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

Abstract: 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.