The Study Of The Optical Emission Spectra Diagnosis Of Cold Plasma

Thanks to the continuous development in the technique generating plasma through gas discharge, the applications of low-temperature plasma are more extensive. Proving to have a promising prospect in the industry, the growing needs as well as specific demands for low-temperature plasma of various characteristics push forward the study of it, making it a valuable topic for scientific research. The prerequisite for experimentally investigating low-temperature plasma is to develop diagnostic methods satisfying simple operation, low cost and proper precision, and thus people can analyze the response of the targeted feature of plasma to the diverse parameters, which is one crucial step before a higher level control of plasma. For that reason, the study of low-temperature plasma generated by coaxial hollow gridded discharge was proposed in this paper by using the optical emission spectroscopy method(OES Method), meanwhile, evaluation of calculating the electron density with absolute continuum intensity measurement(ACI Mesurement)was performed.Firstly, the atomic emission in low-temperature plasma was introduced before discussing the major radiation and excitation processes. The construction and superiority of the coaxial hollow gridded cathode configuration were briefly presented, multiple beforehand measurements were done using Langmuir probe to support the choice of studying the emission spectra of the plasma in the discharge configuration. By changing the discharge conditions, the emission spectra of pure helium and argon plasma were collected and analyzed.Then, the meaning and measurements of four sorts of temperature in plasma were discussed in order to gain insight into the thermal equilibrium condition of the plasma. The theoretical foundation as well as realization process of both excitation temperature diagnosis methods, the Boltzmann Curve Method and the Fermi-Dirac Model, were compared to explain the reason for choosing the former method in this paper. The excitation temperatures of low temperature plasma were measured at helium pressure 75 Pa and argon pressure 25 Pa within the discharge power range1200~2000W, similar measurements were performed at a constant discharge power of 2000 W at different gas pressure, helium 15~75Pa and argon 15~55Pa respectively.A simple model was established to qualitatively study the variation of the excitation temperature.Finally, by making comparison to a reference light source, the method to gainthe absolute value of the spectral emission coefficient of the plasma was demonstrated. According to the radiation transport equation, the relationship between the electron density and the absolute intensity of continuum emission in low ionized plasma was deduced. By performing absolute continuum intensity measurement, the electron density of argon discharge plasma was calibrated.Furthermore, results showed the electron density acquired via ACI measurement was in good agreement with that measured by Langmuir Probe, also in good accordance with the simulation by COMSOL Multiphysics. In the end, the applicability, strength and limitation of the ACI measurement were discussed.