A room-temperature rf undulator, based on helical corrugated waveguide, is designed and produced, in order to obtain coherent X-ray radiation using a relatively low-energy electron beam and pulsed mm-wavelength radiation. Low-power tests with 33 GHz, 0.5 m long undulator section are described. High-power experiments with gigawatt power level BWO, which is planned to feed the undulator, are analyzed.
The flying undulator is a high-power short rf pulse co-propagating together with a relativistic electron bunch in a helically corrugated waveguide. The electrons wiggle in the rf field of the -1st spatial harmonic with the phase velocity directed in the opposite direction in respect to the bunch velocity, so that particles can irradiate high-frequency Compton’s photons. A high group velocity (0.7 of light velocity) ensures long cooperative motion of the particles and the co-propagating rf pulse. So that 10 m of the proposed undulator can substitute 25 m of a conventional DC-magnet undulator (with period 3 cm) providing similar Pierce parameter and requiring almost 2.5 times less energy electron beam. The 0.5 m long undulator section was optimized for frequency 33.3 GHz, in order to minimize all types of losses at as broad frequency band as 1 GHz. The prototype of 0.5 m length, made of the electroformed copper, was produced and tested at low power level for the planned high-power experiment. In this experiment we are going to feed the produced undulator section by ~1 GW, 10 ns rf pulses which we have already obtained in the BWO oscillator driven by 500 kV, 100 A electron beam formed from autoemission cathode.
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