Ablation Impulse Characteristics by Laser Pulse Irradiation

Abstract

For the future space propulsion system, the laser propulsion is expected to be a useful method. The most prominent advantage of the laser propulsion is that the system can generate the impulse without carrying the power supply onboard. This propulsion system has the possibility to improve the payload ratio compared to the conventional chemical propulsion system. Therefore, many researches have been conducted, since Kantrowitz’s proposal of the laser propulsion. Recent studies have shown that an ablative laser propulsion, in which laser ablation of solid propellant is used in a propulsive system, is an optimum method to obtain the impulse more efficiency in space. From 1960s’, the impulse characteristics generated by the single laser have been investigated by many researchers, for the variety of combinations of the laser types and target materials. Recently, the repetitively irradiated laser pulses effect on polymers have been investigated. In 2004, Phispps reported the novel type laser ablation thruster. His thruster could generate thrust by irradiating the laser pulses onto the virgin spots which were slid successively as like tape reel. However, until recently, the impulse characteristics of the repetitively irradiating laser onto the fixed spot have not been investigated. Recntly, Suzuki et.al., investigated the time-integrated impulse characteristics by using the ballistic pendulum system. He investigated the impulse characteristics for the combination of CO2 laser and POM target by the repetitive laser pulse irradiation onto the fixed spot. Figure 1 shows his representative result. For the initial phase(1 ≤ N ≤ 10), the morphology and contamination of the virgin target surface degraded the impulse performance. After initial phase(10 ≤ N ≤ 100), gradual increase of the momentum coupling coefficient with increasing the laser pulse repetition number was observed. Here, N represents the total number of laser pulses. He suggested that this impulse increase was related to the formation of the crater. In a word, the impulse performance can be changed with the variation of the morphology of the surface target. From Suzuki’s result, it is expected that the impulse characteristics of other combinations of laser wave length and the target materials also depends on the total number of laser pulses. However, in Ref., only one combination between the laser and the target material, that is, CO2 laser and POM target was investigated. Therefore, the investigation of the impulse characteristics by the repetitive laser irradiation for other materials is inevitable. Especially, the repetitive laser pulse effect on the aluminum impulse characteristics is expected to offer useful information for not only the laser propulsion performance improvement but also the debris de-orbiting application. The purpose of this study is to investigate the time averaged impulse characteristics of aluminum by the repetitive laser irradiation on the fixed point. This paper introduces the primitive result, as a first step. ∗Assistant Professor, e-mail; matsuda@nuae.nagoya-u.ac.jp, Department of Aerospace Engineering, AIAA Member †Associate Professor, Department of Aerospace Engineering, AIAA Member. ‡Professor, Department of Aerospace Engineering, Associate Fellow.