Ultrafast broadband frequency-modulation reflectometer for density profile measurements in GAMMA 10

A broadband microwave reflectometer was applied to the GAMMA 10 tandem mirror using a fast-sweep hyperabrupt varactor-tuned oscillator (HTO) that can be swept over the full band (12-18 GHz) in less than 5 mu s. Clear fringes are observed during a plasma shot when the resultant frequencies from the reflectometer output are much larger than the ion cyclotron range of frequency (3-10 MHz). The density profile measurement is performed for various sweep times of HTO from 1 ms to 5 mu s in which two reconstruction methods are utilized, zero-cross counting of the fringes and frequency analysis using the maximum entropy method. The reliability of the profile measurement seems to be improved when the sweep time is faster than 10-30 mu s. The integrated value of many reconstructed density profiles agrees well with the profile obtained from a scanning interferometer. The fast-sweep reflectometer is also applied to the measurement of density fluctuations such as plasma movement as well as density profiles. (C) 1997 American Institute of Physics.