Arclike bunch compressors are important components in recirculating accelerator linacs, energy recovery linacs, and high-brightness light sources. However, their design is complicated by longitudinal nonlinear optics and coherent synchrotron radiation (CSR), including both steady-state CSR (ss CSR) in bending magnets and CSR in the subsequent drifts (dr CSR). These effects can degrade the longitudinal compression quality, generate current spikes, and induce transverse emittance growth. In this work, we develop an analytical model for evaluating CSR effects in complex arclike compressors without relying on the small-angle approximation. The proposed method includes both ss and dr CSR contributions and is applied to the mitigation of CSR-induced transverse displacements as well as to longitudinal matching with nonlinear compression. Based on these methods, a two-stage arclike compressor using an asymmetric quadruple-bend-achromat structure is designed. Numerical simulations show that the proposed compressor can compress a 100 pC bunch to nearly 50 fs while maintaining negligible emittance growth. The analytical model is further used to study CSR-induced modifications to the compression process, including rms bunch length and peak-current behavior. Benchmarking against Bmad simulations demonstrates good agreement, confirming the model as an efficient tool for compressor design and optimization.
Seeded free-electron lasers (FELs) have become indispensable tools across numerous scientific fields, owing to their high coherence and stability. Unlike self-amplified spontaneous emission, seeded FELs require a prebunched electron beam to be formed upstream of the radiator, enabling the subsequent generation of fully coherent radiation. This bunching process can be numerically calculated based on the specific components and their parameters used. To facilitate the discovery and optimization of such FELs, in this paper we present an automated optimization method for the microbunching process in seeded FEL schemes. It is demonstrated that the method can automatically reproduce established seeded FELs, such as echo-enabled harmonic generation, eliminating the need for prior physical analysis. Moreover, the method yields new findings in seeded FELs that go beyond mere replication. By incorporating more kinds of components and exploring a broader set of optimization objectives, the optimization can be extended to explore more possibilities.
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.
