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abstract:
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The authors explore the basic physics of the FEL, emphasizing the underlying physical principles with a minimum of mathematics. The description is based on the classical theory of the FEL as that is sufficient to explain most of the physics. The elementary principles of operation of an FEL and the trajectory of an electron in the magnetic field of both helical and planar undulator magnets are presented. The trajectories are then used to calculate the spontaneous emission of an electron in an undulator magnet. The electron equations of motion are extended to include the forces due to both the undulator and the radiation field. If the change in the radiation field is small in one pass through the undulator, one can compute the so-called 'small signal gain' of the FEL using only the particle equations of motion; the radiation field is assumed to remain constant. The potential for increasing the efficiency of an FEL by tapering the period and magnetic field strength of the undulator magnet is discussed.
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