  A new scheme of the THz light source is proposed. The excitation of the
Langmuir wave in a moderately relativistic electron beam via the two-stream
instability and the subsequent interaction of the excited Langmuir wave with
the visible light laser results in THz ligh via the Raman scattering. The cost
of the current scheme could be cheaper than the conventional technologies by
using the electron beam from cathodes and low-intensity infra-red laser.
