A new type of bunch compressor and seeding of a short-wavelength coherent radiation.

Transverse-to-longitudinal emittance exchange was proposed in [1] as a tool for an effective matching of the electron beam phase space to requirements of a possible application. Here we propose a new purpose, namely, use of two consecutive emittance exchanges equipped with the telescope between them for a bunch compression that can be done without the energy chirp in the electron bunch. In principle it allows to reduce the electron peak current in the linac by moving the bunch compressor to the end of the linac and, thus, to relax collective effects associated with high peak currents. It is also possible to have a split-action compression when the first part is done inside the low-energy part of the linac and the second and final part is done after the linac. We also demonstrate how proposed bunch compressor can be used for frequency up-conversion of the energy modulation provided by the laser interacting with the electron beam and thus can prepare a significantly higher frequency seed for seeded free-electron lasers. The same approach can be used for a frequency down-conversion that can be useful for generation of THz radiation