Interpretation of divertor ion temperature measurements from an attached towards a detached regime

We present simultaneous profiles of the poloidal distribution of electron temperature and density (T_e, n_e), the poloidal emissivity distributions of a range of spectral lines together with the corresponding ion temperatures, T_i, in the TCV divertor plasma. These measurements were performed over divertor plasma conditions evolving from strongly attached towards a detached regime. The poloidal n_e and T_e distributions were measured by Divertor Thomson Scattering (DTS) diagnostic and by exploiting TCV’s flexibility to align divertor magnetic configurations to the DTS diagnostic laser path. T_i are inferred from Doppler broadening of C II, C III, and He II ions transitions measured by the high-resolution Divertor Spectroscopy System (DSS). The measured T_i represents an emissivity-weighted ion temperature along the DSS lines of sight. To study the electron/ion thermalisation, we compare T_i of each ion with a corresponding emission-weighted electron temperature, T^∗_e. We find that T_i of C⁺ is lower than its corresponding T^∗_e for all plasma parameters except for near-detachment conditions in the vicinity of the target. Conversely, T_i of C²⁺ and He⁺ are approximately equal to their corresponding T^∗_e, except near the target in attached conditions. To explain these observations, we present a simple model of T_i evolution that includes the thermalisation between plasma species, and the ionisation state evolution with a source of low temperature neutrals. In the framework of this model we also show that T_i of C²⁺ and He⁺ closely follow T_i of ionized deuterium D⁺ in all our plasma parameters and therefore may be used as an indirect measurement of T_i,D+. The results here clearly show the need for a collisional-radiative model that includes local neutral density of deuterium and impurities, to understand T_i of various ion species, and their emission intensities in the divertor plasma.