Edge density X-mode reflectometry of RF-heated plasmas on ASDEX
Richard Schubert, Ph.D. thesis, 139 pages, 74 figures, Max-Planck-Institute f. Plasma Physics & Technical University of Munich / Germany, Apr. 1991


In the present work, microwave reflectometry is extended to the outermost part of tokamak plasmas (ne = 1011 to 1.5 *1013 cm-3), which is 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 is analyzed 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 (E perpendicular B), as only the cut-off frequency of this mode, in contrast to that of the O-mode (E parallel B), takes in finite value (fce) for ne -> 0. Taking advantage of this property, a method is presented to calibrate the measurements on the first reflection, which occurs directly in front of the microwave antennas (1 - 4 mm from the opening) thus giving a high precision even in the outermost part of the plasma close to the microwave antennas. For the calculation of the electron density profile a new and numerically stable algorithm has been developed. Measurements in connection with Lower Hybrid have been made with a set of 2 reflectometer antennas installed in ASDEX 6 cm toroidally away from the LH-grill and having an opening of about 1 cm2 each. The reflection coefficient for the LH-waves calculated with the electron density and the electron density gradient measured at the radial position of the LH-grill antenna (typ. 5*1011 cm-3 and 5*1011 cm-4 resp.) is in good agreement with the measured one. Further, it could be confirmed, that the LH itself can create, under certain experimental conditions, a sufficiently high electron density in front of the grill for good coupling even for large grill separatrix distances. The measured changes in the electron density profile during application of LH depend on whether the RF is injected with the lower or upper of the two waveguide arrays, which constitute the LH-grill antenna on ASDEX. This confirms the theoretically predicted formation of thermal eddies in front of the grill, which lead to a poloidally inhomogeneous density in front of the grill. Finally, a strong increase of the edge turbulence during LH near the grill in high density shots could be detected.

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last modified May/28/01