Researches for development of multi-core superconductors for magnetic system of the International thermonuclear experimental reactor are carried out. Influence of titan concentration in niobic fibers on a microstructure of received intermetallic phases Nb₃Sn and density of a critical current is considered. It is established that density of critical current reach the maximum values >800 А/мм² in a magnetic field of 12 tesla at temperature 4,2 K and electrostatic field strength 0,1 µV/sm at the size of niobic fibers 2–3 µm.