Broadband heterodyne reflectometry on the W7-AS stellarator (abstract).

A broadband heterodyne reflectometer, operating in the frequency range 75-110 GHz, has been installed and operated at the W7-AS Stellarator in Garching for the study of density fluctuations and fast density profile determination. Both local (LO) and signal (rf) oscillators can be swept over the whole frequency range. Due to the balanced detection scheme, all drifts, as well as the broadband noise of the BWO (rf and LO) oscillators, are canceled at the different downconversion stages. After installation on W7-AS, first results reproduced the original findings of the former homodyne system [J. Sanchez, H. J. Hartfuss, E. Anabitarte, A. P. Navarro, et al. Nucl. Fusion 30, 2383 (1990)] as well as new results on the radial distribution of density fluctuations have been obtained. A phase drift due to asymmetries in the spectrum around the last IF has been observed; this phenomenon is strongly dependent on the plasma conditions and could be caused by plasma rotation. It is presently under study. The system offers broadband operation together with the possibility to decouple phase and amplitude oscillations of the reflected beam. The high dynamic range (up to 80 dB) is able to overcome the large amplitude oscillations of the reflected beam. The reflectometer described offers a good basis for further developments on large devices, which will require a very high sensitivity due to the long waveguide runs, together with the advantage of the operation in a continuous frequency range and fast sweeping capability. [ABSTRACT FROM AUTHOR]