A new generation of Neutral Beam systems will be required in future fusion reactors, such as DEMO, able to deliver high power (up to 150 MW) with high neutral energy (>1 MeV). Negative ion beams can attain this performance, resulting in a strong research focus on negative ion production from either surface or volumetric plasma sources. A novel helicon plasma source, employing on a resonant birdcage network antenna, is currently under study at the Swiss Plasma Center (SPC), before installation on the Cybele negative ion source [1] at CEA-IRFM. The source delivers up to 10 kW at 13.56 MHz, and was installed on a linear (1.8 m length) Resonant Antenna Ion Device (RAID). Passive spectroscopic measurements of the Ha, Hb and Hg Balmer lines and of all the diagonal Fulcher-a bands were performed with a sensitive f/2 spectrometer. Multiple viewing lines setup and an absolute intensity calibration were used to determine the plasma radiance profile, with a space resolution < 3 mm. A minimum Fisher regularisation algorithm is then applied to calculate the absolute emissivity profile for each emission line, in cylindrical symmetry as experimentally observed. An error estimate of the Abel-inverted profiles was performed using a Monte Carlo approach. Finally, an RF-compensated Langmuir probe measured the electron temperature and density profiles. The measured absolute line emissivities, electron density and temperature are interpreted using the collisional-radiative code YACORA [2] which estimates the dissociation degree and the distribution of the different atomic and molecular species, including the density of negative ions. In this paper, the results of a power scan up to 5 kW in conditions satisfying DEMO requirements, i.e. p 0:3 Pa and B 150 G, are shown.