X-mode pulsed radar reflectometer for density fluctuation measurements on LHD

A four channel pulsed radar reflectometer system has been installed on the Large Helical Device (LHD). The complicated magnetic structure in LHD causes mode conversion and/or polarization rotation of the microwaves. Pulsed radar reflectometry is a suitable reflectometric technique, because it measures the delay time of the reflected wave, not the phase, and X-mode and O-mode polarized waves can be distinguished. By using X-mode operation of the pulsed radar reflectometer so that each pulse width is about 2 ns, and the repetition rate is up to 200 kHz, the critical density where the microwave is reflected is about 1×1016 m−3. Also it is found that the static natural island affects the X-mode reflectometric measurements.