K.-J. Kim

Basic FEL Physics

This chapter provides a somewhat qualitative introduction to free-electron laser physics. After a brief introduction to how FELs fit in with other sources of coherent radiation and how FEL amplification can work, we derive in Section 3.2 the 1D equations of motion for the electrons in an FEL. We find that the electrons move in a pendulum-type potential formed by the transverse undulator and radiation fields. We then discuss the FEL particle dynamics in the limit that the gain is small in Section 3.3, for which we can approximate the electric field as constant so that we do not need Maxwells equations; this regime of operation is most suited to oscillator FELs that we cover in more detail in Chapter 7.We compute the FEL gain in the small-signal limit, and qualitatively describe the FEL when the radiation power is large. In Section 3.4 we include the self-consistent evolution of the radiation, which extends our equations to include high-gain FELs that experience exponential gain. We show how the basic physics of small signal gain and exponential growth can be derived in terms of three collective variables, and how the FEL Pierce parameter ρ plays a central role in determining the output characteristics. Finally, we make a brief introduction to self-amplified spontaneous radiation, which we will describe more thoroughly in Chapter 4. Introduction Coherent Radiation Sources Powerful, coherent sources are familiar for wavelengths longer than 1 mm – the microwave devices – and also for wavelengths between a few microns and approximately 0.1 μ m – lasers based on atomic and molecular transitions. Microwave devices (including magnetrons, klystrons, and, increasingly, solid-state devices) have found numerous applications including radar, accelerating structures, and food preparation. The applications of lasers operating in the IR, visible, and near UV are incredibly numerous and diverse: uses vary from precise measurements of time and distance to cutting and etching on both large and nano-scales to addressing single atoms for quantum computing.

Beam Optimization Study for an X-ray FEL Oscillator at the LCLS-II

The 4 GeV LCLS-II superconducting linac with high repetition beam rate enables the possibility to drive an X-Ray FEL oscillator at harmonic frequencies. Compared to the regular LCLS-II machine setup, the oscillator mode requires a much longer bunch length with a relatively lower current. Also a flat longitudinal phase space distribution is critical to maintain the FEL gain since the X-ray cavity has extremely narrow bandwidth. In this paper, we study the longitudinal phase space optimization including shaping the initial beam from the injector and optimizing the bunch compressor and dechirper parameters. We obtain a bunch with a flat energy chirp over 400 fs in the core part with current above 100 A. The optimization was based on LiTrack and Elegant simulations using LCLS-II beam parameters.