Ultrafast Microwave Reflectometer for Density Profile Measurements on GAMMA 10

A 12-18 GHz, broad-band reflectometer system has been applied to the central cell of 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 in less than 5 µ s. The high-frequency fringes due to the change of cut-off layer can be distinguished from those due to the existence of density fluctuations, since the fluctuation level with frequency components larger than 20 MHz is much smaller than the average density (≤10-4) in GAMMA 10 plasmas. The reflectometer is applied to determine the density profiles under different operating conditions.