The successful lasing at the SLAC National Accelerator Laboratory of the Linear Coherent Light Source (LCLS), the first X-ray free-electron laser (X-ray FEL), in the wavelength range 1.5 to 15 A, pulse duration of 60 to few femtoseconds, number of coherent photons per pulse from 1013 to 1011, is a landmark event in the development of coherent electromagnetic radiation sources. Until now electrons traversing an undulator magnet in a synchrotron radiation storage ring provided the best X-ray sources. The LCLS has set a new standard, with a peak X-ray brightness higher by ten orders of magnitudes and pulse duration shorter by three orders of magnitudes. LCLS opens a new window in the exploration of matter at the atomic and molecular scales of length and time. Taking a motion picture of chemical processes in a few femtoseconds or less, unraveling the structure and dynamics of complex molecular systems, like proteins, are some of the exciting experiments made possible by LCLS and the other X-ray FELs now being built in Europe and Asia. In this paper, we describe the history of the many theoretical, experimental and technological discoveries and innovations, starting from the 1960s and 1970s, leading to the development of LCLS.

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The History of X-ray Free-Electron Lasers

This article presents a unified formulation and review of an extensive class of radiation effects and devices based on free or quasifree electrons. The effects and devices reviewed include slow-wave radiators [such as \ifmmode \check{C}\else \v{C}\fi{}erenkov, Smith-Purcell, and TWT (traveling-wave tube) effects and devices], periodic bremsstrahlung radiators [such as undulator radiation, magnetic bremsstrahlung FEL's (free-electron lasers), and coherent bremsstrahlung in the crystal lattice], and transverse-binding radiators [such as the CRM (cyclotron resonance maser) and channeling radiation]. Starting from a general quantum-electrodynamic model, both quantum and classical effects and operating regimes of these radiation devices are described. The article provides a unified physical description of the interaction kinematics, and presents equations for the characterization of spontaneous and stimulated radiative emission in these various effects and devices. Universal relations between the spontaneous and stimulated emission parameters are revealed and shown to be related (in the quantum limit) to Einstein relations for atomic radiators and (in the classical limit) to the relations derived by Madey for magnetic bremsstrahlung FEL for on-axis radiative emission. Examples for the application of the formulation are given, estimating the feasibility of channeling radiation x-ray laser and optical regime Smith-Purcell FEL, and deriving the gain equations of magnetic bremsstrahlung FEL and CRM for arbitrary electron propagation direction, structure (wiggler) axis, and radiative emission angle.

Spontaneous and stimulated emission from quasifree electrons

A simple theoretical approach to the free electron laser is reviewed. Extensions of the Lorentz-Maxwell theory are straight forward and can be solved analytically, or on a small computer. A brief discussion looks at high-gain collective effects, short pulse effects, harmonics, energy spread and emittance, transverse diffraction, and noise.

Tutorial on classical free electron laser theory

Spectra have been obtained of the light generated near 10 μm by the Los Alamos free-electron laser. The spectra contain a main line and copious sidebands, mostly to the long-wavelength side of the main line. The spectra vary in overall width as the detuning parameter is altered or as the sidebands are preferentially allowed to escape the optical cavity. A comparison is made between these measurements and the results of numerical simulations.

Raman spectra and the Los Alamos free-electron laser

A general discussion of the free-electron lasers (FEL's) with variable parameter wigglers is presented with a view towards their potential for the production of high power optical radiation at reasonable efficiency. The theoretical analysis is based upon a one-dimensional relativistic Hamiltonian formulation and is developed in a manner to take advantage of the analogy between the FEL process and radio frequency accelerators. Three promising operational modes are identified and analyzed. The first may be thought of as an electron decelerator and is thought to have the most promise for single-pass devices. Both oscillator and amplifier configurations are studied. The second is based upon adiabatic trapping and detrapping, intended to reduce the spread in electron energy typically induced by the FEL process. The third is based upon the method of phase area displacement. It has the advantage of wide gain bandwidth and small induced energy spread, and is thought to have the most promise for storage ring applications. Generally speaking, it is found that high peak power is intrinsic to these modes of operation. Potential problems from parasitic oscillations analogous to the stimulated Raman effect are analyzed, and some others arising from transverse inhomogeneity are identified.

Free electron lasers with variable parameter wigglers

The problem of oscillator evolution and mode competition in free electron lasers is studied. Relativistic quantum field theory is used to calculate electron wave functions, the angular distribution of spontaneous emission, and the transition rates for stimulated emission and absorption in each mode. The photon rate equation for the weak-field regime is presented. This rate equation is applied to oscillator evolution with a conventional undulator, a two-stage optical klystron, and a tapered undulator. The effects of noise are briefly discussed.

P. Bosco

Papers

Quantum/classical mode evolution in free electron laser oscillators