Plasma Boundary Position Measurements Using Frequency Sweep Microwave Reflectometer in LHD

A broadband frequency tunable microwave reflectometer system, which has the ability of fast and stable frequency sweeping operation, is applied in the large helical device (LHD) to measure the boundary of the high-temperature plasma. When the microwave is launched, with extraordinary polarization, from the low field side of the magnetic field with its frequency swept from low to high, the microwave of minimum right-hand cut-off frequency is reflected most outside. We can then estimate the plasma boundary position by measuring the change of the reflected power from the cut-off layer.