Design and mockup tests of the RING photo-neutralizer optical cavity for DEMO NBI

High energy Neutral Beam Injection (NBI) is one of the methods being considered to heat EU DEMO plasma. A major issue of present NBI systems is the limited efficiency of the gas neutralizer (for ITER NBI ~55%), which impacts on the overall system efficiency. An attractive method, but still undemonstrated at full performance, is the photo-neutralization of the negative D-ion beam, with an expected neutralization efficiency for a full-scale DEMO NBI up to 90%. A possible scheme for photo-neutralization is named RING (Recirculation Injection by Nonlinear Gating) where a laser second harmonic is generated and trapped within a non-resonant optical cavity. A mockup of the optical cavity is being operated in Consorzio RFX with a low repetition rate Nd:YAG laser (f = 10 Hz, ? = 1064 nm) to study the feasibility of the RING concept and its potentiality for a full-scale NBI photo-neutralizer. The 2nd harmonic generation efficiency has been measured using a set of 3 Lithium Triborate (LBO) crystals in a disk configuration, confirming the non-linearity of the process with the total crystal thickness. Losses during the 2nd harmonic recirculation in the cavity have been quantified, and the number of 2nd harmonic roundtrips has been estimated with photodiode measurements in order to evaluate the optical cavity performance. Further improvements are considered to increase the optical cavity photon accumulation.