In the gas dynamic trap experiment with 17 keV and 4.5 MW deuterium neutral beam injection the spatial profile of fast ion density has been studied by different methods: MSE spectroscopy, active charge-exchange diagnostic and measurement of DD fusion product fluxes. The characteristic radius of fast ion density profile was found to be about 7 cm at 1/e level mapped onto the GDT midplane, that is close to gyroradius of 10 keV deuteron and less than the estimated region occupied by the captured ions(~15 cm). The analysis of energy balance shows that discrepancy between measured and simulated values (~1.5 times) cannot be explained by enhanced fast ions loses. Simplified theory of fast ion density spatial profiles formation shows that energetically profitable configuration has narrow radial profile. Physical mechanisms of density profile formation are also described.