Laser Propulsion with a Magnetic Nozzle YOSHIHIRO ARAKAWA & KIMITO YOSHIKAWA Summary In laser propulsion devices, propellant is irradiated by a high-energy density laser beam, producing an ionized high temperature, high density plasma. Laser energy is absorbed in the plasma through a process of inverse Bremsstrahlung radiation, producing an explosive expansion which can be used for propulsion. Laser propulsion can be divided into two categories: (i) continuous wave (CW) devices; and (ii) repetitively pulsed (RP) devices. This paper deals with RP devices. The question of which propellant and what type of propellant feed system to use has not yet been answered. In these experiments metal pellets were utilized as propellant and a new propellant feed system was developed for RP laser propulsion experiments. Also, in order to improve the performance of laser propulsion devices, the plasma produced must have a higher temperature and a higher density than present experimental devices allow. With this in mind, methods for protecting the combustion chamber and expansion nozzle surfaces, and obtaining a higher efficiency in the conversion of thermal to propulsive energy were investigated. It is thought that a magnetic nozzle can help to solve these problems. Using the experimental system described, fundamental experiments were performed investigating plasma expansion processes inside a magnetic nozzle. Introduction At present there are several types of advanced space propulsion systems. Electric propulsion systems [1], such as ion engines and Magnetoplasmadynamic (MPD) thrusters, produce high specific impulse and high thrust efficiency. However, they have a common drawback, as their power source must be carried onboard the space vehicle. This results in poor thrust-to-weight ratios and, because of limited power, leads to low thrust levels. By contrast, laser propulsion systems eliminate the need for a heavy onboard power source. The idea of laser propulsion, which is to use a high-power laser as a remote power source, was first proposed by Kantrowitz [2]. Instead of using energy from an onboard power source, energy is supplied by a remote laser station and is used to heat propellant to form a dense plasma, thus providing thrust [3, 4, 5, 6, 7]. Laser propulsion can be divided into two categories: (i) continuous wave (CW) devices; and (ii) repetitively pulsed (RP) devices [8]. This paper deals with RP devices. Experiments so far seem to verify that relatively high specific impulse can be achieved with good efficiency. A major uncertainty, however, is what will happen when Yoshihiro Arakawa is Associate Professor, Department of Aeronautics, University of Tokyo, Japan; and Kimito Yoshikawa is an Aerospace Engineer with Mitsubishi Heavy Industries, Tokyo, Japan. A version of this paper was presented at the Sixth ISAS Space Energy Symposium, 12-13 March 1987.
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