Energetic working fluid polyazide glycidyl GAP, has attracted more and more attention in the field of laser ablation micro thruster[1-3] because it can not only absorb and utilize laser energy, but also fully release its chemical energy and participate in the process of impulse formation. In order to explore the impulse coupling characteristics of solid GAP in vacuum environment, infrared dyes with different mass fraction were doped as absorbent. In a vacuum chamber with a pressure of 30 Pa, a 1064 nm Nd: YAG laser was used to test different laser energy modes (transmissive and reflective) for different thicknesses and different doping concentrations at different laser energy densities. The results show that the doped infrared dye can not significantly change the absorption threshold of laser energy by GAP, but significantly increases the deposition of laser energy in GAP. The doping concentration has a great effect on the coupling performance of the pumping amount within a certain concentration range. When the doping concentration is 5%, the momentum coupling performance of laser ablation GAP is optimal. The thickness of the GAP with the best impulse coupling performance is related to the doping concentration. The higher the concentration, the smaller the thickness of the GAP with the best ablation performance. When doped with infrared dye, GAP exhibits the body absorption characteristic of laser energy, which follows the injection mechanism of “first absorbing laser energy first injection”, so the reflective ablation mode exhibits better propulsion performance.
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 R1 = 5mm, R2 = 25mm~30mm, lens aperture D = 6mm, central thickness TC = 2mm. According to the energy simulation results, the method proposed in this paper is effective.
Laser ablation micro propulsion is a novel space propulsion scheme with minimum impulse bit, high specific impulse, less pollution, simple constructer of thruster and less weight compared to other micro propulsion, with great potential application on precise attitude control and orbit control of spacecraft and formation flying of micro satellites. Because of the splashing of the liquid working fluid drastically reducing the efficiency of laser ablation, it is essential to feed the liquid by micro flow approximately 50μg per pulse. Currently the main difficulty is the micro flow feed and ablation of the liquid energetic polymers. A micro flow system, utilizing piezoelectric valve controlled by pulse signal and piston cylinder, was designed and set up to satisfy the requirement of the continuous ignition and ablation of the liquid GAP. The dynamic characteristics of ablation, impulse, plume and splashing were observed and analyzed. The ablation behavior and splashing is detected by shadowgraphy experiments and the impulse bit is measured by torsional pendulum in vacuum. The experiment demonstrates the stability of the feed system, the jam status of the jet after the laser ablation, and the feasibility of the laser ablation propulsion using liquid energetic polymers in engineering application.
The propulsive properties of the energetic polymer glycidyl azide polymer (GAP) doped with carbon nanoparticles in the transmission mode were studied and a 1064nm Nd:YAG laser with a pulse width of 9ns was selected. According to the Propulsion performance parameter of GAP with different carbon nanoparticles contents and thickness, combined with plume image, the reasons for improving the propulsion performance of laser ablation GAP by carbon nanoparticles doping are analyzed and the design scheme of the structure composition of the polymer target is preliminarily explored. The experimental results show that the 1064nm laser absorption of the polymer GAP is greatly enhanced and the propulsion performance is obviously improved after doping with carbon nanoparticles, but the propulsion performance of the polymer can not be significantly improved by increasing the carbon contents higher than 1%. The GAP after doped with high carbon nanoparticles contents exhibits metal-like surface absorption characteristic and the absorption depth to laser decreases. Under the transmission mode, with the increase of target thickness, the incomplete ablation mass increases, which greatly reduces the utilization ratio of GAP. The optimal ablation efficiency of GAP target with thicknesses of 54μm was more than 250%, and plume images showed that ablation and decomposition were very sufficient. It is appropriate for the target of laser ablation micro thruster.
The photonic laser propulsion is a new propulsion concept, which uses the thrust amplification generated by multiple momentum exchange and photon energy recycling through photon oscillation in external cavity composed of two HR mirrors. The basic theory of the propulsion concept was introduced; the production mechanism of thrust was analyzed and theory modeling was derived. Finally, several typical working parameters (laser parameters, environment parameters and cavity parameters) were set to calculate the theory thrust value. The study results indicated that in condition that all incident photons totally attenuate in cavity without leaping, 1kW laser power, mirror reflectivity 0.99999 and cavity length 10m, the maximum amplification factor 500000 and maximum photon thrust 0.333 N could be achieved, satisfying the needs of space missions.
When using YAG laser emit strong laser pulses to bombardment solid POM, in the areas of interaction will produce a plume. A shadowgraph photography technique was employed for visualizing temporal evolution of plume. The experiment results show that in the plume flow field, multi-density discontinuities were observed. The plume has a high speed towards laser source, and plume uneven expansion makes the shape and position of discontinuities change. Plume velocity affect the propulsive efficiency, enhance the pulse laser energy can speed up the plume, energy increase to a certain degree of plume speed will reach a steady-state value.
Proc. SPIE. 9142, Selected Papers from Conferences of the Photoelectronic Technology Committee of the Chinese Society of Astronautics: Optical Imaging, Remote Sensing, and Laser-Matter Interaction 2013
Splash generation and evolvement have been observed with ns-shadowgraphy time resolved method on the laser micro ablation with liquid working substance. The flow visualization method of splash phenomenon is introduced and the shock wave and particle front velocity were measured and analyzed to research the relationship between material characteristics and splash dynamics. Three different laser energy densities and four different doped mass fractions are adopted for the comparison to find out the energy intensity density and work substance doped mass fraction influence on the splash generation and evolvement. The results show that: the carbon doped in the liquid working substance could weaken the splash process with the smaller droplets and shorter duration time.
Laser propulsion as a new concept propulsion technology, it is paid more and more extensive concern. Laser ablation micro thruster is one of the focus with its high specific impulse, wide dynamic range of impulse, small minimum impulse bit, low power etc, laser ablation micro thruster has wide application prospects on high-precision task of satellite attitudeadjustment, orbit maintain and networking formation control. Due to low thermal conduction, low ablation threshold, polymer material was easily ablated to generate thrust. A computational model of laser ablated polymer was established to simulated the micro-thruster working in vacuum environment. The polymer don’t have fixed fusion point, so build the ablation criterion based on threshold energy, which has observed in many experiments. Put forward the polymer ablation criterion in the numerical model, the target ablation phenomenon happens when inner deposited energy achieve the threshold value. Established the energy distribution equation to describes the ablation process of temperature rise, phase change and the influence of chemical exothermic process. When ablation phenomenon happened the ablation products would ejected, and the target gained recoil impulse from ejection process. According to energy distribution equations we can get the ejection energy, and then get the recoil momentum of target based on momentum conservation law. The propulsion properties of laser ablated polymer was studied through the numerical analysis model. Revealed the relationship between the propulsion capability and laser parameters. Analyzed influence of different propellants to propulsion performance. The numerical analysis model can reflect the propulsion capability of different polymer propellant, revealed the law of propulsion parameters in laser ablation process.
Ablaiton of solid target with high power energy can induce laser vapor plasma, which would impart reverse impulse to the target. The physical processes include target heating, melting, vaporization and formation of plasma plume. In this paper, we presented a new numerical model, which described target heating, melting and evaporation. Meanwhile, the ejection of material formed a plasma plume above the surface and expanded into the ambient vacuum. The formed plasma absorbed the laser energy passing through it. The heating of the target was described with a heat conduction equation, which led to the temperature distribution inside the target, as a function of time. When the temperature rises further, vaporization would appear. The vapor velocity and temperature at the surface were used as input for the boundary conditions of plasma plume, which was described with Navier–Stokes equations, for conservation of total vapor mass density, momentum and energy. We considered two dominant absorption mechanisms in the process of plasma shielding, which were electron–ion and electron–neutral inverse Bremsstrahlung. Based on above assumptions, the left laser energy because of plasma shielding was calculated. Results for an aluminum target with Gaussian profile laser pulse with duration of nanosecond were obtained, including the plasma plume temperature, ionization degree, densities of neutral, ions and electrons and laser absorption energy. Results showed that the energy absorption by plasma plume played an important role in the coupling of laser energy and target.
Plume generation and expansion performance measurements have been performed with ns-shadowgraphy time resolved method on laser micro ablation. The optical display method of micro jet plume characteristics is discussed and the plume character is measured and analyzed to research the relationship between coupling mechanics and plume dynamics. The micro laser ablation properties of different commercial ploymers are compared to find out the ideal micro laser thruster fuel to achieve propulsion performance improvement. The plume generation and expansion character is analyzed by the shock wave and ablation product evolution. Shock wave and ablation product jet could be formed in the air condition, and the velocity is different. Normally, the shock wave is faster than the jet, but the inverse situation is still observed that could be taken as signal of the higher specific impulse. Nine common polymers were tested and compared, the results show that: polyvinyl chloride ( PVC ) material is the best choice of commonly used polymer material. A velocity of 820m/s of shock wave formed by PVC ablation could be obtained, which is highest in the chosen polymers, while the velocity is 844m/s for Al, and there are more ablation product could be found for PVC. The result indicates that ablation efficiency of PVC is the best, and PVC is the priority fuel material for the better propulsion performance, easy machining and storage.
A threat to spacecraft in long-term low earth orbits is the high probability of impacts with small particles of man-made space debris in 1-cm to 10-cm size range. One possible solution for 1-10 cm size debris is to de-orbit the particles with a ground or space based laser. A modified torsional impulse balance system has been developed as a diagnostic tool to study fundamental laser ablative process on different material such as aluminum, titanium, magnesium and carbon fiber composite that are frequently used in spacecraft. Of particular interest is the force due to process of laser ablation as well as the impulse coupling coefficient. It can be concluded from the experimental result that for the experimental materials, with the increasing laser intensity, the coupling coefficient increase firstly and then decrease and it reaches the maximum at some value when the laser intensity varies around 109 W/cm2. And the experimental data compares well with the calculation result according to Phipps' scaling law. As we extend the previous research, it will provide a reference for the study in cleaning man-made space debris by laser.