Spectroscopic investigations of detachment on TCV: Investigating the role of atomic physics on the ion current roll-over and the dynamics of detachment in TCV

Abstract

The process of divertor detachment, whereby heat and particle fluxes to divertor surfaces are strongly reduced, is required to reduce heat loading and erosion in a magnetic fusion reactor. In this thesis the physics leading to the decrease of the divertor ion current It, or ‘roll-over’, is experimentally explored on the TCV tokamak through characterization of the location, magnitude and role of the various divertor ion sinks and sources including a complete measure of particle and power balance. These first measurements of the profiles of divertor ionisation and hydrogenic radiation along the divertor leg are enabled through the development of a TCV divertor spectrometer, together with careful Stark broadening analysis and novel Balmer line spectroscopic techniques. Over a range in core plasma conditions (plasma current, impurity-seeding, density) the It roll-over is caused by a drop in the divertor ion source; recombination remains either small or negligible until later in the detachment process. In agreement with simple analytical predictions, this ion source is limited by a reduction in the power available for ionisation, Precl, sometimes characterised as ‘power starvation’. Concurrent increases in the energy required per ionisation, Eion, further reduce the number of ionizations. The detachment threshold is found experimentally (in agreement with analytic model predictions) to be Precl / Eion It < 2, corresponding to the target electron temperature, Tt ~ Eion/gamma where gamma is the sheath transmission coefficient. Target pressure loss, required for target ion current loss, is observed to be delivered by both volumetric momentum loss, as typically assumed, and by a drop of the upstream pressure. The evolution of measured divertor profiles through detachment of the various ion sources/sinks as well as power losses and charge exchange are quantitatively reproduced through full 2D SOLPS modelling of a ramp of core plasma density through the detachment process