Study On The Microwave Characteristics Of Plasma And Microwave Diagnostic Methods

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.