| Due to its advantages of non-electrode and pollution-free discharge,high ionization rate and wide working pressure,microwave plasma is regarded as a novel technology for the preparation of high-purity materials and gas conversion.The microwave linear plasma source,based on surface wave discharge,can be classified into two forms:internal and outerwall,according to the discharge structure.Among them,the outerwall microwave linear plasma,called Duo-plasmaline,has excellent axial uniformity,which is an effective way to obtain large-area plasma source.The internal microwave linear plasma,called Surfaguide,has the unique advantages of flexibility and high coupling efficiency,which make it an efficient means for converting hazardous gases.Based on these two types of microwave linear plasma sources,a systematic investigation was carried out,and the main works and results are as follows:(1)An asymmetric 3D simulation model of Duo-plasmaline plasma source was built,with the innovative introduction of a reflective antenna and permanent magnets component.The plasma discharge process was analyzed and visualized.The simulation results show tha the dual-port power input mode significantly reforms the axial electron density attenuation.The semi-cylindrical reflective antenna has an excellent homogenization effect on axial plasma parameters by impacting the electromagnetic wave transmission.The external static steady-state magnetic field has a certain constraint and enhancement on the eletron density but reduces the uniformity coefficient.The mode transition of electromagnetic waves from volumetric wave radiation to surface wave sustained in the discharge cavity is visualized by spatiotemporal resolution analysis.The plasma axial electron density uniformity scale was quantified,and the optimal uniformity process parameter was established as dualport power input working with a reflective antenna,in which the theoretical uniformity coefficient was controlled at 97.71%.(2)The Duo-plasmaline plasma discharge and diagnostic platform,with a length of 1000mm,was designed and built.Through axial moving OES diagnosis,the electron excitation temperature,rotational temperature,and electron density were estimated.The experimental results show that the gas temperature was evalueated from the rotational temperature of OH molecule spectrum approximatly,which is less affected by the input power intensity and gas pressure.The electron excitation temperature is in the range of 1-1.2eV,which decreases with the increase of gas pressure.The electron density is about 3-6×1018m3,which significantly improves with an increase in the input power and gas pressure,but shows no obvious difference in order of magnitude.Finally,the optimal experimental uniformity coefficient was achieved under low gas pressure of 30Pa and dual-port powers input of 500W,in which the uniformity coefficients of axial electron density and electron excitation temperature could reach at 81.14%and 88.99%,respectively.(3)In cooperation with the IPP-P4G research group in Germany,we carried out he parameter diagnosis and physical mechanism studies of Surfaguide discharge assisted CO2 gas conversion.The experimental results show that:with the gas pressure in the range of 2-5mbar,the rotational temperature(Trot)regarded as gas temperature is determined by fitting of CO Angstrom system bands.Tgas was obtained between 1500K to 2000K,which had an obvious positive correlation with the gas pressure.The vibrational temperature(Tvib)was estimated by the relative intensity ratio method of the vibrationa transition bands,and the pre-disscociation of CO(B1∑+)state at the high vibrational energy level was found.After theoretical analysis,Tvib was given for reference in the range of 2000K-4000K,which was significantly negatively correlated with gas pressure.The trend of "high conversion and low energy efficiency" of CO2 gas conversion was obtained and calculated by the mass spectrometer.The conversion had high dependence on gas flow,which could reach the maxium of 90%at 20mbar and 100sccm.The energy efficiency had low dependence on discharge parameters and always keeps in the low range of 2%-20%.Finally,the CO2 conversion mechanism in this work was established that:the electron excitation dissociation is dominant at low gas pressure,then the contribution of thermal dissociation contribution cannot be ignored with increasing the gas pressure.(4)Preliminary exploration of electromagnetic field and plasma discharge simulation based on the Surfaguide device were conducted.The results show that,with a reduced height of 10.95mm and a short-circuit position of 91.5mm,the electric field in the reduced waveguide is concentrated and enhanced at the energy coupling position,which met the requirement of internal linear plasma discharge.A 2D axisymmetric model was established,and the plasma with obvious linear extension in the quartz tube was vistualized.The trend of "diffusion at low pressur and contraction at high pressure"was also found,which was consistent with the phenomenon observed in the experiments.The preliminary simulation with external fluid heat transfer showed that the highest temperature region was basically concentrated in the position of the quartz tube where the microwave energy coupled.(5)Through the analysis of axial and radial distribution of electric field of electromagnetic wave and microwave power deposition,we established the commonality of the surface wave sustained mechanism of the two types of microwave linear plamsa.The work and results present in this paper provide a theoretical foundation and important guiding significance for the application scenarios of microwave linear plasma facing different requirments in industrial development. |
