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Unformatted text preview: Basic Physical Processes and Principles of Free Electron Lasers (FELs) Dagnachew W. Workie Department of Physics University of Cincinnati Cincinnati, Ohio 45221 November 26, 01 Abstract A tunable coherent and high peak power sources have been very usefull in many ap- plications. In this paper the physical process of new type of coherent, flexible and very bright radiation source called Free-Electron Lasers ( FEL s ) is descreibed. The motion of electron beam in the presence of the optical and undulator fields leads to energy exchange. This energy exhange leads to formation of bunches of electrons which will emit very bright radiation. The resonance wavelength can, in principle,be tuned to any radiation regions. This is possible by changing the undulatorparameter and/or electron beam energy. The line-shape of the emitted spectrum has the characteristic form of the short time emission process. Different regions of the emitted radiation have found very wide applications. 1 1 Introduction The Free electron lasers ( FEL s ) was invented by by J.M.J. Madey in 1971. Free Electron Lasers are very flixible sources of coherent radiation. Due to their wide range tunabilty and high brghtness they have a growing applications. The FEL s system consists of an electron accelerator, an undulator in which the electrons emit the syncrotron radiation, and an optical resonator. 2 Principles of operation of FEL In an FEL a beam of relativistic electrons produced by an accelerator passess through a transverse periodic field produced by a magnet called undulator and exchanges energy with an electromagnetic field (Fig. 1). As a result of energy exchange, the electrons that gain Figure 1: Basic components of FEL . energy begin to move ahead of the average electron, while the electrons that lose energy begin to fall behind the average . The beam then becomes bunched on the scale of the radiation wavelength and this collective motion of the bunches radiatespowerful coherent synchrotron radiation. Giga watts peak power have been demonstarted. The wavelength of the emitted radiation at the resonance depends on the electron energy and the magnitude and periodicity of the undulator field according to the relation [1, 3] o = u 2 2 (1 + 2 ) (1) where u is the undulator period, is the relativistic factor and is the so called undulator parameter which is proportional tothe magnetic field inside the undulator. Typical values for electron beam enery ranges from few MeV ( 1) toa few GeV ( 1000); and the peak current varies from several ampersto many hundred of ampers. The peak undulator field strength B o is usually several KG ( B o 2- 7 KG ). The undulator period u is in the range of a few centimeters ( u 2- 10 cm ), whereas the undulator length L u = N u extends for a few meters (...
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