| A novel Laser-sustained pulsed plasma thruster(LS-PPT) was proposed and designed in this thesis, considering the disadvantages in pulsed plasma thruster and laser ablated plasma thruser, etc. As a new method to improve the working performance of original pulsed plasma thruster, the LS-PPT was investigated and the following works were finished.The design goals and scheme of LS-PPT was illustrated to improve the thrust performance which is performed by the parameters including impulse bit, specific impulse, thrust efficiency and propellant usaged efficiency, etc. The thurst performance of LS-PPT was firstly evaluated theoretically including of exploring the main factors effecting thrust performance of LS-PPT. The detailed design was performed including the choices of storage energy capacitors, laser source, propellants and the setting of discharge-accelaration channel and power supply. Eventually, the LS-PPT for experiments was presented in this thesis.For the measurement of thrust performance of LS-PPT, a high precision microthrust/micro-impulse measuring system was developed, and the corresponding grounded experimental system was constructed. A novel electromagnetic calibration method based on Ampere force was proposed for high precision calibration of the thrust stand. According to calibration experiments and error analysis, the electromanetic calibration method was proven to be a high precision one. A torsional pendulum based on PSD optical sensor was developed, while the detailed principles and geometrical configuration was introduced. In addition, several solutions for torsional pendulum were proposed to overcome the possible design problems. A novel high precision microimpulse measuring method based on sympathetic resonance was proposed to measure the extreme micro-impulse. It was demonstrated in the corresponding tests that the sympathetic resonance method showed a higer precision than an original single pulse measurement method, especially in meansuring an extreme micro-impulse.Thrust Performance of LS-PPT was tested and some influences of geometrical configuration, capacitors and propellants were investigated. Not only the reasibliblity but also the superiority in working performance of LS-PPT was proven. When the distance between electrodes was adapted to be 30 mm and the distance between ceramic nozzle and cathode electrode was 15 mm while the capacitors were charged energy of about 25 J, the LS-PPT using metal aluminum as propellant was tested. Some results such as high thrust performance with an impulse bit of about 600μN·s, a specific impulse 7838 s, and thrust efficiency about 89.8% were achieved. According to comparisons with the laser ignited APPT, the LS-PPT without a ceramic insulator, and the LS-PPT with a ceramic insulator, it was shown that propellants, geometrical configuration and electrical parameters were possible factors which influence the thrust performance of LS-PPT. These results include as following:(a) In contrast with a PTFE propellant, higher thrust performance was achieved for the LS-PPT using metal aluminum as propellant.(b)It was beneficial to set a ceramic insulator onto the LS-PPT for improving the stability and thrust performance.(c) Within a range of stored energy 0~25J, highest thrust performance of LS-PPT will be achieved with an optimal value between 20~40mm for the geometrical parameter d1 and an optimal value between 10~20mm for the geometrical parameter d2. It was found that the ablated mass of propellants was influenced significantly by the accuracy of laser focusing and setting of laser parameters. With the same laser parameters, the ablated mass per shot of PTFE was approximately twice or triple of metal aluminum. In addition, the ablated mass of ceramic nozzle and ceramic insulator was considered to be negligible.The addition of magnetic field inside the LS-PPT was proven to be effective in improving thrust performance. It was demonstrated that with an addition of magnetic field the impulse bits were improved to be about twice of the original values, which was in accordance with the conclusions of Zaidi et al. Besides, the propellants polymer PTFE and metal aluminum were compared with each other. It was shown that higher impulse bits can be obtained for the LS-PPT using PTFE. However, higher specific impulse and thrust efficiency were presented for LS-PPT using metal aluminum as propellant.The characteristics of electromagnetic acceleration of plasma inside the dischargeacceleration channel of LS-PPT were diagnosed using a spectrum method. The following facts were found including :(a) the electrical discharge and ionization became more intensive and more high valence ions can be produced with higher charged voltages of capacitors,(b) a velocity of about 3km/s was achieved for the laser produced plasma when expanding to a depth of 5mm of ceramic nozzle,(c)under charged voltages of 1500 V and 2000 V, the velocity of plasma was accelerated to be 16.7km/s and 66.7 km/s respectively, with the corresponding specific impulses of 1670 s and 6670 s, and the corresponding thrust efficiency of 8.9~15.4% and 72~91%. It was worthy to point out that the above results by spectrum diagnosis were in accordance with the previous results by micro-impulse measurement,(d) under a low charged voltage of capacitors, the plasma was not accelerated effectively with too long electrodes. Hence, the adaption between geometrical configurations and electrical parameters should be considered while optimizing a LS-PPT.Based on the above experimental investigations, several basic physical processes in the working of LS-PPT were modeled and simulated, including the production of plasma under the irridiation of an intensive laser and the expansion of plasma inside a ceramic nozzle.Firstly, the ablation of polymer PTFE under irridiation of intensive laser was simulated numerically. A dynamic dual layers of propellant consisted of amorphous and gel phases during phase change were considered, and the ablation processes were divided into two stages. Besides, non-fourier effect of heat conduction, reflectivity and volumeric absorption of propellant in laser ablation were also considered. It was demonstrated that laser ablation of phase change of PTFE were influenced significantly by non-fourier effect, parameters of laser intensity and absorption coefficient, etc.Secondly, the intensive ns-laser ablation of metal aluminum was modeled and simulated. The non-fourier heat conduction and phase change under an intensive nslaser were investigated while considering the thermal vaporization and phaser explosion mechanisms. For simplicity, an enthalpy method was used to model the non-fourier heat conduction. The laser intensity was tested first and used in simulations. The laser ablation of propellant was simulated by coupling with the dynamic absorption of aluminum plasma plume. The shielding effect of aluminum plasma, the laser parameters, and the non-fourier effect were all investigated. The numerical results demonstrated that the enthalpy method was simply and feasible when solving the problem of complicated interfaces between solid-liquid or liquid-gas phases. The numerical results for laser fluence threshold in laser ablation were in good accordance with the experimental results. In addition, the effects of laser fluence, laser wavelength and background pressure were also inverstigated.Finally, the production and expansion of laser ablated aluminum plasma inside a ceramic nozzle were modelled and simulated when the thermochemical non-equilibrium effect was considered. A two-temperature model was applied to model the nonequilibrium between translational and electronical energy modes. The numerical results of thrust and impulse produced by the laser ablated plasma were demonstrated to be in well accordance with the experimental results. |
PDF Full Text Request