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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.

Removal of the undesired time-energy correlations in the electron beam is of paramount importance for efficient lasing of a high-gain free-electron laser. Recently, it has been theoretically and experimentally demonstrated that the longitudinal wakefield excited by the electrons themselves in a corrugated structure allows for precise control of the electron beam phase space. In this Letter, we report the first utilization of a corrugated structure as a beam linearizer in the operation of a seeded free-electron laser driven by a 140 MeV linear accelerator, where a gain of ~10000 over spontaneous emission was achieved at the second harmonic of the 1047 nm seed laser, and a free-electron laser bandwidth narrowing by 50% was observed, in good agreement with the theoretical expectations. [ABSTRACT FROM AUTHOR]