Limitations on the efficiency of frequency conversion by reflection of electromagnetic waves from relativistic electron bunches or moving plasma

It is shown that the feasibility of obtaining high-power short-wavelength radiation by reflection of electromagnetic waves from a bunch of relativistic electrons is limited by the high transparency of these bunches and the finite dimensions of the leading edge of the electron buildup in such bunches. A comparison is made of coherent (across an electron beam) and noncoherent scattering processes and it is shown that the latter predominate. Simple expressions are obtained for estimating the experimental feasibility of such frequency conversion.