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P: Fachverband Plasmaphysik

P 15: Helmholtz Graduate School II

P 15.23: Poster

Dienstag, 14. März 2017, 16:30–18:30, HS Foyer

First steps in analyzing the role of the outer divertor in the L-H transition power threshold in ASDEX Upgrade — •Ou Pan, Tilmann Lunt, Daniel Carralero, Andrea Scarabosio, Marco Wischmeier, Ulrich Stroth, and ASDEX Upgrade Team — Max-Planck-Institute for Plasmaphysics, Boltzmannstr. 2, 85748 Garching, Germany

The transition from L-mode to H-mode occurs generally above a certain heating power threshold (P_L-H). The widely used ITPA scaling for P_L-H in deuterium plasmas depends on the line-averaged density, magnetic field and the surface area of the separatrix. However, in ASDEX Upgrade, P_L-H shows a significant dependence on the material of the plasma facing components (PFCs). The threshold in full tungsten (W) wall discharges is lower by 25%-30% compared to that with a graphite (C) wall. In this work, the role of the outer divertor for the reduction of P_L-H was investigated by comparing the electron temperature (T_e) and density (n_e) measured by the divertor Langmuir probes near the outer strike point (OSP) as well as the neutral flux density measured by the ionization pressure gauges in the private flux region. It is found that in the case of a W wall T_e and n_e near the OSP rise substantially before the L-H transition and show a more peaked profile along the target. Since the radial electric field (E_r) in the scrape-off layer is related to the gradient of the target temperature, the pronounced peaking of the T_e profile at the outer target likely leads to a stronger E_r in the SOL. The E×B flows induced by these fields may be important for the access to H-mode.