Layout and first measurements of the RING optical cavity for laser NBI photoneutralization

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 (?=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.