Feasibility of applying the lower cut-off frequency for the density radial coverage extension in EAST reflectometry measurement

The extraordinary mode（X-mode） lower cut-off frequency is proposed for use in the reflectometry diagnostic on ITER for the electron density profile measurement, which is a trade-off between extreme plasma parameters and the accessible probing frequency. In contemporary experiments, the lower cutoff frequency can be identified at the probing frequency below the electron cyclotron frequency( fce) under certain plasma conditions. We provide here, for the first time, the experimental validation of the use of the lower cut-off frequency for the density profiles via the reflectometry measurement on EAST. The corresponding group delay of the lower cut-off frequency evolves continuously with the upper one, revealing a reasonable radial coverage extension of reflectometry measurement toward the plasma core. It is concluded that the lower cut-off frequency can be used as a supplement to the upper one in the density profile inversion process, which is of particular interest in the high magnetic field and/or density discharge to extend the radial coverage of reflectometry measurement.