Speaker
Description
See full abstract here
http://ocs.ciemat.es/EPS2019ABS/pdf/P2.1026.pdf
In divertor tokamaks heat is flowing in a narrow channel of width q towards the divertor targets. Having a reliable extrapolation of q for ITER and DEMO is essential for the development of power exhaust scenarios, which led to the development of experimental multi machine scaling laws for q obtained from low edge density H-modes [1]. However, physics based models explaining the experimental results have only been partially successful. These models require assumptions for the parallel heat transport, which are not yet fully understood. Langmuir probes can help to disentangle the contributions from electrons and ions to the total heat flux, leading to a more complete picture of the parallel heat flux.
In this contribution the role of parallel electric currents in the Scrape-Off Layer (SOL) of the tokamak ASDEX Upgrade is investigated. These currents were measured in L and H-mode discharges by Langmuir probes located at the outer divertor target. It is shown that for low plasma densities the measured electric current can be in the order of the ion saturation current, while for high densities it becomes much smaller. By using an analytical SOL current model [2] the measured radial current profile can be largely explained by electric currents driven by the plasma potential difference created by the different electron temperatures between the outer and inner divertor targets. A comparison between the outer target heat flux calculated from the Langmuir probes with the one from thermography shows that the heat flux is underestimated considerably if the electric current to the target is neglected. Therefore, a significant part of the heat to the outer target is carried by the electrons in the current for such low density L and H-mode plasmas.
References
[1] Eich, T., et al., Nuclear Fusion 53, 9 (2013) [2] Staebler, G. M., Hinton, F. L., Nuclear Fusion 29, 10 (1989)
