Validation of 2D Te and n e measurements made with Helium imaging spectroscopy in the volume of the TCV divertor

Multi-spectral imaging of helium atomic emission (HeMSI) has been used to create 2D poloidal maps of Te and n e in TCV's divertor. To achieve these measurements, TCV's MANTIS multispectral cameras (Perek et al 2019 Rev. Sci. Instrum. 90 123514) simultaneously imaged four He I lines (two singlet and two triplet) and a He II line (468 nm) from passively present He and He+. The images, which were absolutely calibrated and covered the whole divertor region, were inverted through the assumption of toroidal symmetry to create emissivity profiles and, consequently, line-ratio profiles. A collisional-radiative model (CRM) was applied to the line-ratio profiles to produce 2D poloidal maps of Te and n e. The collisional-radiative modeling was accomplished with the Goto helium CRM code (Zholobenko et al 2018 Nucl. Fusion 58 126006, Zholobenko et al 2018 Technical Report, Goto 2003 J. Quant. Spectrosc. Radiat. Transfer 76 331-44) which accounts for electron-impact excitation (EIE) and deexcitation, and electron-ion recombination (EIR) with He+. The HeMSI Te and n e measurements were compared with co-local Thomson scattering measurements. The two sets of measurements exhibited good agreement for ionizing plasmas: (5 eV <= Te <= 60 eV, and 2 x 10 18 m-3 <= n e <= 3 x 10 19 m-3) in the case of majority helium plasmas, and (10 eV <= Te <= 40 eV, 2 x 10 18 m-3 <= n e <= 3 x 10 19 m-3) in the case of majority deuterium plasmas. However, there were instances where HeMSI measurements diverged from Thomson scattering. When Te <= 10 eV in majority deuterium plasmas, HeMSI deduced inaccurately high values of Te. This disagreement cannot be rectified within the CRM's EIE and EIR framework. Second, on sporadic occasions within the private flux region, HeMSI produced erroneously high measurements of n e. Multi-spectral imaging of Helium emission has been demonstrated to produce accurate 2D poloidal maps of Te and n e within the divertor of a tokamak for plasma conditions relevant to contemporary divertor studies.