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P: Plasmaphysik
P 11: Plasma Wall Interaction
P 11.3: Talk
Wednesday, March 19, 2003, 12:10–12:30, FO1
Angular resolved energy flux measurements in a magnetized plasma — •Bernd Koch, Werner Bohmeyer, and Gerd Fußmann — MPI f. Plasmaphysik, Berlin, Germany
In fusion experiments the energy flux to the vessel walls and the target plates is an important issue. In order to spread the heat load, surfaces are usually designed to intersect magnetic field lines at very shallow angles. The experimental results presented within this work were obtained at the PSI-2 facility, a linear discharge arrangement with moderate field strength (B≈0.1 T, ne≈1016−1019 m−3, Te≈1−15 eV). A flat, turnable probe capable of simultaneous energy flux and current density measurements as functions of an applied voltage was exposed to a number of different plasma conditions. It was attempted to cover a significant range of parameters (Hall parameter h, gyro radii ri,e and Debye length λD) by variation of the discharge parameters (mi, ne, Ti,e). In our experiments the probe dimension d was significantly larger than the electron gyro radius; for the ions a range of ri/d≈0.03…0.6 was covered. A pronounced reduction of the floating potential was found experimentally at shallow angles between the magnetic field and the probe surface. As the energy gain within this potential accounts for the major part of the ion energy at the surface (e.g. ≈60% for H+) this also leads to a large reduction of the energy flux density beyond the effects of mere particle flux density reduction, which is also observed. Furthermore, it is shown how important parameters of plasma wall interaction can be deduced from the combination of j(U) and q(U) characteristics.