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In order to survive the high power densities being studied in the current generation of tokamaks, limiters and divertor plates must be placed within a few degrees of parallel to the magnetic field. For the same reason, any probes employed to measure the plasma parameters near these surfaces must also be nearly parallel to the field. It has been found that such probes often fail to show saturation of the ion current and that the electron saturation current cannot be explained by current models. An understanding of these effects is essential for the correct interpretation of Langmuir probes. In addition, a model which successfully explains the current voltage characteristic of a probe can also be used with confidence to predict other properties of the plasma surface interaction.
The presented model will be proved using data from flush mounted probes and different other diagnostics at the ASDEX Upgrade tokamak. Further on the evaluated data will be used to describe the plasmas behaviour in front of the target plates in situations where a strong additional gas puff increases the neutral density in the divertor region. It will become evident that flush mounted probes can be used as well as domed probes (within the usual limitations of probe measurements) to determine the plasma parameters in front of the target plates. The flush mounted probes have the potential for use as a standard diagnostic in almost every mechanical part inside the vacuum vessel.

Additional information

Authors: WEINLICH M, Max-Planck-Institut für Plasmaphysik Garching bei München (DE)
Bibliographic Reference: Report: IPP 5/64 DE (1995) 105pp.
Availability: Available from Max-Planck-Institut für Plasmaphysik, 85748 Garching bei München (DE)
Record Number: 199610324 / Last updated on: 1996-03-29
Original language: de,en
Available languages: de,en