"Relativistic mirror" experiment with frequency tuning and energy gain

The front of an intense relativistic electron beam has been used to reflect a counterstreaming electromagnetic wave (fᵢ=9.3 GHz, Pᵢ=250 kW). A doubly-Doppler-shifted reflected wave with a pulse duration on the order of a nanosecond has been produced with a sixfold increase in frequency, a 13-dB power gain, and a doubling of wave energy. The output has been analyzed with a multichannel grating spectrometer and found to have a spectral width Δλλ of {~}5%. The reflected-wave frequency was tunable over a range of {~}±{}20% by simply varying the external magnetic field, and over a much broader range by simultaneously changing the beam rise time. All these observations are in good agreement with a simple theoretical model of beam-front scattering. This mechanism represents a new kind of short-pulse, high-power, tunable source of millimeter (and probably submillimeter) wavelength radiation, and it could be immediately useful as an electron-beam velocity diagnostic.