In this issue the problem of the development and utilization of lasers, these unique sources of coherent radiation, has been given great consideration. The problem of the development of lasers working at various wavelengths has also been investigated, together with the problem of frequency tuning and that of the simultaneous generation of several frequencies. This is connected with the necessity of the utilization of lasers for the solution of an increasingly expanding range of problems which have a great scientific and applicative value. In this journal the question was also exposed in detail of the utilization of chargedparticle accelerators, in particular of electron accelerators. Many papers were dedicated to the investigation of synchrotron and undulator radiation generated by the motion of the particles in circular and spiral trajectories. Of even greater importance is not the spontaneous~ but the induced undu!ator radiation. The devices in which such a radiation is produced have received the designation of free-electron lasers (FEL). Such a name is used in the sense that here, unlike for conventional lasers, as the active medium one utilizes an electron beam. The particles' trajectory is determined by the external magnetic fields, and, in this sense, the electron beam is not free; the name does not stress so much the freedom of the electron, as the difference of this device from a conventional laser, in which the electrons of the atomic shells are used. The deep interest in the FEL is explained by the properties of the radiation produced by such a device. Among those properties are, primarily, the possibility of tuning it over a wide range of wavelengths and the possibility of obtaining radiation with very large peak and average power, with a high conversion efficiency of electron energy into radiation energy. Already in 1933 Kapitsa and Dirac [i] theoretically demonstrated the possibility of generating stimulated Compton radiation through the interaction of electrons with an external electromagnetic wave. Many theoretical and experimental studies on this device, which can be considered a forerunner of the recent FEL's, were conducted at the end of the 1940s and the beginning of the 1950s. One should mention that, since conventional lasers did not exist yet, the possibility was investigated mainly of generating waves in the millimeter and submillimeter band. A large number of theoretical studies of this type was performed by Ginzburg and co-workers [2]. In particular, he showed that for this purpose one could use the Cherenkov radiation generated by the transit of charged particles through a hole in a dielectric, and he established the criteria for the construction of those holes without reducing the generated radiation. An important study was performed by Motz and collaborators [3] on the Ubitron, which is the device closest to the FEL (Fig. i). Later the undulator radiation was studied at the Erevyan Institute of Physics, the Institute of Physics of the Academy of Sciences, at the Tomsk Polytechnical Institute of the Institute for Scientific Research, Nuclear Physics Branch [4-7]. It is interesting to mention that, in order to test the possibility of generating waves in the millimeter band, it was necessary to have well-bunched beams, but since there were none, then to test the principle of generation itself the Stanford linear electron accelerator was used, with an energy of 100-120 MeV. The performed experiment confirmed the simultaneous occurrence of visible light. But, of course, this was not induced radiation, and
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Jul 1, 1992
D.J Larson + 1
