Energy balance in divertor plasma detachment
Plasma detachment from the divertor target promises efficient reduction of power loading on the targets in a tokamak-reactor. The experimental manifestation of detachment is a rollover, followed by a significant reduction, of the ion saturation current onto the target along with the increase of the fueling (gas puffing) rate. Original theoretical studies of this phenomenon lead to considering “momentum removal” – that is, some force acting on the plasma along the magnetic field – in the scrape-off layer (SOL), producing a drop of the plasma pressure between the mid-plane (“upstream”) and the target (“downstream”), as the principal cause of detachment. Although the reduction of the pressure near the target, accompanied by the decrease of the plasma temperature there, implies the reduction of the particle flux to the target, the story is not that simple. The flows in the edge are sustained by the sources and sinks of particles and the pressure profile adjusts itself to ensure the required flow pattern that can be far from the simple, laminar flow from upstream to downstream. Particle balance in the detaching plasma was considered in  where the volumetric recombination of the ions and electrons was identified as the mechanism responsible for the detachment. Globally, this balance means the equality of the fueling and pumping fluxes, since ionization and recombination do not affect the total particle content in the system. However, these flows are orders of magnitude lower than the recycling ones, so they can hardly affect the plasma state directly, controlling the particle content (or the plasma density) instead. The recycling fluxes depend in turn on the power available for ionization and recombination of the particles. Therefore, it is natural to consider the detachment process from the viewpoint of power balance, taking the particle content in the edge as the parameter characterizing the density.
This paper presents the results of theoretical and modelling analyses of the role of power balance in the recycling and recombining plasmas, following the approach of . On the modeling side, the SOLPS4.3 code suite  was applied to a simplified geometry model built around a DIII-D-like magnetic equilibrium. In particular, the ratio of the plasma pressure upstream to the power flux entering the recycling region as the principal parameter controlling the jsat rollover is identified in the theory and confirmed by modelling, Fig. 1. The results show that volumetric recombination and the power loss with radiation from impurities are indeed directly responsible for detachment, whereas the “momentum removal” plays a secondary, although important, role in providing the conditions for these processes.
Fig. 1. jsat vs. the Pup/qrecycl ratio for different power input to the SOL. DIII-D-like configuration, pure deuterium plasma.
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