A Doppler Backscattering System In The Keda Space Plasma Experiment

Doppler backscattering system?DBS?is a microwave diagnostic system which combination of the microwave technology and scattering measurement technology.It has higher resolution in time,in space and in wave number.It can simultaneously measure the plasma density fluctuation and the perpendicular rotation velocity distribution,and is proved to be a powerful tool for analyzing plasma turbulence.A certain frequency microwave beam is incident into the plasma by the DBS and scattered at the cut-off layer.The density fluctuation and vertical rotation velocity at the cut-off layer are obtained by measuring the amplitude and phase changes of the scattered signal.The main research content of this paper is to install the DBS on KSPEX,which is used to simultaneously measure the perpendicular rotation velocity and density fluctuation of plasma.The DBS is the O mode system with the frequency range of1?10 GHz,which is the superheterodyne measurement.In the system,the local oscillator signal of 3 GHz is mixed with the detection signal to generate a differential frequency for the superheterodyne measurement.The magnitude and direction of Doppler shift could be measured by superheterodyne detection.The system uses a double-ridged horn antenna to transmit and receive microwave signals.In the typical oxide coated cathode plasma of KSPEX with a peak density about 3.88×10¹⁷m^-3.Results of ray tracing show that the measurement range can cover almost the whole minor radius.A clear Doppler shifted peak is obtained in laboratory tests when the microwave is vertically incident on an iron plate moving back and forth.Phase derivative method are used to extract the Doppler shifted frequency,and experimental results show that the power spectrum of perpendicular rotation velocity.The above results show that the DBS has obtained effective experimental data.Another research content of this paper is the design of 140 GHz frequency selective surface?FSS?for the DBS on EAST.The DBS on East contains two systems:Q-band?33?50 GHz?and V-band?50?75 GHz?.In order to avoid the microwave overflowing from the high frequency microwave heating system may cause damage to the microwave devices of the DBS.This paper presents the design of a circular ring FSS with reflecting 140 GHz microwave signal?D-band?while transmitting through Q-band and V-band signals.Relationship between the size of the ring inner diameter and the resonance frequency of the FSS,and the influence of the microwave incident angle on the transmission loss at the resonance frequency?140 GHz?and the passbands?33?75 GHz?,are surveyed.Result shows that the resonance frequency decreases with the ring inner diameter increases,which is consistent with the theoretical calculation results.Moreover,the maximum transmission losses at different resonance frequencies are all below-16 dB.For the candidate size,the transmission losses at the resonance frequency and the passbands are about-25 d B and-0.1 dB.The results of passband attenuation and resonant frequency at different incident angles show that the FSS can work normally in the range of 8°.