Efficiency enhancement in power conversion is discussed for a dielectric-loaded Raman-type free-electron laser. To analyze the problem, we divide the model of a Raman-type free-electron laser into many small segments along it, in each of which the drift velocity of the electron beam is assumed to be constant. In each segment, we solve numerically the dispersion relation for the coupled electromagnetic and space charge waves to get the spatial growth rate for them. Using the spatial growth rate obtained above, We calculate the increase in the power carried by the electromagnetic wave and the decrease in the drift velocity of the electron beam in that segment. By repeating the above calculation step by step, we follow the growth of the electromagnetic wave along the electron beam. To keep the electromagnetic wave synchronized with the electron beam, we gradually increase the permittivity of the dielectric loaded on the waveguide or gradually decrease the period of the wiggler magnetic field in the direction of wave propagation in accordance with the decrease in the drift velocity of the electron beam. Thus the efficiency of power transfer from the electron beam to the electromagnetic wave can greatly be enhanced.
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