The ITER Neutral Beam Test Facility (NBTF), called PRIMA (Padova Research on ITER Megavolt Accelerator), is hosted in Padova, Italy and includes two experiments: MITICA, the full-scale prototype of the ITER heating neutral beam injector, and SPIDER, the full-size radio frequency negative-ions source. The NBTF realization and the exploitation of SPIDER and MITICA have been recognized as necessary to make the future operation of the ITER heating neutral beam injectors efficient and reliable, fundamental to the achievement of thermonuclear-relevant plasma parameters in ITER. This paper reports on design and R&D carried out to construct PRIMA, SPIDER and MITICA, and highlights the huge progress made in just a few years, from the signature of the agreement for the NBTF realization in 2011, up to now-when the buildings and relevant infrastructures have been completed, SPIDER is entering the integrated commissioning phase and the procurements of several MITICA components are at a well advanced stage.

The first three moments of the energy distributions of products from fusion reactions in thermonuclear plasmas with Maxwellian ion velocity distributions are determined analytically. Relativistic kinematics is used allowing the desired accuracy to be reached, which is 2 to 3 orders of magnitude better than previous analytical results. In particular, neutron spectra of the reactions D(d,n)³He and D(t,n)α for plasma ion temperatures 0 < T_i < 100 keV are studied for which the results are also given in tabulated form with interpolation formulas for the purpose of practical use. The neutron energy distributions are also calculated with numerical methods, in order to assess the analytical results. High accuracy calculations are motivated by the crucial role that neutron measurements are envisaged to play in the next step fusion experiments on burning plasmas, which are also discussed.