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 (Te, ne), the poloidal emissivity distributions of a range of spectral lines together with the corresponding ion temperatures, Ti, in the TCV divertor plasma. These measurements were performed over divertor plasma conditions evolving from strongly attached towards a detached regime. The poloidal ne and Te 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. Ti 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 Ti represents an emissivity-weighted ion temperature along the DSS lines of sight. To study the electron/ion thermalisation, we compare Ti of each ion with a corresponding emission-weighted electron temperature, T∗e. We find that Ti 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, Ti 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 Ti 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 Ti of C2+ and He+ closely follow Ti of ionized deuterium D+ in all our plasma parameters and therefore may be used as an indirect measurement of Ti,D+. The results here clearly show the need for a collisional-radiative model that includes local neutral density of deuterium and impurities, to understand Ti of various ion species, and their emission intensities in the divertor plasma.