Free‐electron laser (FEL) theory and experiments are reviewed. The physical mechanism responsible for the generation of coherent radiation in the FEL is described and the fundamental role of the ponderomotive wave in bunching and trapping the beam is emphasized. The relationship of the FEL interaction to the beam–plasma interaction is pointed out. Various FEL operating regimes are discussed. These include the high‐gain Compton and Raman regimes, both with and without an axial guiding magnetic field. The linear and nonlinear regimes are examined in detail, with particular emphasis on techniques for achieving efficiency enhancement. The quality of the electron beam used to drive FEL’s is a critical factor in determining their gain and efficiency. The subject of electron beam quality, for different accelerators, is discussed. Key proof‐of‐principle experiments for FELs in an axial guiding magnetic field, as well as those driven by induction linacs, rf linacs, electrostatic accelerators, and storage rings, are reviewed. Finally, the requirements on wigglers and resonators are discussed.

A review of free‐electron lasers

Physics of the high-gain FEL and superradiance

Kinematic and dynamic properties of a waveguide FEL

The stability of single‐mode operation of free electron laser (FEL) oscillators is investigated. Two models of an untapered FEL oscillator are considered. The first model is called the klystron model. In the klystron model the FEL interaction occurs at two points: a prebunching point and an energy extraction point. In this model the nonlinear electron dynamics are solvable exactly, leading to a complex delay equation for the wave fields. The stability of single‐mode operation can then be determined easily as a function of a single‐pass gain, energy mismatch–frequency, and the difference between the group velocity of the radiation and the beam velocity. The second, more realistic model has a distributed interaction region of finite length: Stability of single‐mode operation in this device must be determined numerically. The results of the models in the low gain regime are compared and the parameter regimes where stable single‐mode operation is possible are determined. It is found that the time to reach a single‐mode state can be extremely long.

Mode competition and suppression in free electron laser oscillators

Vacuum electronic sources have shown marked improvement since the invention of the magnetron before World War II, and dramatic increases in both average powers and frequencies have been achieved. Of course, many of these gains have been achieved by the development of different devices. The typical development pattern for a given device exhibits and initial period of rapid improvement followed by a plateau determined by technological or physical limitations on the concept. Slow wave devices such as magnetrons and/or klystrons operate efficiently at frequencies up through X-band or somewhat higher. Helix or coupled cavity traveling-wave tubes are used for various applications at frequencies ranging up through W-band. At still higher frequencies, fast wave devices such as gyrotrons and free-electron lasers are required for high-power operation. The free-electron laser concept is unique in that the mechanism is applicable across the entire electromagnetic spectrum; free-electron lasers have been built at wavelengths from microwaves through the ultraviolet, and plans are under development for X-ray systems. Our purpose in this paper is to describe the principal directions of free-electron laser research at the present time. To this end, we first give a brief tutorial of the physics underlying the concept and then describe the principal development paths under way.

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Free-electron lasers: vacuum electronic generators of coherent radiation

The frequency offset of the sideband instability in a free-electron laser (FEL) should depend on (1-v/sub X//v/sub g/)/sup -1/, where v/sub X/ is the average longitudinal velocity of the electrons and v/sub g/ is the group velocity of the electromagnetic waves. We have tested the v/sub X//v/sub g/ dependence of the sideband shift in a 2-mm, Raman-regime FEL oscillator. A change of v/sub X//v/sub g/ from 0.93 to 0.98, accomplished by an increase in the undulator period, resulted in the measured sideband shift increasing from 6% to 40%, in approximate agreement with theory.

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Sideband control in a millimeter-wave free-electron laser.

A theoretical analysis is given of an FEL operating in the millimeter range and, hence, operating in a waveguide. For study of the situation in which the average longitudinal velocity of electrons v/sub parallel/ is close to the electromagnetic pulse group velocity v/sub g/, it is shown that the usual paraxial approximation is invalid. The complete analysis is then specialized to a single particle model which, for operation in a steady state, yields a dispersion relation. The resulting dispersion relation is analyzed analytically and numerically. It is shown that when v/sub parallel/ greater than or equal to v/sub g/ there is no sideband instability, and an analytic expression is given for the stability boundary. 10 refs., 7 figs.

E.J. Sternbach

Papers

Sideband control in a millimeter-wave free-electron laser.

Waveguide suppression of the free electron laser sideband instability