FREE ELECTRON LASERS

The free electron laser (FEL) uses a high quality relativistic beam of electrons passing through a periodic magnetic field to amplify a copropagating optical wave (1-4). In an oscillator configuration, the light is stored between the mirrors of an open optical resonator as shown in Figure 1. In an amplifier configuration, the optical wave and an intense electron beam pass through the undulator field to achieve high gain. In either case, the electrons must overlap the optical mode for good coupling. Typically, the peak electron beam current varies from several amperes to many hundreds of amperes and the electron energy ranges from a few MeV to a few GeV. The electrons are the power source in an FEL, and provide from a megawatt to more than a gigawatt flowing through the resonator or amplifier system. The undulator resonantly couples the electrons to the transverse electrical field of the optical wave in vacuum. The basic mechanism of the coherent energy exchange is the bunching of the electrons at optical wavelengths. Since the power source is large, even small coupling can result in a powerful laser. Energy extraction of 5% of the electron beam energy has already been demonstrated. The electron beam quality is crucial in maintaining the coupling over a significant interaction distance and of central importance to all FEL systems is the magnetic undulator. The peak undulator field strength is usually several kG and can be constructed from coil windings or permanent magnets. In the top part of Figure 2, the Halbach undulator design is shown for one period. The field can be achieved, to a good approximation, using permanent magnets made out of rare earth compounds; a technique developed by K. Halbach (5), and now employed in most undulators. The undulator wavelength is in the range of a few centimeters and the undulator length extends for a few meters, so that there are several hundred periods for the interaction (6-8). The polarization of the undulator can be either linear or circular or a combination (9). The optical wave has the same polarization as the undulator driving it. This is an illustration of the FELs most important attribute, the flexibility of its design characteristics