  A scheme of soft-xray radiation is proposed. In the scheme, a moderately
intense laser excites plasmons via the two-plasmon decay in a relativistic
electron beam. As the second laser encounters the electron beam in the opposite
direction, it emits soft x-rays via the backward Raman scattering. Our analysis
suggests that the effective cross-section of this scattering is higher than the
Thomson scattering and that the conversion efficiency from the pump laser to
the soft x-ray could be very high in the optimal scenario. USing the plasmon
pump with duration of 1-100 pico-second and the electron beam with density of
10^18-10^19 per cc and energy of 1-20 MeV, soft x-ray of 5 nm to 300 nm with
the duration of 10 femto seconds to 1 pico-second can be emitted in the
direction of the electron beam. Advantages of the scheme of other schemes are
discussed.
