Spectroscopic characterisation of TCV divertor towards a detached regime

The challenge of power exhaust of the high heat and particle fluxes foreseen for ITER and the forthcoming nuclear fusion reactors can be mitigated by operating in a detached divertor regime.This regime has been the object of three decades of studies, as its achievement leads to a simultaneous reduction of target temperature and particle fluxes, relieving the wall’s materials from melting and erosion processes.The effect of the physical processes leading to detachment can be summed into loss coefficients within 0-D models (e.g. the two-point model, 2PM) describing the heat transport through the divertor region.In this thesis, we present the hardware upgrade to TCV’s Divertor Spectroscopy System (DSS), which consisted in the installation of a mirror-based collection optics coupled to a new triple fibre bundle, conveying the light emitted by the same plasma volume to three independent spectrometers.This upgrade included the installation of two high-resolution spectrometers (i.e. ∆λ(FWHM) ≈ 0.2 Ã), capable of resolving line-shape broadening contributions of several spectral lines.This enabled a new estimation of ion and neutral species temperatures in the divertor region based on the measurement of the Doppler broadening contribution to the observed spectral line-shapes.This thesis presents detachment-relevant experiments based on deuterium density ramps, in which three spatially resolved diagnostics came together for a detailed characterisation of the divertor leg during its evolution from an attached towards a detached regime.These diagnostics are the DSS, the Multispectral Advanced Narrowband Tokamak Imaging System (MANTIS), which provides 2-D emissivity maps of spectral lines of interest across the whole divertor region, and the Thomson Scattering system (TS), which provides Te and ne measurements along the path length of three vertical laser beams.Exploiting TCV’s shaping capabilities over several repeat discharges, we were able to reconstruct radial profiles of ne, Te and pe along the divertor leg and across the density ramps using TS data, while the DSS provided the ion temperatures associated with C II, C III, He II impurity ion species and the neutral temperature associated with atomic deuterium.The essential integration of MANTIS data allowed for the comparison of ion temperature with effective electron temperatures Te*(α) associated with the ion species α to observe electron-ions thermalisation in the highly collisional plasma characterising the divertor leg in detachment relevant experiments.One first observation from these experiments regarded the need to consider a cold influx of neutral particles (i.e. D and C) released in the target vicinity in order to match the measured Ti(α) < Te*(α).This hypothesis was tested by considering a simple model that included a collisional-radiative model for low temperature and density C and He particles to calculate their charge state distributions, emissivity and thermalisation times with plasma D ions.A second measurement carried out in this thesis regarded the determination of characteristic temperatures associated with the C III radiation front, a proxy for the achievement of low Te in the divertor.Values of peak Te and average Te are presented.Lastly, these experiments allow for qualitative estimations of the role of the main loss coefficients in the 2PM and their contribution to the divertor evolution towards a detached regime.