  Longitudinal space charge (LSC) driven microbunching instability in electron
beam formation systems of X-ray FELs is a recently discovered effect hampering
beam instrumentation and FEL operation. The instability was observed in
different facilities in infrared and visible wavelength ranges. In this paper
we propose to use such an instability for generation of VUV and X-ray
radiation. A typical longitudinal space charge amplifier (LSCA) consists of few
amplification cascades (drift space plus chicane) with a short undulator behind
the last cascade. If the amplifier starts up from the shot noise, the amplified
density modulation has a wide band, on the order of unity. The bandwidth of the
radiation within the central cone is given by inverse number of undulator
periods. A wavelength compression could be an attractive option for LSCA since
the process is broadband, and a high compression stability is not required.
LSCA can be used as a cheap addition to the existing or planned
short-wavelength FELs. In particular, it can produce the second color for a
pump-probe experiment. It is also possible to generate attosecond pulses in the
VUV and X-ray regimes. Some user experiments can profit from a relatively large
bandwidth of the radiation, and this is easy to obtain in LSCA scheme. Finally,
since the amplification mechanism is broadband and robust, LSCA can be an
interesting alternative to self-amplified spontaneous emission free electron
laser (SASE FEL) in the case of using laser-plasma accelerators as drivers of
light sources.
