Properties of Laser Ablation Products of Delrin with CO₂ Laser

Abstract

A requirement for efficient pulsed laser propulsion from ground to LEO is the achievement of a specific impulse of up to 800's at a jet efficiency of at least 50%. With classical polymers like Delrin or POM these numbers cannot be obtained. The suggestion to use polymers with included metal powder for a better deposition of the laser energy per unit volume and thus the achievement of a higher jet velocity together with a lower ablation rate has been experimentally investigated. Samples of different polymers and different metal contents (Al and Mg in the range of 3% to 60% by weight) have been formulated and irradiated with CO(sub 2) laser pulses of about 12 microseconds in duration and energies of 40 J to 280 J (fluences of 20 to 150 J/sq cm). The samples were mounted to a pendulum for measuring the excerted impulse and the mass loss per pulse was determined by weighing. These experiments have been carried out in ambient atmosphere as well as at various pressure levels down to vacuum of less than 1 mbar in a vessel. In general the expansion of the ablated material (vapor) was forced to be one-dimensional by applying a guiding tube. But three- dimensional expansions were investigated as well. Furthermore the efficacy of certain selected formulations as a propellant have been tested in a bell shaped light concentrating structure. The samples have been inspected under an electron microscope before and after the laser interaction. Because a suspicion existed from the beginning of the experiments that a certain amount of the incident laser energy might be absorbed in a decoupled plasma wave (breakdown) time resolved power measurements of the incident enemy of the reflected energy from the target and at a later stage of the energy transmitted through a hole in the target were made.