Search

We present a spatially hybrid fluid–kinetic neutral model that consists of a fluid model for the hydrogen atoms in the plasma grid region coupled to a kinetic model for atoms sampled at the plasma–void interfaces and a fully kinetic model for the hydrogen molecules. The atoms resulting from molecular dissociation are either treated kinetically (approach 1) or are incorporated in the fluid model (approach 2). For a low-density JET L-mode case, the hybrid method reduces the maximum fluid–kinetic discrepancies for the divertor strike-point electron densities and electron temperatures from approximately 150% to approximately 20% for approach 1 and to approximately 40% for approach 2. Although the simulations with purely fluid neutral model become more accurate for increasing upstream plasma density, we still observe a significant improvement by using the hybrid approach. When consuming the same CPU time in averaging the electron strike-point densities and temperatures over multiple iterations as for the simulations with fully kinetic neutrals, hybrid approach 1 reduces the statistical error with on average a factor 2.5. Hybrid approach 2 further increases this factor to approximately 3.3, at the expense of accuracy.

Pairs of ASDEX Upgrade L-mode discharges with the toroidal magnetic field, BT, in the forward and reverse directions have been used to study the impact of neoclassical drifts on the divertor plasma conditions and detachment. The evolution of the peak heat flux and the total power loads onto both the outer and the inner targets depends significantly on the toroidal field direction: increasing the core plasma density affects mainly the heat loads in the BT 0 (favourable). Ion saturation current measurements show similar trends to those of the IR heat flux data. These discrepancies are not only caused by drifts but also by different levels of radiated power in the core, thus the power across the separatrix, Psep. Tomographic reconstructions show that Psep is not constant within the entire dataset. Finally, at Ip=0.8MA, a significant reduction of the peak heat flux is observed at both targets for both field directions. On the other hand, at Ip=0.6MA, a reduction of the peak heat flux is only observed for BT

An H-mode database of ASDEX Upgrade plasmas with improved diagnostics for a variety of different plasma conditions has been analysed to study the momentum removal in the Scrape-Off Layer (SOL). A strong reduction, up to a factor of 100, of the electron pressure is observed close to the separatrix. Sets of L-mode and H-mode like plasma simulations with the SOLPS5.0 code package have supported the interpretation of the experimental observations (e.g., apparent momentum gain, strong radial position dependence) and helped to clarify the role of ion-neutral interactions. The experimental data are in turn used to check the SOLPS predictions, which differ quantitatively, suggesting that important pieces of physics are still not successfully captured by the code. Keywords: Momentum removal, Scrape-off layer, SOLPS, Divertor detachment

A database with attached, partially detached and completely detached divertors has been constructed in ASDEX Upgrade discharges in both H-mode and L-mode plasmas with Thomson Scattering data suitable for the analysis of the upstream SOL electron profiles. By comparing upstream temperature decay width, with the scaling of the SOL power decay width, based on the downstream IR measurements, it is found that a simple relation based on classical electron conduction can relate and well. The combined dataset can be described by both a single scaling and a separate scaling for H-modes and L-modes. For the single scaling, a strong inverse dependence of, on the separatrix temperature, is found, suggesting the classical parallel Spitzer–Harm conductivity as dominant mechanism controlling the SOL width in both L-mode and H-mode over a large set of plasma parameters. This dependence on explains why, for the same global plasma parameters, in L-mode is approximately twice that in H-mode and under detached conditions, the SOL upstream electron profile broadens when the density reaches a critical value. Comparing the derived scaling from experimental data with power balance, gives the cross-field thermal diffusivity as consistent with earlier studies on Compass-D, JET and Alcator C-Mod. However, the possibility of the separate scalings for different regimes cannot be excluded, which gives results similar to those previously reported for the H-mode, but here the wider SOL width for L-mode plasmas is explained simply by the larger premultiplying coefficient. The relative merits of the two scalings in representing the data and their theoretical implications are discussed. [ABSTRACT FROM AUTHOR]