  Comparing with ITER, the experimental fusion machine under constraction, the
next step test fusion power plant, DEMO will be characterized by very long
pulse/steady-state operation and much higher plasma volume and fusion power.
The substantially increased level of neutron and gamma fluxes will require
reducing the physical access to the plant. It means some conventional
diagnostics for the fusion plasma control will be not suitable in DEMO.
Development of diagnostics along with the machine design is a primary task for
the test plant. The deuterium- tritium fuel ratio and temperature are among
important parameters, which should be under control. In this letter a novel
technique for the core fuel ratio and temperature diagnostics is proposed. It
is based on measurements and comparison of the rates T(p,{\gamma})4He and
D(T,{\gamma})5He nuclear reactions that take place in the hot deuterium-tritium
plasma. Based on detection of high energy gamma-rays, this diagnostic is
robust, efficient and does not require direct access to the plasma. It could be
included in the loop of the burning plasma control system. A feasibility of the
diagnostic in experiments on JET and ITER is also discussed.
