  The process of divertor detachment, whereby heat and particle fluxes to
divertor surfaces are strongly diminished, is required to reduce heat loading
and erosion in a magnetic fusion reactor to acceptable levels. In this paper
the physics leading to the decrease of the total 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 analysis 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. Over a range in TCV plasma conditions (plasma current
and electron density, with/without impurity-seeding) the $I_t$ roll-over is
ascribed to a drop in the divertor ion source; recombination remains small or
negligible farther into 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, $E_{ion}$: often
described as 'power starvation' (or 'power limitation'). The detachment
threshold is found experimentally (in agreement with analytic model
predictions) to be $\sim P_{recl}/I_t {E_{ion}} \sim 2$, corresponding to a
target electron temperature, $T_t \sim E_{ion}/{\gamma}$ where ${\gamma}$ is
the sheath transmission coefficient. The target pressure reduction, required to
reduce the target ion current, is driven both by volumetric momentum loss as
well as upstream pressure loss. The measured evolution through detachment of
the divertor profile of various ion sources/sinks as well as power losses are
quantitatively reproduced through full 2D SOLPS modelling through the
detachment process as the core density is varied.
