Study On The Spectral Diagnostics Of Plasma Jet Generated By Gaseous Discharge

Plasma jet provided a new method for synthesis of new materials; it was applied in surface treatment of metals and surface coating. Gaseous discharge, as a mean path of obtaining plasma, played a more important role in both civil and industrial applications. Accordingly, optical diagnostics as a non-intrusive method has important foundation and application significance in the related areas.One of the focuses of this dissertation is to describe the emission optical spectroscopy of low pressure discharge plasmas and presents the laws observed by the corresponding processes in the plasmas. Some diagnostic methods based on plasma static-electricity and spectroscopy are theoretically outlined at first, and then some investigations are theoretically made on the applicability of the diagnostic methods. As results of the investigations,some new analytical methods of vibration-rotation temperature are derived for different resolved spectral and some spectral simulation was made by relevant calculation program and a new diagnostic method of discharge characteristic based on the 1f/2f detection of concentration modulation have been achieved.Usually it was thought concentration modulation spectroscopic technique is based on that transient molecular concentration was in direct ratio of the absolute value of discharge current-that is to say modulation frequency of transient molecules concentration is twice the discharge frequency f. However, in our experiment the signal could be detected using both 1f and 2f. The reason was the discharge was not continuous, but occurred as a pulse only once in every negative half cycle. Fourier transform analysis was operated on the signal pulse, it was found that besides the original frequency, there were a large amount of harmonic frequency compositions. So the optical signal could be demodulated using both 1f and 2f. The rational explanation was given for the phenomena that the pulsed discharge presented with continuous AC voltageThe dissertation is composed of 10chapters and one appendix.The main results of the dissertation are:A new equipment of AC discharge was developed for producing molecular ion beam.The N₂,O₂and Ar plasma jet was generated from a nozzle using an AC discharge of about 15kV. A very high ratio of N₂⁺/N₂,up to 6:1 was achieved in N₂ discharge, which is much higher than that obtained with conventional nitrogen ion source. The effects and processes of N₂ discharge and concentration modulation spectroscopy were studied in N₂ discharge。The vibrational temperature and rotational temperature of N₂ molecular ions beam excited was caculated as 3310Kand 282K from the emission spectra.the spectral intensities and width of some Ar lines produced in plasma jet was adopted to calculate the electron temperature and electron density .it is respectively 18714 Kand1.37×10¹⁶cm^-3Nonmonotonic characteristics was found that the vibrational temperature decreased first and then increased along the downstream from the nozzle by measuring the emission spectrum from the different axial position in the jet. On the basis of experiment condition a theoretical model was presented to fit with experimental data. Meanwhile,the vibrational temperature variation with the different axial position z and its generation mechanism is analyzed. The N₂⁺/ N₂ ratio variated along the axis and the property of discharge in the experimental system was studied.The dynamic study of the generation of the nitrogen molecular ions produced in the AC discharge of cw nitrogen jet by the means of the concentration modulation spectroscopy was presented. Unexpectedly, the nitrogen ions was generated in pulse mode when the discharge voltage was higher than 9.6 kV (peak-to-peak), and the molecular ions pulse arised only in every negative cycle of the discharge, meanwhile an opposite peak appeared in the discharge current sine waveform. This peak was formed by the inertia movement of ejecting nitrogen ion in jet. Additionally, the intensity of the ions was found to be proportional to the area of this opposite peak.This phenomena was interpreted by a proposed theoretical model based on the 1f/2f detection of concentration modulation spectroscopy and further confirmed by the experiments.In order to probe into the dependence of the emission spectral intensity on discharge voltage and gas pressure, the N₂ spectral in glass capillary by dielectrical barrier discharge at discharge frequency 20kHz are measured via concentration modulation spectroscopy. The spectral line at 357.7nm of N₂C³âˆ_u—B³âˆ_g and at 391.4nm N₂⁺B²âˆ‘_u⁺—X²âˆ‘_g⁺ was recorded experimently to study the dependence of its the emission spectral intensity on discharge voltage and gas pressure. The measured results show that the intensity increases with increasing discharge voltage at lower voltage, but increases slowly at higher voltage for constant gas pressure p=130Pa; and the intensity decreases with increasing gas pressure at constant discharge voltage U=6.5kV. Furthermore, a theoretical model was proposed according to electronic excitation function and ionization function ,which can basically interpret the relation of spectral intensity varing with discharge voltage and gas pressure, goodly fit the experimental data by a nonlinear least square fitting ,the correlation coefficient R>0.9.The mechanism of the dependence of the emission spectral intensity on discharge voltage and gas pressure in the discharge plasma was revealed further.The intermediate decomposition products of methanol (CH₃OH) in an AC discharge have been diagnosticated via concentration modulation spectroscopy . Several main vibrational bands of CO Angstr(o|¨)m B¹âˆ‘⁺—A¹âˆsystem, CH A²â–³â€”X²âˆsystem at 430nm and B²âˆ‘^-—X²âˆat 390nm, as well as CHO(329.82nm), CH₂O(369.8nm), CH₃O(347.8nm) and Balmer series spectra of H atom are identified in the region between 300nm and 670nm. Furthermore, the dependences of the emission spectral intensities of the intermediate decomposition products on the discharge voltage and parent gas pressure are investigated in detail.. The experiments indicated that the relative population ratio of CO(B¹âˆ‘¹) and H increases with increasing discharge voltage more quickly than that of CH(A²â–³). Several possible reaction passages are given and discussed. Additionally, the vibrational and rotational temperatures of CH (A²â–³) were determined to be about 4200K and 1100K respectively, and the vibrational temperatures of CO(B¹âˆ‘⁺) were determined to be about 2500K by analyzing the intensity distribution using LIFEBASE computer program. The decomposition mechanism of methanol in the discharge plasma was discussed in this paper as well.