field distribution allows strategic placement of dampers at radial nodes, effectively suppressing higher-order modes within a compact structure. However, the radiofrequency performance of cavity is highly sensitive to the placement of the higher-order modes coupling slot and the Electromagnetic(EM) field symmetry. These factors can easily lead to leakage of the accelerating mode, with the leaked energy absorbed by the absorber, whose power handling capacity is limited. Consequently, the leakage rate of the TM020 mode damping cavity becomes critical in limiting the achievable accelerating cavity voltage. This paper proposes a novel extraction scheme utilizing a circular waveguide structure and a waveguide-coaxial input coupler to significantly reduce accelerating field leakage, thereby achieving a high-fidelity TM020 cavity. A detailed design of a 500 MHz TM020 HOM-damped cavity is presented. Simulation results demonstrate that the energy leakage of the accelerating mode can be suppressed below 1% during operation.

In this report, we present our recent progress in the design and high-power testing of the 2nd harmonic cavity for the China Spallation Neutron Source upgrade project. To achieve optimal performance, high-performance magnetic alloy (MA) cores with dimensions of Phi 850mm x Phi 316mm x 25mm were meticulously developed and fabricated to serve as the load material for the radio-frequency (RF) cavity. Through rigorous testing, we were able to achieve a remarkable cavity accelerating gradient of over 40 kV/m under 15% duty cycle. To ensure optimal cooling efficiency, we conducted a comprehensive fluid dynamics simulation analysis and verified our results through experiments. Finally, to assess the long-term stability and performance of the cavity, we conducted a series of extended operation tests. These experiments successfully confirmed the high-performance capabilities and exceptional stability of the 2nd harmonic cavity.

The second phase of the China Spallation Neutron Source (CSNS-II) will employ a dual-harmonic RF system to mitigate the significant space charge effects associated with 500 kW beam power. To address this, three magnetic alloy loaded cavities, characterized by their wideband and high-gradient properties, will be integrated as multi-harmonic cavities in the upgrade project. Additionally, the current LLRF control system hardware, based on the CPCI platform, is encountering issues related to discontinuation and performance limitations. This paper presents the testing and verification of hardware upgrades and multi-harmonic control system algorithms for the magnetic alloy loaded cavity LLRF control system, ensuring the effective implementation of the multi-harmonic control algorithm within the new hardware framework.

The fixed-field alternating gradient (FFAG) accelerator, with its compact structure and the combined advantages of high energy from synchrotrons and high beam intensity from cyclotrons, offers unparalleled benefits in accelerating protons, heavy ions, and short-lived particles such as muons and unstable nuclei. The China Spallation Neutron Source plans to establish an FFAG accelerator with a diameter of 20 meters at the end of the negative hydrogen linac for various purposes, including experiments in nuclear physics and medical applications. For this design, a scaling scheme using spiral magnets as the basic focusing structure is investigated to provide the proton beam with kinetic energy ranging from 300 MeV to 600 MeV. We present the design results for the scaling FFAG accelerator and discuss the optimization of the cell tune. A detailed beam dynamics verification and discussion of the proposed FFAG accelerator lattice are presented using the ray-tracing code Zgoubi, including off-axis optical characteristics, large amplitude transverse motion, and full-cycle longitudinal acceleration. The FFAG accelerator is demonstrated to provide a large transverse and longitudinal acceptance for the tracking beam in the designed lattice. A new simulation code based on Python for the study of FFAG accelerator beam dynamics has been developed, and the corresponding verification results are also presented.

The China Spallation Neutron Source (CSNS) upgrade project (CSNS-II) aims to enhance the beam power from 100 to 500 kW. A dual-harmonic accelerating method has been adopted to alleviate the stronger space-charge effect in rapid-cycling synchrotrons owing to the increased beam intensity. To satisfy the requirements of dual-harmonic acceleration, a new radiofrequency (RF) system based on a magnetic alloy-loaded cavity is proposed. This paper presents design considerations and experimental results regarding the performance evaluation of the proposed RF system through high-power tests and beam commissioning. The test results demonstrate that the RF system satisfies the desired specifications and affords significant benefits for CSNS-II.

Two high-power tetrode tubes operated in push-pull mode were employed as the final stage amplifier for the wideband magnetic alloy (MA) loaded cavity in the China Spallation Neutron Source Phase II (CSNS-II). However, the anode voltage of the tetrode tubes exhibits significant waveform distortion due to the wideband load impedance and dynamic tube characteristics. This distortion poses a threat to beam stability and results in additional power dissipation within the cavity. Consequently, it is imperative to address the compensation of higher harmonics. Prior to achieving this, a comprehensive and intricate analysis of tetrode tube operation is required, involving the simultaneous resolution of numerous variables to obtain accurate solutions for anode voltage and anode current. This article introduces a new method for analyzing tetrode tube operation in the presence of multiharmonics. The efficacy of this method is verified through high-power tests.

Here, we report our recent progress in the design, fluid thermodynamics simulation, and high-power test of the2nd harmonic cavity for the China Spallation Neutron Source Phase II. A high-performance and large-size magnetic alloy (MA) core was developed as the load material for the radiofrequency cavity to achieve a high gradient of 40 kV/m. The water-cooling structure and cooling efficiency were studied and improved through numerical analysis and thermal experiments. The long-term stability of the cavity, especially the waterproofness of the MA cores with high heat load, was verified by high power tests.