Reflectometry for density fluctuation and profile measurements in TST-2

A reflectometer in Ka-band was designed, constructed and applied to the TST-2 spherical tokamak in order to detect density fluctuations induced by the radio frequency (RF) heating wave, and to measure the density profile. The optics of the reflectometer consists of two concave mirrors to make a small microwave beam spot inside the plasma. Using Kirchhoff integration, the parameters of the optics were optimized to achieve a high throughput (i.e. the ratio of the received to the launched powers) in various measurement conditions. Using the reflectometer, the density fluctuations in the frequency range of RF were detected and the electric field excited by RF (21MHz/260 kW) was estimated to be about 1.3 kV/m.