The role of particle, energy and momentum losses in 1D simulations of divertor detachment

Abstract

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 ionisation source and (b) access low-target temperature, Ttarget, conditions for volumetric momentum losses. Recombination is found to play a small role at flux rollover, but as Ttarget drops to temperatures around 1 eV, 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 Ttarget. This is also linked to thresholds in Ttarget 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 Ttarget (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 Ttarget where there is no solution and the plasma ‘jumps’ from high to low Ttarget 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.