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Funding: This work was supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC-02-06CH11357.
An XFELO proposed recently* requires a continuous sequence of electron bunches with ultra-low transverse emittance less than 0.1 mm-mr, a bunch charge of 40 pC, an rms energy spread of 1.4 MeV, repeating at a rate between 1 MHz to 100 MHz. The bunches are to be compressed to an rms lengths less than 2 ps at the final energy of 7 GeV. Following the successful commissioning of the pulsed injector based on a thermionic gun** we discuss a concept for ultra-low emittance injector to produce 100 MHz CW electron bunches. The electron beam is extracted by ~1MV rf voltage using low frequency ~100 MHz room temperature rf cavity. The injector also includes a chicane and slits to form a short ~1 nsec bunch, a pre-buncher a booster buncher to form low longitudinal emittance of the bunched beam, an accelerating section to ~50 MeV using higher harmonic cavities, and an rf cosine-wave chopper to form any required bunch repetition rate between 1 MHz and 100 MHz. The results of initial optimizations of the beam dynamics with the focus on extracting and preserving ultra-low emittance will be presented.
*K.-J. Kim, Y. Shvyd'ko, and S. Reiche, to be published in Physical Review Letters (2008)
**K. Togawa, et al., Phys. Rev. STAB 10, 020703 (2007)
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