An improved understanding of the roles of atomic processes and power balance in divertor 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 paper 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 novel spectroscopic techniques which are introduced. Over a range in TCV plasma conditions (different levels of plasma current/electron density with/without impurity-seeding) the It roll-over is due to a drop in the divertor ion source; recombination remains either small or negligible until later in the detachment process. The ion source reduction is driven by both a reduction in the power available for ionization, Precl, and concurrent increase in the energy required per ionisation, Eion: sometimes characterised as 'power starvation'. The detachment threshold is found experimentally (in agreement with analytic model predictions) to be ~ Precl/ItEion~ 2, which corresponds to the target electron temperature, Tt~ Eion/? where ? is the sheath transmission coefficient. The loss in target pressure, required for target ion current loss, is driven not by just volumetric momentum loss as typically assumed but also due to a drop of upstream pressure. The measured evolution through detachment of the divertor profile of various ion sources/sinks as well as power losses, charge exchange and molecular/atomic components of the Da emission are quantitatively reproduced through full 2D SOLPS modelling of a ramp of core plasma density through the detachment process.