First characterization by Beam Emission and Cavity Ring-Down Spectroscopy of the SPIDER negative ion beam produced in the plasma volume

The Neutral Beam Injector (NBI) foreseen for ITER will produce a H/D beam based on H-/Dions provided by an accelerator capable to deliver a current density of 350 A/m² (H) / 285 A/m² (D), an extracted beam inhomogeneity <10 %, and a co-extracted electron fraction below 1 (H) / 0.5 (D) [1, 2]. The full size negative ion source SPIDER has recently completed an extensive characterization in volume operation mode in preparation to Cs operations. This phase is crucial to assess source readiness for long pulses (1h at least), the stable full power operation, as well as to test the beam and source diagnostic tools required to investigate the physics of SPIDER. In particular, to study the physics of negative ion production in the plasma volume and their extraction and acceleration, the Beam Emission Spectroscopy (BES) and Cavity Ring-Down Spectroscopy (CRDS) recently came fully operational and were extensively employed. BES measures beam deflection, divergence, intensity and stripping losses from the emission spectra [3] of the currently available 80 H-/D- beamlets, whereas, CRDS measures the volume density of negative ions in front of the plasma grid [4]. We discuss herein relevant recent results on negative ion beam characterization both in hydrogen and deuterium by providing an insight on the correlation between negative ion density and the extracted beam. For instance, beam features such as divergence and spatial intensity profiles were investigated by varying extraction grid voltage and acceleration voltage. The results presented in this contribution represent an important reference for SPIDER characterization in Cs-free operation as well as for a comparison with upcoming campaigns in the presence of Cs.