Fig. 10. Free electron laser process: electron velocity opposite to electric field of electromagnetic (EM) wave. The EM wave propagates to the right, past the electron. The electron gains, and the wave loses, energy. the rate at which the electron passes wavelengths of the periodic magnet) will cause the bulk of electrons to accumulate in the Fig. 11 situation. Once that accumulation occurs, and it occurs toward the right side of the periodic magnet, there will be a net transfer of energy to the wave. Hence, the wave will grow, i.e., laser action will take place. By changing the speed of the entering electrons, the frequency of the amplified wave can be varied. In other words, the FEL is tunable. The tunability of the FEL is one of its important advantages. Other advantages are its potentially high efficiency (30%^50%) (16), its potentially high power (17), and the fact that heating of an output window is not a problem because the FEL does not need an output window (18). The output window in a laser serves to contain the lasant while permitting the output radiation to exit the laser cavity. In a FEL, the lasant consists of relativistic electrons that are “contained” by the magnetic field. Due to this magnetic containment, no output window is necessary, and the output radiation can exit around the perimeter of a 100%-reflecting mirror that would replace the partially reflecting mirror on the right side of Fig. 8 and would be of smaller diameter than the mirror on the left. The above advantages of the FEL in the SPS context can be contrasted with the following disadvantages: [1] solar-to-electric power conversion is required, [2] the FEL is in the early stages of development, and [3] a stable, efficient electron beam energy recovery system has not yet been developed (17).
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