Numerical and Experimental Study of the Trichel Pulses in Needle-plane Geometry
One of the unique aspects of the negative corona discharges in the air is the regular train of pulses that form the discharge current, called Trichel pulses. These pulses are the result of the combination of several phenomena such as the avalanche ionization of the neutral molecules by the impact of the energized electrons, formation of the cloud of positive ions close to the cathode, and formation of the cloud of negative ions at a farther distance from the corona electrode compared to their positive counterparts. In this thesis, the results of a detailed numerical investigation of the formation of Trichel pulses in a needle-plane negative corona discharge, as well as a simulation of the transition of the discharge from Trichel pulse regime to the glow discharge regime, is presented. All presented numerical models in this thesis were three-species models including the motion, generation, and dissipation of three charged species: electrons, positive ions, and negative ions. Also, all models were built using COMSOL multiphysics. Photoionization as the main mechanism for sustaining the positive corona discharge was included in the numerical analysis for both the positive and negative corona discharges using the three exponential approximation. A parametric study of the impact of different model coefficients on the characteristic of the Trichel pulses including the repetition frequency, average DC current and pulse rise time was investigated. The studied parameters include coefficients of the two ionization, and attachment reactions, the mobilities of the three charged species considered, electrons, positive ions, and negative ions, and the coefficient of the secondary electrons emitted from the needle. It was shown that two reactions, the recombination of positive and negative ions, and the recombination of electrons and positive ions play a minor role in the calculated characteristics of the Trichel pulses. Finally, an experimental study of the characteristics of the Trichel pulses in air at room temperature, pressure, and relative humidity has been conducted. The impact of different parameters: the needle voltage, needle-plane distance and the radius of curvature of the needle’s tip on the frequency, DC current, and the temporal characteristics of the pulses (rise time, fall time, and the pulse width) was studied. Four different needles with radii of curvature ranging from 19 to 55 microns were used. Applied voltage on the needle was varied from the onset voltage (-4 kV to -6 kV) to -10 kV. It was observed that the temporal characteristics of the pulses such as rise time, was not a function of needle tip radius of curvature, voltage level, or needle-plane distance. The experimental data were compared with the results of a numerical simulation. The experimental findings were in a good agreement with the results of the numerical model.
Corona discharge, Trichel pulse, Photoionization, Non-thermal plasma, Drift-diffusion equation, Poisson’s equation
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