arXiv subjects

Zheqiao Geng

Publications and source records attributed to Zheqiao Geng.

3 recordsLinked to original sources

arXiv2026

Beam loading analysis and control in standing wave cavities

The interaction between a particle beam and the accelerating mode of a radiofrequency (RF) cavity cause beam loading, representing the beam-induced cavity fields. Beam loading leads to amplitude and phase errors in the cavity fields and reduces the beam quality, especially in accelerators with large beam currents, wideband RF cavities, or circular machines where particles stay for multiple turns. Insight into the principle of beam loading is helpful to understand the beam measurement results and propose efficient compensation methods in low-level RF systems. In this work, the beam loading effects are studied with the equivalent circuit model of standing wave cavities. Analytical results of beam-induced cavity voltages are derived for both a single bunch and a bunch train using the phasor Laplace transform method. The results are general for wideband cavities with a bandwidth that may cover multiple harmonics of the bunch repetition frequency. Based on the analysis, control methods in form of feedforward and feedback are proposed to compensate for the beam loading. Simulation studies are carried out to validate these control methods with a cavity simulator including both the RF drive and beam loading. The analysis and control methods are also applicable to the beam in a circular accelerator with coupled-bunch instabilities, which are discussed in the last part of this paper. This work also acts as a supplementary material to another work of the author, in which the beam loading effects are analyzed only for narrow-band cavities with only one beam harmonic appearing in the cavity bandwidth.

physics.acc-ph↗

arXiv2025

Femtosecond-Scale MeV-UED Beamline Using a Stand-Alone Multi-Cell RF Photogun

The temporal resolution of MeV ultrafast electron diffraction (UED) is fundamentally constrained by the electron bunch length at the sample, motivating the development of new electron sources capable of producing femtosecond scale bunches. In this work, we propose a multicell RF photogun that has a tailored phase velocity profile to generate 5 to 15 fs rms MeV electron bunches directly from the electron gun, eliminating the need for downstream compression. This approach achieves comparable performance to conventional one and a half cell photoguns with downstream compression, while reducing system size, complexity, and power requirements. We examine two implementations: a standing wave (SW) and a travelling wave (TW) design. The TW variant demonstrates over an order of magnitude lower power dissipation than typical SW structures, enabling potential kHz operation. When paired with SwissFEL style C band RF sources, which offer high amplitude and phase stability, the TW photogun is projected to deliver a temporal resolution of 26 fs rms.

physics.acc-ph↗

arXiv2019

Robustness Issues of Timing and Synchronization for Free Electron Lasers

Free electron lasers (FEL) require strict time relations for the electron bunch and RF field interaction, which must be precise and deterministic in time. This is guaranteed by the timing and synchronization systems that should be robust under the situations like a power cycle in the master oscillator, timing master, reference frequency distribution devices or low-level radio frequency (LLRF) devices. After the power cycles, the time relations should be kept or be capable to recover quickly to improve the availability of the FEL machine. This article focuses on the robustness of the timing and synchronization systems, such as the time uncertainty of the RF pulse related with the trigger, the phase uncertainty of frequency dividers after power cycles and the race condition between the trigger and clock. The possible solutions to achieve a robust design of the timing and synchronization systems are also discussed.

physics.acc-ph↗