Research On Fundamental Theory Of Microwave Plasma Torch(MPT) And Development Of Kilowatt MPT Spectrometer

It has been about 30 years since the development of Microwave Plasma Torch (MPT), this type of plasma source has the advantages of both Inductively Coupled Plasma (ICP) and traditional Microwave Plasmas (MWPs) with powerful excitation capability. With the MPT, an argon, helium, nitrogen, or even air plasma discharge can be sustained at atmospheric pressure and at very low flow rate of supporting gas and low microwave power. So the MPT source has rapidly gained widespread attention in analytical chemistry field, and gradually been improved in the development process. However, the major improvements of the torch itself are mainly based on the needs of the practice and using the trial and error method. There is little theoretical exploration, resulting in insufficient to solve the new problems caused by the raise in power from one hundred watts up to the kilowatt level in the development of kilowatt MPT (kW-MPT) spectrometer. Based on this, different MPT torch designs are analyzed and studied from a theoretical point of view by modeling, simulation and high-speed photographic method/technology in this thesis, and the related research achievements has been successfully applied to practice at last, and the related issues have been addressed. The main innovative achievements made are as follows:1) The equivalent circuit model of the structure of the MPT are for the first time established on the transmission line theory, and the roles played by each adjustment mechanism of the MPT in term of the impedance matching and tuning are analyzed as well. The theoretical model and the experimental results agree well, which can provide a theoretical basis for further improvement of the torch structure.2) Spatiotemporal motion characteristics of the MPT source are for the first time investigated with a high-speed camera. It was found that the inverted funnel shape with a central channel of the Ar MPT is formed by the regular and high speed rotational motion of the discharge filament(s), and it is also the origin of the unique performance compared with other microwave plasma sources. The effects of different operating conditions on the plasma high-speed rotational motion are investigated, and the excitation process of MgCl2 dry aerosol in the MPT is also preliminary studied, this work makes the understanding of the MPT go from macroscopic time scale into the microscopic time scale.3) The regular and high speed rotational motions of the plasma filament(s) are regard as the origin of the unique noise characterization of MPT, and a mathematical model for describing the relationship between them is established based on this point of view. This model can explain the noise spectrum variation under different conditions, and also provide a good reference for the optimal conditions in the spectral detection field.4) The electromagnetic field distribution characteristics of MPT cold cavity was simulated in this paper. The effect of inner tube and the location of the metallic spacer between inner and intermediate tubes on the electromagnetic distribution in the torch was discussed in depth and verified with experiments. Based on abovementioned results a novel microwave plasma torch with double resonance configuration was developed. A very stable plasma could be sustained not only at relatively low microwave forward power, but also at kilowatt microwave forward power with this modified MPT torch. The results laid a good theoretical and technological foundation for the development of high-performance kW-MPT atomic spectrometry, and also accelerated the development progress of the high-performance kilowatt MPT spectrometer.5) A highly stable high-power microwave power source with a stability better than ± 0.5%is developed. On the basis of reasonable selection of components, the kilowatt MPT spectrometer is set up and the basic functions set by the National Major Scientific Equipment Development Projects of China (Grant No:2013YQ470781) has been realized. Direct introduction of wet aerosols produced by solution nebulization into ArMPT is for the first time achieved. The performance of the instrument has meet the goal of the Project, and successfully passed the medium-term evaluation of the MOST (Ministry of Science and Technology, Government of China).