Speaker
Description
A new Tangential Divertor Spectrometer System (TDSS) on the TCV tokamak provides, for the first time, simultaneous measurements of ion temperature and parallel flow poloidal and radial profiles in the divertor Scrape-Off Layer (SOL). High spectral resolution enables the identification of the emission region through Zeeman splitting, allowing for the reconstruction of radial profiles using the unique plasma shaping capabilities of TCV. Understanding the main ion and impurity kinetics is essential, as they significantly affect the in-out symmetry of radiation in the divertor and can lead to increased core contamination. In addition to previously presented temperature and density measurements using divertor spectroscopy, dedicated experiments were carried out in Ohmic L-mode plasmas with divertor conditions ranging from detached to attached, combining TDSS with Thomson Scattering, multispectral imaging (MANTIS) and reciprocating probe measurements to characterise the SOL over a range of densities. The magnetic field and plasma current directions were changed to identify the impact of magnetic drifts on the radial profiles of flow and temperature. Measurements of C$^{2+}$, He$^{+}$, and N$^{+}$ line emissions revealed that these impurity species exhibited similar flow velocities and temperatures along the divertor leg. However, C$^{+}$ was systematically colder and slower, consistent with weaker entrainment and thermalisation with the main ion population. For the first time, radial profiles of ion $T_i$ and electron temperature $T_e$ revealed $T_i> T_e$ in the divertor far SOL and $T_i \leq T_e$ near the separatrix. Magnetic drifts had a profound effect on the ion flow profile, resulting in a significant change in ion energy transport along the leg. With increasing core density, the impurity flow increases for the so-called favourable (for H-mode) magnetic field direction but is unchanged for the unfavourable magnetic field direction. SOLPS-ITER simulations with magnetic drifts reproduce the upstream and divertor electron density and temperature profiles, and capture the experimentally observed dependence of the C$^{2+}$ flow profile on core density and magnetic field direction. This combined experimental and modelling study revealed that ion kinetics are somewhat decoupled from the electrons in the outer divertor leg and depend strongly on plasma conditions. These findings underscore that ion-driven energy fluxes can constitute a substantial fraction of the total SOL power balance, making their accurate characterisation essential for understanding divertor energy fluxes.