An improved understanding of the roles of atomic processes and power balance in target ion current loss during detachment

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 talk the physics leading to the decrease of the TCV tokamak divertor ion flux, or ‘roll-over’ (corresponding to partial detachment), is experimentally explored through characterisation of the location, magnitude and role of the various divertor ion sinks and sources including a complete measure of particle and power balance. This is achieved using a novel, validated, experimental probabilistic analysis of the Balmer line spectra from multi-chord visible spectroscopy.

Over a range in core plasma conditions (current, impurity-seeding, density) the divertor target ion current (I_t) loss, measured by Langmuir probes, is caused by a drop in the measured divertor ion source; volumetric recombination remains small to negligible. ‘Power limitation’ results in ion source reduction when 50% or more of the power entering the recycling region (P_recl) is spent on ionisation, in agreement with analytical predictions. This pivot point corresponded to target temperatures ~ 4-6 eV and to a critical upstream pressure to recycling heat flux, estimated using IR heat deposition profiles and P_recl, ratio (p_u/q_recl): both in agreement with analytical predictions.

The profiles of the various ion sources and sinks together with power losses are tracked experimentally as detachment proceeds and were quantitatively consistent with SOLPS modelling. As the power reaching the near-target region decreases, the ionisation and impurity radiation regions move towards the x-point, leaving behind a region with strong molecular (Dα) emission and elevated charge exchange to ionisation ratios. From those ratios momentum losses (up to 70%) are implied that develop concurrently with power limitation of the ion source.