High-resolution visible spectroscopy for ion temperature and flow velocity measurements of the TCV divertor plasma

A high-resolution spectroscopic system was installed on the Tokamak à Configuration Variable (TCV), whose toroidal lines-of-sight (LOS) cross the divertor region. This system simultaneously measures the Doppler broadening and Doppler shift of several spectral transitions to infer their associated ion temperatures (Ti) and the toroidal component of their flow velocities (Vi). Herein, TCV's Tangential Divertor Spectroscopy System (TDSS) is described, along with its data analysis procedure and the first simultaneous measurements of temperature and flow velocity of C2+, He+, and N+ ions. The TDSS can be configured with a range of LOS that can be tailored to different divertor magnetic configurations. To obtain high sensitivity and accuracy in the kinetic ion parameters, each LOS is complemented by a tangentially opposed LOS to achieve a velocity sensitivity of 2.5 km/s/pixel that implicitly annuls any spectroscopic first-order drifts. A spectral resolution of 0.2 Å allows for reliable temperature determination down to ∼1 eV for He ions and ∼3 eV for the heavier C and N ions. These abilities, together with TCV's plasma shaping and positioning flexibility, were used to obtain the poloidal distributions of Ti and Vi for several impurity ions across the divertor. These measurements, together with the TS measurements of Te and ne, can be used to study the convective vs conductive partitions of energy transport across the scrape-off-layer plasma, which is crucial to the physics of energy transport in the divertor.