The thermal coupling between laser and matter has always been a hot topic. The temperature characteristics of aluminum target irradiated by pulsed laser and the target vapor plume ejection were studied. The heat conduction equation was used to describe the temperature change characteristics of Al target irradiated by laser. The mass conservation equation and momentum conservation equation were used to describe the temperature and density of melted fluid. The mass conservation equation was used to describe the density and velocity characteristics of vapor after gasification. The simulation results showed that the target temperature raised and the liquid and vapor phases changed due to the laser irradiation. The model considered the thermal convection effect caused by the liquefaction flow of target material, and the physical model can describe the temperature change of metal target under laser irradiation. The phase change of solid target was obtained by setting the phase indicator, and the process of target steam injection was analyzed. The physical model can be used to analyze the thermal coupling of laser-matter interaction, thus laying a theoretical foundation for the study of laser-matter interaction.
Laser micro thruster based on semiconductor laser has great advantages in volume. However, the design of optical collimating and focusing system is very challenging under highly integrated conditions. The operating distance from the laser exit to the working ablation target needs to be tens of millimeters. Based on the ray tracing method, a convenient and fast design method of the optical collimating and focusing system is proposed under the small working distance. The method is verified by an example. The design conditions are as follows: laser wavelength is 940 nm, fiber core diameter is 105 microns, and the numerical aperture is 0.22; focusing spot diameter is 50 microns; fiber head and ablation surface distance is not more than 20 mm; only one lens is used. Using the method proposed in this paper, the final design results are as follows: doubly convex lens R<sub>1</sub> = 5mm, R<sub>2</sub> = 25mm~30mm, lens aperture D = 6mm, central thickness T<sub>C</sub> = 2mm. According to the energy simulation results, the method proposed in this paper is effective.
In order to meet the requirement of frequent in-situ calibration before micro-thrust measurement, a new calibration method based on laser interference is proposed. Based on common optical elements, the interference light path is set up, and the light path adjustment process is simplified by using the visible light of 532nm wavelength as the light source. The calibration principle is that the capacitive displacement sensor and laser interferometer simultaneously measure the movable pyramid prism’s position change resulted by adjusting displacement table, and the measurement result of laser interferometer is viewed as the reference displacement to carry on the sensor's calibration. By comparing with the sensor output, the method of calculating the number of fringes corresponding to optical path difference is analyzed. The practicability and accuracy of the calibration device is verified by experiment, and the calibration results and the relative error are analyzed at last.
Laser ablation micro-propulsion technology is the most promising field in laser propulsion. The specific impulse is the important evaluate indicator of the micro propulsion performance. It represents the impulse generation by consuming unit weight of the working medium. Its accurate measurement can not only help to analyze the mechanism of laser ablation matter, but also help the design of micro laser ablation thruster. This paper presents a measurement method, which can not only obtain the impulse by single pulsed laser ablation the working medium, but also measure the ablation mass, which can directly measure the specific impulse. The method is based on the model of torsion pendulum in vertical direction movement, which is the direction of impulse by pulsed laser ablation and the gravitation direction caused by the loss of the ablation mass, to obtain the corresponding impulse and ablative weight. The paper deduced the measuring principle, pointed out the error and analysed the design principle of the torsion balance, the results show that, according to the torsion angle change due to the impulse by pulsed laser ablation and the mass loss by laser ablation, the maximum angle can be used to calculate the impulse and the stable rotation angle can be used to calculate the loss weight when the torsion period is much greater than four times the width of a single pulse.