Impact of electron beam quality on nonlinear harmonic generation in high-gain free-electron lasers

Nonlinear harmonic generation can be a very useful and important phenomenon for single-pass free-electron lasers (FELs) operating in the high-gain regime. Strong bunching at the fundamental wavelength and its associated higher harmonic content allow significant radiation at shorter wavelengths to be emitted without serious effects upon the output power at the fundamental. Here, we analyze the relative sensitivities to beam quality variations of the output fundamental and harmonic powers for a visible-wavelength FEL operating in the high-gain regime

Toward Creating a Coherent, Next-Generation Light Source with special emphasis on nonlinear harmonic generation in single-pass, high-gain free-electron lasers

There is a strong desire for short wavelength (~1 Å), short pulsewidth (<100 fs), high-brightness, transverse and longitudinally coherent light pulses for use by the synchrotron radiation community. These requirements exceed the limits achievable by existing, so-called, "third-generation" light sources, such as the Advanced Photon Source (APS) at Argonne National Laboratory (ANL), USA, the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, and SPring-8 in Harima Science Garden City, Japan. Single-pass, high-gain free-electron laser (FEL) methods have the ability to fulfill these requirements and have been proposed as the next-generation light source. Such arrangements include, but are not limited to, straight amplifier configurations, self-amplified spontaneous emission (SASE), the two-undulator harmonic generation amplifier scheme (TUHGS), and high-gain harmonic generation (HGHG). These single-pass, high-gain FELs typically employ planar undulators and can all generate nonlinear spectral harmonics with significant power levels. Of notable interest is the combination of the accelerator and traditional lasers, since existing laser technologies may be used for seeding the amplifier, TUHGS, and HGHG cases. This work examines single-pass, high-gain free-electron lasers analytically, via numerical simulations, and experimentally. An existing code, MEDUSA, was further developed to simulate relevant mechanisms as will be described. Along with a review of the respective theories, a three-step SASE FEL experiment at the APS, a two-step FEL experiment at the Accelerator Test Facility (ATF) at Brookhaven National Laboratory (BNL) involving both the SASE and HGHG methods, and the characteristics of nonlinear harmonic generation in both of these experiments are discussed. Finally, a modular approach to the next-generation light source is described. The above proof-of-principle experiments represent necessary steps toward achieving the next-generation light source. The ATF experiment tests the SASE and HGHG theories at a mid-infrared wavelength, while the APS experiment examines the SASE theory first at visible and then at ultraviolet wavelengths. The extension from the visible/ultraviolet wavelengths to the x-ray wavelength regime is not trivial, and there still remains much work before achieving the final goal based on the single-pass, high-gain free-electron laser theories and experiments. Of the topics elucidated within this manuscript, nonlinear harmonic generation in single-pass, high-gain FELs is perhaps the most significant. Although there is a reduction, compared to the fundamental wavelength, in the resultant output photon power when using the nonlinear harmonics to achieve shorter wavelengths, the harmonics do permit the use of an electron bunch of both lower energy and lesser quality. Therefore, smaller, less expensive machines could be developed, allowing many more facilities to be constructed and ultimately benefit from these bright, coherent, next-generation light sources

The Effect of Wiggler Imperfections on Nonlinear Harmonic Generation in Free-Electron Lasers

The generation of harmonics through a nonlinear mechanism driven by bunching at the fundamental has sparked interest in using this process as a path toward an X-ray free-electron laser (FEL). An important issue in this regard is the sensitivity of the nonlinear harmonic generation to wiggler imperfections. Typically, linear instabilities in FELs are characterized by increasing sensitivity to both electron beam and wiggler quality with increasing harmonic number. However, since the nonlinear harmonic generation mechanism is driven by the growth of the fundamental, the sensitivity of the nonlinear harmonic mechanism is not severely greater than that of the fundamental. In this paper, we study the effects of wiggler imperfections on the nonlinear harmonics in a 1.5- FEL, and show that the decline in the third harmonic emission with increasing levels of wiggler imperfections roughly tracks that of the fundamental

A compact free electron laser device operating in the UV-soft X-ray region

We discuss the design elements of a compact free electron laser device, operating, in an oscillator-amplifier configuration, up to the soft X-ray region. We show that, the source can provide radiation from extreme UV to soft X-ray region, with significantly large output peak and average laser power with undulator lengths not exceeding 7 in

Evolution of transverse modes in a high-gain free-electron laser

At the point of saturation in a high-gain free-electron laser (FEL) the light is fully transversely coherent. The number and evolution of the transverse modes is important for the effective tune-up and subsequent operation of FELs based on the photon beam characterization and in designing multi-module devices that rely on relatively stable saturation distances in each module. In the latter, this is particularly critical since each section will seed another module. Overall, in a single- or multi-module device, experimental users will desire stability in power and in photon beam quality. Using a numerical simulation code, the evolution of the transverse modes in the high-gain free-electron laser (FEL) is examined and is discussed. In addition, the transverse modes in the first few higher nonlinear harmonics are investigated

The Sensitivity of Nonlinear Harmonic Generation to Electron Beam Quality in Free Electron Lasers

The generation ofharmonics through a nonlinear mechanis driven by bunching at the fundamentalhas ssdam interes as a path toward enhancing and extending the uspOzOFTH of an X-ray free-electronlase (FEL) facility. The seTLkk-zTH of the nonlinear harmonic generation to undulatorimperfections electron beam energy sergyT peak current, and emittanceis important in an evaluation of theprocesW Typically, linear insrTDDzJLTH inFELs are characterized by increasp sreaspTHp to both electron beam and undulator quality withincreas-T harmonic number. However,swev the nonlinear harmonic generationmechanis is driven by the growth of the fundamental, thesTDkLJFTHp of the nonlinear harmonic mechanis is not expected to besp-OF-THpJO greater than that of the fundamental. Inthis paper, wesTLW the effects of electron beam quality, moreseTJkzFpTHp emittance, energy sergyT and peak current, on the nonlinear harmonics in a 1.5- A FEL, and sdT that the decline in the harmonic emisnic roughlyfollows that of the fundamental