  We develop a theoretical model for a tunable coherent terahertz radiation
source based on the long-lived Bose condensate of photons. In the device we
propose, the original photon pumping is performed incoherently by a blackbody
radiation emitter. The photons thus produced Bose-condense by the inelastic
relaxation on a two-dimensional electron gas in a perpendicular magnetostatic
field. The process involves neither population inversion nor light wave
amplification the standard laser sources are built on. The coherence and
tunability of the light emitted by such a photon condensate are provided and
supported by the discrete spectrum of the electron gas in the quantizing
magnetic field. The device is a compact-size semiconductor crystal. We propose
the design and perform the realistic calculations of the physical properties
and limiting factors for the terahertz photon Bose-condensate resonator. We
show that our terahertz source can deliver the highly coherent light emission
in the frequency range of 3-30 THz for the magnetic field induction of the
order of 2 T, with the upper emission frequency limit adjustable by the
strength of the magnetic field applied.
