Abstract
The increasing demands for electron beams with high quality and low timing jitter for accelerator-based applications such as ultrafast electron microscopy (UEM) and ultrafast electron diffraction (UED) have stimulated active efforts to develop suitable injectors for these accelerators. One promising direction in this regard is using of a bimodal rf gun operating in fundamental mode and a phase-locked harmonically-related higher mode. Work reported in this paper is on analysis and design of a bimodal rf gun operating in S-band as the fundamental and C-band as the second-harmonic modes. The rf design for such an rf gun includes the new design like directional coupler and rectangular mode launcher, which presented good transmission, low reflective cross-talk, and allowed multiple iterations of the modification. Compared to the fundamental mode, the new design achieved reduction in rf emittance and energy spread for MeV UEM initial distribution, and reduction in slope of flight time as function of injection phase for MeV UED applications.
Highlights
Accelerator-based electron beam applications like ultrafast electron diffraction (UED) and microscopy (UEM) have been proposed as promising tools to study substances at femtosecond and picosecond temporal scale [1]
UED application In MeV UED [4], one of the most potential methods of visualization of atomic dynamics, the sample is driven to a nonequilibrium state by a laser, and probed by a delayed electron pulse produced in a photocathode rf gun
The design of a bimodal gun operating at S-band and C-band was developed in a collaboration between Wright Laboratory, Department of Physics, Yale University and Shanghai Advanced Research Institute, Chinese Academy of Sciences
Summary
Accelerator-based electron beam applications like ultrafast electron diffraction (UED) and microscopy (UEM) have been proposed as promising tools to study substances at femtosecond and picosecond temporal scale [1]. By selecting a proper amplitude ratio and phase relationship between the fundamental and the harmonic rf field in the gun cavity, the superposition of the two frequencies can provide a temporally-flat-top-like rf profile, the electrons at the head and the tail of the electron bunch are influenced by similar rf kicks, decreasing the rf transverse emittance, the energy spread, and phase jitter. The further research design is being implemented to investigate the possibility of designing a relatively simple and costeffective bimodal gun that is capable of generating high quality beams with a small transverse emittance and low energy spread for potential application like UEM and low timing jitter for UED.
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