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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]

To improve temporal coherence in electron beam based light sources, various techniques employ frequency up conversion of external seed sources via electron beam density modulation; however, the energy spread of the beam may hinder the harmonic generation efficiency. In this Letter, a method is described for cooling the electron beam energy spread by off-resonance seed laser modulation, through the use of a transversely dispersed electron beam and a modulator undulator with an appropriate transverse field gradient. With this novel mechanism, it is shown that the frequency up-conversion efficiency can be significantly enhanced. We present theoretical analysis and numerical simulations for seeded soft x-ray free-electron laser and storage ring based coherent harmonic generation in the extreme ultraviolet spectral region. [ABSTRACT FROM AUTHOR]

The article proposes the use of a crystal-based cavity resonant at a higher harmonic of the undulator radiation, along with phase shifting to allow harmonic lasing of the x-ray free electron laser oscillator (XFELO) and the generation of hard x-ray radiation at a reduced electron beam energy. A comparison between SASE, low-gain XFELO operated at the fundamental frequency, and harmonic lasing of an XFELO is also presented.

High quality electron beams with flat distributions in both energy and current are critical for many accelerator-based scientific facihties such as free-electron lasers and MeV ultrafast electron diffraction and microscopes. In this Letter, we report on using corrugated structures to compensate for the beam nonlinear energy chirp imprinted by the curvature of the radio-frequency field, leading to a significant reduction in beam energy spread. By using a pair of corrugated structures with orthogonal orientations, we show that the quadrupole wakefields, which, otherwise, increase beam emittance, can be effectively canceled. This work also extends the applications of corrugated structures to the low beam charge (a few pC) and low beam energy (a few MeV) regime and may have a strong impact in many accelerator-based facilities. [ABSTRACT FROM AUTHOR]