Mostacci, A. Variola, A. JACoW Geneva, Switzerland The Extreme Light Infrastructure-Nuclear Physics (ELI-NP) facility is currently under construction near Bucharest (Romania); it will focus on laser-based fundamental research on nuclear physics. The facility will host two 10 PW laser systems and an advanced gamma beam source, called Gamma Beam System (GBS). GBS is a photon electron collider producing gamma rays from Compton back scattering of a laser light off a high charge, low emittance electron beam from a 720 MeV warm linac. The gamma rays will have tunable energy (1-20MeV) with worldwide outstanding performances, such as narrow bandwidth (0.3%) and high spectral density (10⁴ photons/s/eV). To achieve such challenging performances, the luminosity will be raised by colliding up to 32 electron bunches with a properly recirculated laser beam. New class diagnostics need to be developed at the interaction point to allow efficient photon electron collisions. This infrastructure will create a new European laboratory with a broad range of science covering frontier fundamental physics, new nuclear physics and astrophysics as well as applications in nuclear materials, radioactive waste management, material science and life sciences.