Free Electron Lasers
TL;DR: In this paper, the authors present a survey of the free electron laser (FEL) design and its operating modes, including phase displacement, adiabatic capture, and detrapping.
Abstract: It is the purpose of this paper to explain the various operating modes of the \"free electron laser\" in a manner that is easy for accelerator designers to understand.1 The successful operation of the \"free electron laser\" by the group at Stanford, directed by John Madey,2'3 along with the availability of high power electron beams has stimulated a great deal of interest in the use of the \"free electron laser\" (FEL) to produce high power tunable laser beams. At the last National Accelerator Conference held in San Francisco, Pellegrini4 presented a paper on the FEL. Since his paper contains an excellent discussion of the work preceding and following Madey's experiments, we will not repeat it here. Often the analysis of the FEL, as well as the methods envisioned for the FEL operation, started with the assumptions that the period and amplitude of the wiggler field were constant and the magnetic field uniform in the transverse direction. A great deal of the earlier work, using the techniques of the laser and plasma physics community, was presented at the 1977 Telluride Conference.5 Here Colson developed the equations of motion for a single electron moving through a wiggler in the presence of an electromagnetic wave propagating along the wiggler axis. The similarity of these equations of motion to those used by accelerator physicists, desitning radio frequency accelerating systems, allowed us to use the ideas developed for the acceleration of charged particles to guide the design of the FEL. The FEL is viewed as a decelerator in which the usual longitudinal accelerating field of a microwave cavity is replaced by the transverse decelerating field of a laser. The FEL uses a periodic transverse magnetic field (wiggler field) to provide the coupling between the longitudinally directed electrons and the transverse laser field. At the 1979 Telluride Conference7 many of the papers used terms like: coupling between the betatron and synchrotron oscillations, stationary and decelerating buckets, phase displacement, adiabatic capture, and detrapping. Accelerator physics has both influenced FEL design and introduced a new jargon into the field. Since the equations of motion have been derived before,6 the main emphasis of this paper is to demonstrate the similarity of these equations to those studied by the accelerator physicist for many years. In the following survey of the field, we will use accelerator methods to discuss the present FEL designs. Because this survey is mainly tutorial the derivation of the equations is presented in a physically intuitive fashion rather than in a strictly rigorous manner.
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01 Jan 1988
56 citations
TL;DR: In this article, the problem of the development and utilization of free-electron lasers (FEL) has been given great consideration, and the question was also exposed in detail of the utilization of chargedparticle accelerators, in particular of electron accelerators.
Abstract: 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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TL;DR: A free-electron laser oscillator has been operated above threshold at a wavelength of 3.4 µm as discussed by the authors, where µm is the number of free electrons in a single photon.
Abstract: A free-electron laser oscillator has been operated above threshold at a wavelength of 3.4 \ensuremath{\mu}m.
865 citations
TL;DR: In this article, a tunable high-power free-electron laser with a relativistic electron beam in a constant spatially periodic transverse magnetic field has been demonstrated, achieving a gain of 7% per pass at an electron current of 70 mA.
Abstract: Gain has been observed for optical radiation at 10.6 \ensuremath{\mu}m due to stimulated radiation by a relativistic electron beam in a constant spatially periodic transverse magnetic field. A gain of 7% per pass was obtained at an electron current of 70 mA. The experiments indicate the possibility of a new class of tunable high-power free-electron lasers.
496 citations
01 Jun 1975
TL;DR: In this paper, gain has been observed at 10.6 micrometers due to stimulated emission of radiation by relativistic electrons in a spatially periodic transverse magnetic field.
Abstract: : Gain has been observed at 10.6 micrometers due to stimulated emission of radiation by relativistic electrons in a spatially periodic transverse magnetic field. The magnitude of the measured gain is close to the theoretical value for the stimulated emission process. The dependence of the gain on electron energy and current and on the polarization of the stimulating radiation are also consistent with the stimulated emission hypothesis. The results raise the possibility that this mechanism can be used in the development of a new class of tuneable high power lasers.
492 citations
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TL;DR: In this article, a theory of the coupling between the negative energy plasma wave and the electromagnetic radiation by means of the rippled magnetic field is presented, and the saturation mechanism is found to be the trapping of the beam by the unstable plasma wave.
Abstract: The possibility of a new type of laser has been investigated by computer simulation using a fully relativistic electromagnetic particle code which has one spatial and three velocity dimensions. By passing a relativistic electron beam over a rippled static magnetic field, high frequency electromagnetic radiation is generated. If the ripple wavelength is λ0, the lasing wavelength is roughly λ0/2γ2. Thus, such a laser is continuously tunable by varying γ. It has been observed in simulation that as much as 35% of the beam energy can be converted into radiation, of which as much as nearly 90% can be in the most rapidly growing mode. A theory of the coupling between the negative energy plasma wave and the electromagnetic radiation by means of the rippled magnetic field is presented. Good agreements have been obtained between the simulation and the theory. The saturation mechanism is found to be the trapping of the beam by the unstable plasma wave. A theoretical estimate of the amount of energy that can be converted into radiation from the electron beam is also given.
205 citations
TL;DR: The classical mean energy radiated by electrons in a free-electron laser with a symmetric magnet is equal to one-half the derivative, with respect to energy, of the classical mean squared radiated energy as mentioned in this paper.
Abstract: To lowest order in the electric field and the inverse electron energy, the classical mean energy radiated by electrons in a free-electron laser with a symmetric magnet is equal to one-half the derivative, with respect to energy, of the classical mean squared radiated energy. The integral for the mean squared energy is also shown to be identical to the integral for the classical spontaneous power spectrum.
201 citations