The Design Of A Reflectometry System On The J-TEXT Tokamak

Microwave diagnostics are essential methods to get the experiment data with high resolutions in magnetic confinement fusion research.Microwave reflectometer is a kind of radar-like method for the measurement of electron density profile in the inhomogeneous fusion plasma.This paper mainly focuses on the development of a new multi-channel and multi-mode microwave reflectometry instrument on the J-TEXT tokamak.This paper firstly introduces some common methods of electron density diagnosis in the high temperature plasma and their advantages and disadvantages.By contrast,the microwave reflectometry shows the advantages of higher spatial and temporal resolutions,and less window area used.Then the development history and research status of the reflectometry are introduced.Through calculation and simulation,this paper illustrates the shortcomings of the pre-existing Q band O mode reflectometry on the J-TEXT Tokamak,and proposes the design of new multi-channel and multi-mode microwave reflectometry system to improve the measuring capacity of the density profile.Finally,the working modes of the system are determined as:Q band with X mode microwave,V band with O mode and V band with X mode.The measurable density range extends from 1.3×10¹⁹8)^-33.1×10¹⁹8)^-3 to 1.3×10¹⁹8)^-37.0×10¹⁹8)^-3.The reflectometry transmits microwaves into plasma with sweep period of 20?s.The output frequencies are swept in Q band?33⁵0GHz?and V band?50⁷5GHz?respectively.To ensure that each receiver can get effective reflected signal,the output power of the horns range from 15dBm to 19dBm.Then the reflected signal is mixed with homodyne mixing.A data acquisition system with a sampling rate of 60M/s is selected to get the beat frequency and upload the data to the server.The density inversion program reads the data on the server and output the density profile.In the process of the design and construction of the new microwave reflectometer system,the system performances are improved in the following aspects.First,a digital frequency synthesizer?DDS?is used instead of the traditional Voltage-Controlled Oscillation source?VCO?,so the purity,accuracy,stability and phase noise level of the source signal in the system has been significantly improved.Second,on the premise of ensuring the system reliability,a single-antenna design has been adopted in to save the window space.At last,the acquisition system has been upgraded and optimized to ensure fast data acquisition and uploading.