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We explore the analytical dependence of the free-electron laser (FEL) saturated power on the cavity detuning and discuss the dependence of the higher harmonics generated power on this quantity. We show that a suitable modulation of the cavity detuning may provide a pulsed behaviour of the laser, superimposed to the natural mode locking. The new temporal characteristics of the laser are discussed and it is shown that they can be controlled by controlling the cavity-length modulation period. We also study the effect of this modulation on the harmonic generation. The possibility of extending the same mechanism to a free electron laser operating with a storage ring is finally discussed and we show that, under certain conditions, it may modify the characteristics of the stationary configuration.

The potential advantages and the physical properties of several electron-transparent mirrors (e.g., metal meshes, thin foils, pierced mirrors, and Bragg reflectors) to be used in infrared free-electron laser resonators are reviewed. The conditions under which the effect on the electron beam quality can be kept small are discussed, and experimental results on the angular spread produced by a 2.5- mu m-thick metal mesh on a 5 MeV electron beam are presented. The experimental test of a resonator with two different electron transparent output couplers is reported. >

The preliminary design of an electron storage ring dedicated to the free electron laser operation, is presented. The main feature of the proposed laser is the complete tunability in the wavelength range 0.3 ?m < ? < 20 ?m with an output average power of the order of 1 kW and an efficiency, electron beam-laser beam, of ~ 2%.

The steady state features of the free electron laser amplifier operating in a storage ring is presented. The electron distribution relative to the longitudinal motion and the amplifier output are evaluated as functions of the storage ring parameters and the laser power density. It is shown that the energy transferred from the electrons to the laser beam is proportional to the energy emitted via synchrotron radiation in the whole machine.

We present the essential features of the 20 ÷ 30 MeV microtron single pass FEL (oscillator) project developed at the C. N. E. N. Frascati Center for high average power infrared radiation generation (? = 10 ÷ 30 ?m).

A simple argument to maximize the average output laser power in a single passage FEL device is presented. The optimization parameter is the undulator number of periods. The optimum values to get both maximum gain and average power are indicated and are shown to be different.

The limitations on the FEL performances due to the electron beam length, energy spread and emittance are examined.