Ming-Yang Huang

Scaling fixed-field alternating-gradient accelerators (scaling FFAs) are promising candidates for high-intensity proton drivers, for which space charge effects are a key limitation of beam performance. For long bunches in circular accelerators, 2.5D space charge methods provide an efficient alternative to full 3D PIC methods by exploiting the large longitudinal-to-transverse aspect ratio of the bunch. In scaling FFAs, however, the reference closed orbit varies with particle energy, and the transverse and longitudinal directions are not directly available in cylindrical coordinates. Therefore, the longitudinal slicing and transverse planes required by a 2.5D model must be reconstructed dynamically, rather than defined from a single fixed reference orbit, which poses a central challenge for efficient 2.5D modeling in high-intensity scaling FFAs. This paper presents a new 2.5D space charge modeling method for high-intensity scaling FFAs and its implementation in the PyFFA code. The central idea is to replace the fixed reference orbit used in conventional 2.5D models with a dynamically reconstructed local reference frame based on a precomputed family of energy-dependent closed orbits. During tracking, this frame is updated according to the instantaneous particle state, enabling transverse Poisson equations to be solved consistently in this dynamic local coordinate system. The resulting space charge field is then transformed back to global cylindrical coordinates and applied in the tracking equations. Benchmarks of space charge fields, tune shifts, rms envelope evolution, and longitudinal phase-space evolution show good agreement with analytical results, validating the field calculation and its coupling to beam dynamics for long, arc-shaped bunches in scaling FFAs. The implemented model enables efficient studies of space charge effects on beam dynamics in high-intensity scaling FFAs.

In a rapid cycling synchrotron (RCS), closed-orbit correction is an important task for maintaining high beam quality and machine stability. Since the orbits in RCS change during energy ramping, response matrix-based orbit correction is performed in segments, which can result in the corrector current ramp rates exceeding safety limits and accuracy loss in orbits. To address these issues, this paper employs machine learning for orbit correction in RCS. An adjacent-difference-constrained neural network (ADCNN) was built and tested in both simulations and machine experiments at the China Spallation Neutron Source (CSNS). The correction results demonstrate the feasibility of using neural networks for RCS orbit correction. The ADCNN exhibits a correction effect equivalent to that of response matrix-based correction while effectively limiting the current ramp rates of the correctors.

A 300–600 MeV high-intensity scaling fixed-field alternating gradient (FFA) facility is proposed as a multipurpose proton driver for the China Spallation Neutron Source (CSNS) Phase-II upgrade. To identify a viable lattice configuration under strict engineering constraints, a statistical analysis methodology based on large-sample ray-tracing simulations comprising over 360,000 parameter configurations is applied. A two-stage scanning pipeline evaluates the topological limits of both FDF triplet and FD doublet structures under realistic multisource error budgets. Factoring in phase-space geometry, error tolerance, and facility integration, the findings indicate that a spiral FD doublet lattice with a super-periodicity of N = 16 is preferred as the structural baseline; it avoids parameter bottlenecks and yields a large, resonance-sparse stable region. To characterize the coupled high-dimensional dynamics quantitatively, a machine learning feature attribution framework is applied. The framework identifies the spiral edge angle and the full-ring vertical tune as the primary drivers of beam survivability, and maps the underlying resonance stopbands from tracking data alone. Based on these results, a baseline configuration is established with a wide dynamic tune allowance that maintains long-term beam survivability within the physical and engineering boundaries of the facility.

For the China Spallation Neutron Source (CSNS), the rapid cycling synchrotron (RCS) accumulates and accelerates the injection beam to the design energy of 1.6 GeV before extracting it to the target. For the second phase, CSNS-II, which aims to achieve a higher target beam power of 500 kW. The injection energy of the CSNS-II will be increased from 80 MeV to 300 MeV and the injection beam power will be increased about 20 times. This paper first systematically outlines the key challenges faced by the CSNS-II RCS and their corresponding solutions. Subsequently, the upgrade scheme for the CSNS-II RCS are comprehensively investigated and analyzed, including the development of new injection system, optimization of longitudinal dynamics, mitigation of beam instability, RCS closed orbit correction, beam loss control, and the design of new collimation systems. Based on the extensive numerical simulation results and beam commissioning experimental data, the upgrade schemes for the critical systems of the CSNS-II RCS have been experimentally verified and demonstrated to be technically feasible.

The Rapid Cycling Synchrotron (RCS) functions as the principal accelerator for the China Spallation Neutron Source Phase II project (CSNS-II). Designed for a beam power of 500 kW, the RCS achieves the highest circulating beam current worldwide at this power level. The impedance has been upgraded in detail based on the findings outlined in the reference [1] during the design phase of CSNS, particularly regarding the new critical components in the RCS of CSNS-II, which are attributed to coupled impedance. This paper presents a systematic investigation of the impedances of vacuum components through analytical calculations, numerical simulations, and bench measurements, developing a comprehensive impedance model for the RCS. By utilizing this model, we conduct a thorough evaluation of potential beam instabilities within the ring, identifying impedance thresholds and growth rates for various instabilities, with special emphasis on critical instabilities, which are rigorously analyzed. The bunch length stretching induced by magnetic alloy cavities is extensively simulated, and a range of effective suppression techniques is proposed to mitigate transverse coupled-bunch instability, thereby facilitating the successful realization of the design objectives.