【作者】 李斌；

【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， 2010， 博士

【摘要】 本论文主要研究了等离子体的微波特性以及几种新型的微波诊断方法。在等离子体微波特性研究方面,我们对微波在等离子体中的传播特性进行了理论分析和数值计算,并在此基础上展开了实验研究；而在微波诊断方法研究方面,我们对等离子体吸收探针、微波截止探针和微波反射计分别进行了理论分析和实验研究,并对这些诊断方法进行了优化和改进。本论文首先采用抛物线型分层模型对等离子体与微波的相互作用进行了理论分析,获得了微波在非均匀等离子体中传播时的反射系数与吸收系数的理论模型,并对这个模型进行了典型参数的数值计算,计算结果表明,高密度、强碰撞的等离子体对在其中传播的微波有明显的吸收效果。相关的实验研究是在等离子体微波特性实验室中进行的,实验结果表明,等离子体覆盖可使等离子体目标反射系数在2～18GHz频率范围内减小10-20dB,这个结论与理论分析结果一致。此外,我们在微波暗室环境中利用脉冲压缩雷达反射法对复杂背景等离子体目标进行了密度剖面测量。本论文在不采取准静电近似的前提下,对标准型等离子体吸收探针(PAP)进行了理论分析,提出了表面波色散关系的电磁理论模型,并通过实验研究和数值模拟对此模型进行了验证分析,两方面的结果都验证了PAP的表面波电磁理论模型是有效的。基于以上理论分析结果,我们提出了一种新型的PAP结构——电磁PAP。此外,我们还对敏感型PAP的鞘层效应对等离子体密度测量结果的影响进行了实验研究,提出了双功能PAP的测量方案。这种PAP同时具有静电探针和敏感型PAP的功能。利用双功能PAP的双重功能可在进行电子密度测量之前完成鞘层厚度的标定。这对于降低数据处理过程中因不准确的预设鞘层厚度值引起的测量误差而言是很重要的。本论文提出了一种用于等离子体密度测量的新方法——微波截止探针。它的测量原理是基于微波在等离子体中所发生的截止现象。我们首先对微波截止探针的测量原理进行了理论分析和数值模拟,两方面的结果均表明,微波透射系数频谱数据的临界点所对应的频率值即为接收天线所处位置的等离子体频率。所以只需从实验上测得入射微波在等离子体中的透射系数频谱数据,即可从中确定测量点的等离子体频率,并利用等离子体密度与等离子体频率之间的关系获得电子密度值。实验研究中,通过改变等离子体参数研究了微波截止探针的适用范围。实验结果表明,微波截止探针可用于大参数范围的等离子体电子密度诊断。本论文研究和分析了用于EAST托卡马克等离子体密度剖面诊断的频率调制反射计的研制。为了综合利用寻常波模式和非寻常波模式各自的优点,本系统采用双线极化的测量模式,可同时进行寻常波截止和右旋截止反射信号的测量。对于一定频率范围的微波源而言,这种极化方式可扩大微波反射计可测等离子体密度范围。为了保持测量数据的信息完整性,本系统采用数字检波方案,利用数据采集系统记录中频信号波形数据,之后可利用数据处理方法对此进行相应的数值分析,常用的数据处理方法有最佳路径方法、突发模式分析方法和自适应时频分析方法等。更多还原

【Abstract】 The microwave characteristics of plasma and several novel microwave diagnos-tic methods have been studied in this thesis. The propagation characteristics of mi-crowaves in plasma are investigated through theoretical analysis and numerical calcu-lations. Based on the theoretical and numerical results, these characteristics are stud-ied subsequently in experiments. In addition, several microwave diagnostic methods including the plasma absorption probe, the microwave cutoff probe and the microwave reflectometry are analyzed theoretically and experimentally. Then these methods are optimized and improved.The plasma-microwave interaction has been analyzed theoretically by using a parabolic hierarchical model. Theoretical models of the reflection and absorption co-efficients for the propagation of electromagnetic waves in a nonuniform plasma are obtained from the theoretical analysis. The numerical calculations using the typical parameters are completed for these theoretical models. Relevant results show that the high-density and highly collisional plasma is effective for the absorption of the electro-magnetic waves. Related experimental studies are carried out in a microwave anechoic chamber. The object for measurement is a plasma target. When the plasma target is covered by a plasma layer, the reflection coefficient of the incident microwave is re-duced 10～20 dB sensitively in the frequency range 2～18 GHz. It means that the highly collisional plasma is a good absorber of the microwave. This result is in a good agree-ment with the theoretical result. Moreover, the pulse compression radar reflectometry is used for measuring the electron density profile in the plasma target with parasitic reflections.The standard plasma absorption probe (PAP) has been analyzed theoretically with-out using the quasi-static approximation. A theoretical electromagnetic model for the dispersion relation of surface waves is presented in this thesis. The principle of this model is verified in experiments and numerical simulations. Both experimental and numerical results show that the electromagnetic model of surface waves is valid. Then a new type of PAP, which is named as the electromagnetic PAP, is proposed. In addi-tion, the sheath effects of the sensitive PAP on the measurement of electron density is investigated in experiments. A dual-role PAP has been proposed for the investigation. It can be used as the Langmuir probe and the sensitive PAP simultaneously. Using both functions of the dual-role PAP, the sheath thickness can be calibrated before the measurement of electron density. It is useful to reduce the measurement error caused by an inaccurate assigned value of sheath thickness in the data processing.A novel diagnostic method for the measurement of electron density using a mi-crowave cutoff probe has been proposed in this thesis. It is based on the cutoff phe-nomenon when the microwaves propagate in the plasma. The measurement principle of this method is analyzed in theories and simulations. There is an obvious critical point in the frequency spectrum of microwave transmission coefficient. Both the theo-retical and simulation results show that the abscissa value of this critical point equals the plasma frequency in the position of the receiving antenna. Once the transmission power spectrum of the microwaves in the plasma is obtained from experiments, the value of plasma frequency can be easily determined from the spectrum curve. Then the value of electron density is determined directly by using the relationship between the electron density and the plasma frequency. The measurable range of the microwave cutoff probe is investigated by changing the plasma parameters. Experimental results show that the microwave cutoff probe can be used for diagnosing plasmas with a wide variable range of parameters.The research and development of a frequency modulated continuous wave (FM-CW) reflectometry, which is used for the measurement of the density profile in Experimental Advanced Superconducting Tokamak (EAST), are studied and analyzed in this thesis. In order to combine the respective merits of both the 0-mode and X-mode reflectometry systems, the dual-polarization is chosen for our reflectometry. This mode of polarization has an ability to receive the reflection signals of both O-and X-modes simultaneously. It can be used for extending the measurable range of electron density for a microwave source with a fixed frequency range. The digital detection is employed in this system to keep the integrity of the acquired data. The waveform data of the intermediate-frequency (IF) signal is recorded by using the data acquisition system (DAQ). Then this data can be processed numerically by using several kinds of data processing algorithms, such as the best path technique, the burst-mode analysis and the adaptive spectrogram analysis.更多还原