Due to the high heat flux to the target plates of present day divertor tokamaks, the use of thermally robust flush mounted Langmuir probes becomes more and more important. The surface normal of the probe defines a direction in space which is generally neither parallel nor perpendicular to the magnetic field, and the angle between these two vectors plays an important role for the physics in front of the probe. An analytic description of the sheath physics in front of a target plate is presented, and a model for the analysis of the I–V characteristics of such probes is derived therefrom. The model includes, on the one hand, considerably more physics than previous descriptions, and is, on the other hand, much simpler and more practical than numerical solutions and simulations. Subsequently the application of this model to triple probes is discussed. It will become evident that flush mounted probes can be used as reliably as domed probes to determine the plasma parameters in front of the target plates.
In 1997 the new `LYRA' divertor went into operation at ASDEX Upgrade and, in parallel, the neutral beam heating power was increased to 20 MW by installation of a second injector leading to a P/R value of 12 MW/m. Experiments have shown that the ASDEX Upgrade LYRA divertor is capable of handling such high heating powers. There is an overall reduction of the maximum heat flux in the LYRA divertor by about a factor of 2 compared with the previous open divertor Div I. This reduction is mainly due to increased radiative losses inside the divertor region, which are caused by an effective reflection of hydrogen neutrals into the hot separatrix region. The main channel of radiative loss is carbon radiation, which cools the divertor plasma down to a few electronvolts, where hydrogen radiation losses become significant. The radiative losses preferentially reduce the power flux at the separatrix, leading to early detachment around the strike point position. With increasing density, the detached region extends upwards on the vertical target. The power fraction radiated in the LYRA divertor is around 45% and nearly independent of the heating power. This value is a factor of 2 higher than the typical radiation fraction in Div I. B2-EIRENE modelling of the performed experiments supports the experimental finding and refines the understanding of loss processes in the divertor region.
High density operation in the ASDEX Upgrade divertor I with horizontal target plates is reported. Density rampup experiments were carried out to characterize detached plasma conditions in the divertor. During the detached phases, hydrogen continua and spectral line emission from high-n shells were observed in the divertor due to the volume recombination. The spectroscopic measurements provide a consistent picture of the evolution of the divertor plasma parameters during the density ramp. By means of the ADAS atomic physics program package, the rate of volume recombination was evaluated, including the effect of opacity. The relative importance of volume recombination in comparison with the target plasma sink is discussed. Observations indicating differences in volume recombination between the two divertor legs are presented, and the connection of volume recombination to divertor detachment is addressed.
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