Reflectometric density profile measurement using an ultrafast sweep oscillator

A broad band frequency modulation reflectometer system has been installed and operated on the GAMMA 10 tandem mirror. A fast sweep hyperabrupt varactor-tuned oscillator (HTO) is used as a source, which can be swept over the full frequency band (12–18 GHz) in less than 5 μs. The high frequency fringes due to the change in the cut-off layer can be distinguished from those due to the existence of density fluctuations, since the fluctuation level with frequency components larger than the ion cyclotron range of frequency (10 MHz) is much smaller than the average density (10−4 or less). The reliability of density profile measurement is investigated for various sweep times of HTO from 5 μs to 5 ms. It is observed that the reliability is greatly improved and the reconstructed density profiles agree well with that obtained from an interferometer when the sweep time is faster than 10 μs. The present reflectometer is routinely applied to determine the density profiles of the central cell plasma in GAMMA 10.