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Several concepts for room-temperature RF undulators, fed by cm- or mm- wavelength radiation and aimed to produce ~1 nm wavelength radiation using relatively low energy electron beams, are considered. The analysis shows that requirements to electron beam quality and to RF power become more severe with wavelength reduction. In order to satisfy these requirements, we suggest to build an undulator of several separate segments. In accordance with first concept each segment introduces a high-Q cavity fed by its own multi-megawatt RF source. The 30 GHz pulsed gyroklystron or gyrotrons at 30 - 300 GHz, already elaborated and tested in IAP RAS, are appealing candidates. These sources can provide the effective undulator period of 0.5 - 0.05 cm, the undulator parameter up to 1, and the effective field length of 50 cm for each segment. The second concept implies using a high-power short pulse propagating in a long corrugated waveguide where -1st field harmonic is responsible for particle wiggling. High group velocity of this pulse allows to provide long interaction of particles with RF fields.
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