One of the most used systems to heat the plasma to thermonuclear temperatures in fusion experiments is the injection of energetic neutral beams (Neutral Beam Injection - NBI). In the path toward the European demonstrative fusion reactor DEMO, the efficient neutralization of a high energy (~1MeV) negative ion beam in a NBI system is one of the major issues. Current gas neutralizers limit the neutralization efficiency to <60%, requiring also strong pumping to avoid the gas spreading along the injector. A promising, and yet undemonstrated method alternative to gas-neutralization is photo-neutralization, where a negative ion beam (D- typically) is neutralized by the interaction with photons. Several systems have been proposed with expected neutralization efficiencies in the range of 70-90%. In this work we describe a novel photoneutralization scheme in which the second harmonic of a Nd:YAG laser beam is generated and trapped within a non-resonant optical cavity (Recirculation Injection by Nonlinear Gating—RING concept). After a conceptual study of the RING concept, a mock-up of the optical cavity has been realized, with a Lithium Triborate (LBO) crystal as second harmonic generator. The layout of the optical cavity mock-up is presented together with the first measurements of the intensity of the trapped multi-folded laser beam. The measurements are taken with a low repetition rate (10 Hz) Nd:YAG laser (λ=1064 nm). The efficiency of the second harmonic generation is measured and compared with the theoretical value. The measurement of the laser beam intensity after a round trip (10 reflections) allows to estimate the overall performances of the optical cavity.