Research On Exciting Characteristics Of High Power Radio-Frequency CO₂ Laser

Radio-frequency (RF) excited high power CO₂ laser is the main light source and shows an important trend for the high power laser welding and cutting equipment in the world nowadays. In the thesis, systematically theoreticl analysis and experimental researches were done on the aspect of the exciting power supply, RF transmission, RF excited gas discharge plasma, uniformity of RF excited gas discharge and characteristics of RF power modulation of high power RF excited CO2 laser. The main research contents of the thesis are as flows:(1) The 30kW self resonant oscillated RF power supply was analysed and designed. The oscillating circuit with three-point capacitance was designed based on the RS3041 RF vacuum tube. The static and dynamic parameters were analysed. The component parameters of the anode resonant circuit and the grid feedback circuit were calculated. And the simulation model of the RF vacuum tube oscillating and amplifying circuit was established. The simulation result showed that 30kW output power with 83MHz RF output can be obtained by the power supply circuit.(2) A 2-dimensional theory model of voltage distribution for large area RF-excited slab discharge was presented. The 2-D function curves of voltage distribution based on 40mmx500mm electrodes at 2kW input RF power and 200mm×1000mm electrode at 30kW input RF power were calculated. The result showed that the maximum voltage fluctuations were 25% and 65% respectively. Further more, in order to resolve the nonuniformity of the 2-D voltage distribution, a theory model of installing parallel resonant inductors in the longitudinal direction and reactance components in the transverse direction was presented.(3) Theory model of making the 2-D votage distribution of the large area gas discharge uniform was established and experiments were done to verify the uniformity of RF excited gas discharge. When RF energy was injected at the middle in the longitudinal direction, and at the middle or the side in the transverse direction, uniform voltage distribution was obtained by using symmetricl or asymmetricl parallel inductors on the two sides of the transverse direction. For the electrodes of 40mm×500mm and 200mm×1000mm, when the middle feed point and side feed point were adopted respectively on the transvers direction, the voltage fluctuations of the former were 1.5% and 2% respectively, and the voltage fluctuation of the latter were 2.5% and 3% respectively. Experiments were done to study on how the intensity of the glow discharges varying with the input RF power or the gas pressure. And the influence of the parallel inductors on the uniformity of the gas discharge was found to accord with the theory model.(4) Transmission of the RF power and impedance matching characteristics of the load were studied. Equivalent impedance of gas discharge on electrodes of 40mm×500mm and 200mm×1000mm, and RF transmission impedance matching network were ananysed and calculated. WhenΠ-shaped or L-shaped matching network was adopted, the variation rules of the parameters of the impedance matching network with the load were deduced by using the Smith circle-diagram method. The best matching network type and parameters of the components were determined. And the voltage standing wave ratio is controlled within 1.2 by using a self-adaptive adjusting installation.(5) The power modulation characteristics of the RF slab laser were studied. Parameters of the modulation pulse on ignition, maintenance, continuous mode and pulse mode were analysed. The output waveform according to the requirement of high power RF laser was obtained by developing the hardware circuit and software program of the DSP pulse width modulator.(6) Influence relation between the distortion of the resonator mirror and the output laser beam with the change of the input RF power was studied. Relation curve of the synchronously heating compensation for the mirror with the variation of the input RF power was established by simulating and analyzing the temperature distribution and distortion of the reflecting mirror using the method of finite element analysis. And compensation result of the curvature rate distortion less than 0.02% of the reflecting mirror was obtained.The research contents of the thesis has important theoretical significance and practical value to conquer the key technology of the RF excited high power CO₂ laser and to meet the urgent need for the development of the high power laser on equipment manufacturing industry in our country.