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In this report microwave reflectometry is extended to the outermost parts of tokamak plasmas, which are subject to strong electron density fluctuations. The perturbations of electron density profile measurements by these fluctuations, which lead to strong modulations in intensity and phase of the reflected signal, are analysed in detail. By increasing the frequency of the interference fringes to values between 800 kHz and 2.4 MHz, it is possible to make reliable profile measurements even in the region of very strong fluctuations. Measurements in the low density region are only possible with reasonable errors in the X-mode, as only the cut-off frequency of this mode, in contrast to that of the O-mode, takes a finite value as n(e) tends to 0. Taking advantage of this property, a method to calibrate the measurements on the first reflection is presented. This reflection occurs directly in front of the microwave antennae (1-4 mm from the opening), thus giving a high degree of precision even in the outermost part of the plasma.

Additional information

Authors: SCHUBERT R, Max-Planck-Institut für Plasmaphysik, Garching bei München (DE)
Bibliographic Reference: Report: IPP 4/249 DE (1991) 136 pp.
Availability: Available from Max-Planck-Institut für Plasmaphysik, 8046 Garching bei München (DE)
Record Number: 199210172 / Last updated on: 1994-12-02
Original language: de
Available languages: de