微波反射计在磁约束等离子体中的应用

【作者】 张重阳；

【导师】 刘万东； 李弘；

【作者基本信息】 中国科学技术大学 ， 等离子体物理， 2014， 博士

【摘要】 本论文围绕着微波反射计的研究工作展开,微波反射计是一项等离子体诊断,它能够提供磁约束装置内等离子体密度剖面分布以及等离子体密度涨落,得益于其较高的时间空间分辨能力以及较少的窗口资源占用,它已经广泛的应用于各种类型的磁约束装置上。本文的主要工作包括：微波反射计方案设计,微波反射计的平台实验研究,微波反射计数据处理方法研究,最后介绍了微波反射计在托卡马克上的实验测量工作。通过这些研究工作,本论文给出了微波反射计系统的一些关键参数,包括测量目标,时间空间分辨,测量误差,测量范围等。本论文首先分析比较了三种微波反射计设计方案,包括幅度调制反射计,频率调制造续波反射计,脉冲雷达反射计。在EAST上我们采用频率调制作为微波反射计的设计方案,由于采用了HTO固态源作为该反射计的微波源,该微波反射计的全波段扫描周期大大缩短(10微秒),在一个扫描周期内等离子体被’冻结’。为增加反射计的密度覆盖范围,该微波反射计采用了双极化测量,它的发射天线极化方向与托卡马克中磁场方向成四十五度夹角,可以同时发射寻常波极化与非寻常波极化的微波信号,在接收端采用一对接收天线分别接收两种极化信号。该反射计的微波发射频率覆盖33GHz～72GHz之间,与之对应的电子密度测量范围包括0～6.0×1019m.,这使得该反射计能够覆盖EAST托卡马克整个低场侧等离子体。本论文通过平台实验对影响微波反射计测量的几个因素做了实验分析,其中包括：微波源的频率输出特性,微波源的扫频速率,反射板与天线之间的距离与夹角,过模波导的色散效应。我们在平台实验中改变这些参数,进而观察这些参数对于微波反射信号幅度与频率的影响,最终通过平台实验来确定微波反射计的时间分辨与空间分辨,以及波导的色散效应,发射天线的有效角度。接下来我们根据平台实验得到的结果对微波反射计的硬件进行调整,对微波源做校准使得其输出频率随时间线性扫描,通过平台实验的定标来扣除波导色散对中频频率的影响,确定发射天线与等离子体截止层夹角的有效范围,调整延迟线长度使得中频频率处在最优的探测范围内。本论文根据频率调制反射计的特点在数据处理中采用了两种分析方法-时频分析方法与数字复解调方法。时频分析方法是对中频数据在不同的时间点上做频谱分析,得到随时间变化的中频频谱,然后根据中频频率计算得到时间延迟,再通过时间延迟得到相位随频率变化,最终根据寻常波以及非寻常波在等离子体中的色散关系分别得到两种模式对应的密度剖面。在频谱分析过程中由于寄生散射的干扰较多,因此可以采用一些能够剔除寄生散射的寻峰算法,包括：最佳路径法,突发模式法,自适应频谱法等。而数字复解调也就是CDM方法通过中频频谱移动以及数字滤波处理可以直接获得中频信号的相位延迟信息。这两种的适用范围不同,CDM适用于中频频谱分布在一个较窄频率范围内的情况,而时频分析方法可以在更广的范围内使用。在本论文的最后介绍了微波反射计的三次实验情况,这三次实验分别在EAST与J-TEXT上进行。在EAST首次实验中采用的是V波段非寻常波模式,也就是单一通道测量,得到了部分密度剖面的分布;在J-TEXT上采用了双极化测量,密度测量范围覆盖整个低场侧等离子体,得到了完整的密度剖面分布与左旋波模式下微波在内壁处的反射信号；在EAST第二次实验中微波反射计采用了双极化测量,因此能够覆盖整个EAST低场侧等离子体,同时为了降低反射计对窗口资源的占用我们将天线安装在真空室内部,在微波传输过程中采用了低损耗的过模波导,在真空室内部使用了角锥天线作为接收与发射天线。更多还原

【Abstract】 The thesis is basic on research of microwave reflectometry, the microwave re-flectometer is a diagnosis equipment and it can provide the plasma density profile and plasma Density fluctuation in magnetic confinement devices, benefit from its high tem-poral and spatial resolution and relatively small window consuming, it has been widely used in a lot of magnetic confinement devices around the world.The main work of this thesis was arranged as follow:microwave reflectometer scheme design, microwave re-flectometer table experiment research, microwave reflectometry data analysis method, microwave reflectometry experiment measurements at tokamak.Through the table ex-periment research, we can indicate key parameters of microwave reflectometer such as:measurement target, electron density range, temporal and spatial resolution, range resolution, range accuracy.At first, the thesis introduce three kinds of microwave reflectometer scheme, which includ amplitude modulation reflectometry, frequency modulated continuous wave reflectometry, pulse radar reflectometry.At EAST tokamak,frequency modula-tion was chosen as the reflectometry schematic, HTO was utility as reflectometry source, the reflectometry can sweep the full band in ten microseconds, plasma can be seen as ’frozen’ in the sweep period. In order to increase the plasma density cov-erage, the transmit antenna was arranged45degree with magnetic field, extraordinary wave and extraordinary wave polarization antenna were utility as received antenna. The reflectometer frequency range cover33GHz～72GHz, so corresponding elec-tron density measurement range is0～6.0×1019m-3. which makes the reflectometer can cover the low field side plasma at EAST Tokamak.In the thesis, several parameters which can affect reflected signal has been re-searched at table experiment. The parameters include:the output frequency character-istics of microwave source,Sweep rate of the microwave source, the distance between the transmit and receive antenna, angle between antenna and metal panel, the disper-sion effect at waveguide. By adjusting these parameters in table experiment, we can observer beat frequency spectrum, according to table experiment result, we can con- firm spatial and temporal resolution of the reflectometer. After table experiment, we will optimize reflectometry hardware parament such as microwave source calibration to reduce the nonlinear frequency output of the source, reduce the effect of waveguide dispersion through the table experiment calibration, determine the valid angle range between transmitting antenna and plasma cutoff layer, adjust the length of delay line to makes the intermediate frequency falls in the optimal detection range.Based on the characteristics of frequency modulation reflectometer, the thesis us-ing two analytical methods in data processing:time-frequency analysis method and CDM method. The time-frequency analysis do a lot of spectrum analysis to IF data acquisition in a sweep period, then getting IF frequency over time, then calculating the time delay through transmitting frequency, after that the time delay is utilize to get phase delay, at last density profile was acquired according to ordinary mode and extraordinary mode dispersion relation in plasma. In data process, there are some parasitic reflect interference the process of spectral analysis, we used three different peak search algorithms:best path method, the burst-mode method, adaptive spectrum method. Digital complex demodulation (CDM) method move spectrum of IF sig-nal,then filtering them which can directly obtained IF signal phase delay. This two methods utilize at different environment, the CDM can apply in the intermediate fre-quency spectrum with a narrow frequency range, while the time-frequency analysis can be used in a broader frequency spectrum.At last, the thesis describes three experiments of the reflectometer, these are at EAST and J-TEXT respectively. In EAST tokamak single V-band extraordinary wave mode was apply at first time, it has been get part density profile at the first experiment; With the dual-polarized reflectometer at J-TEXT,profile density can cover entire low-field side Plasma density of the tokamak, at the same time the Left hand wave mode microwave was observer at J-TEXT; The second experiment at EAST also used a dual-polarized measurement, while In order to improve signal to noise ratio we will install the antenna inside the vacuum chamber, and a low-loss over mode waveguide was apply as microwave transmission line.更多还原