No abstract

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.

The erosion yield of carbon by chemical hydrocarbon generation is investigated using spectroscopic particle flux measurements in the divertor of the ASDEX Upgrade tokamak. The methane formation at the graphite surface in hydrogen and deuterium plasmas is derived from CH/CD molecular band emission, and the corresponding hydrogenic fluxes are obtained from Hβ/Dβ spectroscopy, supported by Langmuir probe measurements in many cases. Under conditions of high particle flux densities above 1022 m-2·s-1, which are typical for high recycling divertor operation, a strong decrease of the apparent chemical erosion yield derived from CH release with increasing hydrogen flux density (and decreasing impact energy) is observed, YCH ∝ ΓH-0.8. The erosion yields depend on the hydrogen isotope; the values for deuterium are about a factor of 2 higher than those for hydrogen but exhibit the same flux dependence. These results are obtained for relatively low target temperatures between 300 and 360 K; the electron temperature at the target for the lowest yields/highest fluxes is about 5 eV. The yields at the highest fluxes are much lower than the results obtained from extrapolation of laboratory experiments performed under low flux density conditions. Three explanations are possible for the observed reduction of CH fluxes with increasing hydrogen flux densities: the flux dependence of the underlying chemical erosion yield Ychem, the energy dependence of Ychem entering via the experimental correlation of E and 1/Γ, and the plasma parameter dependence of the used molecular photon efficiency arising from the increasing prompt redeposition of CH4 fragmentation products with rising ΓH.

1 See appendix. 2 See the author list of 'Overview of progress in European Medium Sized Tokamaks towards an integrated plasma-edge/wall solution' by Meyer [22].

Tungsten-coated tiles, manufactured by plasma spray on graphite, were mounted in the divertor of the ASDEX Upgrade tokamak and cover almost 90% of the surface facing the plasma in the strike zone. Over 600 plasma discharges have been performed to date, around 300 of which were auxiliary heated with heating powers up to 10 MW. The production of tungsten in the divertor was monitored by a W I line at 400.8 nm. In the plasma centre an array of spectral lines at 5 nm emitted by ionization states around W XXX was measured. From the intensity of these lines the W content was derived. Under normal discharge conditions W-concentrations around 10 −5 or even lower were found. The influence on the main plasma parameters was found to be negligible. The maximum concentrations observed decrease with increasing heating power. In several low power discharges accumulation of tungsten occurred and the temperature profile was flattened. The concentrations of the intrinsic impurities carbon and oxygen were comparable to the discharges with the graphite divertor. Furthermore, the density and the β limits remained unchanged and no negative influence on the energy confinement or on the Hmode threshold was found. Discharges with neon radiative cooling showed the same behaviour as in the graphite divertor case.