Comparative Study of Plastics as Propellants for Laser Ablation Plasma Thrusters

A NEW analysis and comparison of laser ablation propulsion (LAP) parameters like specific impulse and coupling coef-ficient obtained while ablating different prospective plastic materials is presented in view of selecting the most suitable of them. The tests involved a medium-low energy pulsed laser along with a free-moving ballistic pendulum with a target pasted on it. In a novel application of 2-D position-sensitive detectors (PSDs) in LAP, the displacement measurements were taken by the noncontact optical triangulation with a laser. Their use gave the advantages of detecting any unwanted lateral movement of the pendulum. The study should have high utilization for the future LAP applications. Laser ablation propulsion technique, introduced by Arthur Kantrowitz in 1972, depends upon the characteristics of laser source as well as those of the target material, as the momentum of the target is produced in reaction to the generated momentum of the ejecting particles [1-3]. Specific impulse values up to several thousand seconds are possible by properly choosing the propellant, that is, target material and the laser pulse characteristics. The momentum of the object may well also depend on the coupling coefficient of material-laser interaction. Earlier works include ablation with some of the materials included in this study, but here a list of three commonly available plastics is analyzed and compared [3-6]. Additionally, most researchers have employed mechanical force sensing for thrust measurement or charge-coupled device imager for displacement measurement [5], whereas others have used electromagnetic coils for sensing torque on the suspended target [3]. On the other hand, here a noncontact optical triangulation method is being employed using a precision PSD with 1 mum resolution and a stable He-Ne laser. This yields direct measurement and better accuracy along with lateral-shift error reduction, and thus useful output curves have been obtained.