X-Mode Frequency Modulated Density Profile Reflectometer on EAST Tokamak

An extraordinary-mode (X-mode) frequency-modulated continuous-wave (FMCW) profile reflectometer has been built on EAST. In the reflectometer, continuous waves with fre- quency sweeping from 12.5 GHz to 18 GHz were generated through a Hyperabrupt Tuned-varactor Oscillator (HTO) source and a four times active multiplier was used to increase the frequency to V-band (50 GHz to 72 GHz). The polarization of horn lens antenna is perpendicular to the mag- netic field line at the edge plasmas. According to the V-band frequency range and polarization, the system cover density range from 0.5 × 10 ¹⁹ m ⁻³ to 3.0 × 10 ¹⁹ m ⁻³ (when toroidal magnetic field is 1.8 T), with time resolution of 12.5 ∼ 50 µs. The density profile could be calculated by assuming the edge profile through an empirical equation. The maximum spatial error deduced by the method is about 4 cm. This reflectometer has been successfully applied in 2010 autumn EAST campaign, the temporal evolution of density profiles was acquired during the low confine- ment mode to high confinement mode transition. The density pedestal of EAST Tokamak was observed and the top value and gradient of the density pedestal were estimated.