  A new 1D divertor plasma code, SD1D, has been used to examine the role of
recombination, radiation, and momentum exchange in detachment. Neither momentum
or power losses by themselves are found to be sufficient to produce a reduction
in target ion flux in detachment (flux rollover); radiative power losses are
required to a) limit and reduce the ionization source and b) access low-target
temperature, T_target, conditions for volumetric momentum losses. Recombination
is found to play little role at flux rollover, but as T_target drops to
temperatures around 1eV, it becomes a strong ion sink. In the case where
radiative losses are dominated by hydrogen, the detachment threshold is
identified as a minimum gradient of the energy cost per ionisation with respect
to T_target. This is also linked to thresholds in T_target and in the ratio of
upstream pressure to power flux.
  A system of determining the detached condition is developed such that the
divertor solution at a given T_target (or lack of one) is determined by the
simultaneous solution of two equations for target ion current - one dependent
on power losses and the other on momentum. Depending on the detailed momentum
and power loss dependence on temperature there are regions of T_target where
there is no solution and the plasma 'jumps' from high to low T_target states.
The novel analysis methods developed here provide an intuitive way to
understand complex detachment phenomena, and can potentially be used to predict
how changes in the seeding impurity used or recycling aspects of the divertor
can be utilised to modify the development of detachment.

Plasma Physics