Numerical Simulation Study Of Microwave Signal Propagation For EAST Ultrashort Pulse Reflectometer

Microwave reflectometer is a popular non-intrusive plasma density diagnostic instrument on tokamaks,which provides sub-centimeter and millisecond level resolution.The ultra-short pulse reflectometer(USPR)achieves plasma density measurement by emitting a chirped wave containing a broadband signal and measuring the time of flight from different frequency components.Compared with standard swept-frequency microwave reflectometers,the USPR can provide improved temporal resolution at the level of microsecond.An USPR system is currently being built on EAST(Experimental Advanced Superconductive Tokamak).In order to predict the denstiy reconstruction of EAST USPR system,this study developed a numerical simulation program based on Python and use it to simulate the beam propagation and reflection of the chirped wave of extraordinary waves(Xmode)in the plasma.The main findings of this work include:1)Refer to the existing full-wave simulation model,enhance its calculations stability by introducing the current continuity equation,so that solve the problem of electric field divergence when testing the program.2)The electron density profile from plasma core to SOL(Scrape-Off Layer)region has been successfully reconstructed by the reflection signal interpretation.3)Under the setting of a local electron density disturbance profile,simulation results of USPR density inversion show that although a large local density depression or elevation can cause the inversion to fail,it is possible to qualitatively determine the existence of distortions in the actual profile through analysis of the microwave time-of-flight map.4)This study also proposes a method for data analysis and processing,which calculates the location and size of local density disturbances through iterative calculation of the cut-off density formula.5)Another study found that the small gap between right-hand cutoff layer and electron cyclotron resonance layer,due to the low plasma density on the plasma edge,brings unexpected leakage from transmitting microwave beam to pedestal and core region.This kind of "tunneling" effect will cause the reflected signal to have energy loss in the low frequency band.6)The study also discusses the influence of the poloidal magnetic field on the reflected signal.The spatial variation of the poloidal magnetic field will lead to the conversion between extraordinary(X)waves and ordinary(O)waves,which leads the energy loss in the reflected signals.The simulation results show that the "tunneling" effect and the O-X mode conversion effect have little effect on EAST USPR system,so the currently designed transmit power meets the working requirements.To sum up,the above simulation research predicts the future experimental results of the USPR system on EAST and discusses the influence of some physical phenomena on the experimental results,so as to provide a basis for optimizing experiments.