Experimental Study Of High-Confinement Mode Density Fluctuation Measured By Microwave Reflectometry On EAST Tokamak

Microwave reflectometer is widely used in the measurement of density profile and density fluctuation,and it is a non-perturbative measurement tool that produces relatively little perturbation to the plasma.So,it is an ideal detection tool for tokamak devices.An 8-channel O-mode polarized correlation reflectometer has been installed at EAST tokamak to measure density fluctuations from edge to core.The PCR launches8 different frequency microwaves([20.4,24.8,33,40,42.4,48,52.6,57.2] GHz)into the plasma from the low field side and two poloidally separated antennae are used to receive the reflected waves.As a result,this diagnostic can measure fluctuations in8(radial)×2(poloidal)spatial positions.The 8-channel O-mode polarized correlation reflectometer comprises a 4-channel([20.4,24.8,33,40] GHz)O-mode polarized correlation reflectometer and a 4-channel([42.4,48,52.6,57.2] GHz)O-mode U-band correlation reflectometer.The principle of the U band correlation reflectometer system on EAST and the antenna parameters including the antenna position and transmitting angle are introduced and the actual performance of the main components such as the balance mixer and the bandpass filter is tested.The high-confinement mode density fluctuation during inter-ELM is studied by this reflectometry on EAST tokamak.This inter-ELM phase can be divided into two stages.In the first stage,a low frequency(<50 k Hz)broad-band fluctuation occurs in the pedestal gradient region.In the second stage,the broad-band fluctuation is suppressed,the density gradient becomes larger,and quasicoherent fluctuations(QCFs)appear.The increase of density gradient indicates the pedestal particle transport is improved in the second stage and could be attributed to the suppression of the low frequency broad-band fluctuation.During the whole interELM period,the fluctuation of core region has no significant change.