A CO<sub>2</sub> laser of 300 ns pulse length, operating at 10.6 μm wavelength and from 1-4 J pulse energy was used to ablate
carbon-doped Delrin<sup>®</sup> (polyoxymethylene, or POM) targets in a set of conical aluminum minithrusters at standard
temperature and pressure. Nozzles with lengths ranging from 0.5 - 5 cm were used (corresponding to expansion ratios of
about 4 to 16), as well as a bare sample with no nozzle. A piezoelectric force sensor was used to record the imparted
impulse for fluences in the range of 1-100 J/cm<sup>2</sup> for each thruster. The effect of increasing the expansion ratio on the
impulse generation for single pulse laser propulsion experiments will be described. The study will also clarify the effect
of confining air from an ambient atmosphere in augmenting impulse generation.
The fluence dependence of the laser ablation of selected polymers was studied within the range from 1-150 J/cm<sup>2</sup>. A
TEA CO<sub>2</sub> laser operating at 10.6 μm with 300 ns main pulse length and up to 20 J pulse energy was used to ablate
prepared polymer samples with single pulses of laser energy. Measurements of parameters such as the ablated mass per
spot area (Δm<sub>a</sub>), momentum coupling coefficient (C<sub>m</sub>), specific impulse (I<sub>sp</sub>), and internal efficiency (η<sub>i</sub>) will be plotted
as functions of fluence. Critical threshold effects observed throughout the experiments will be described in detail.
One of the many challenges faced by laser propulsion is the long-term performance of the propellant. The chemical
changes that can take place at the propellant surface during ablation can greatly modify the in-flight performance
characteristics. For stable regimes for propulsion, such chemical action should be minimized. A TEA (Transverse
Electrical discharge in gas at Atmospheric pressure) CO<sub>2</sub> laser of 10.6 μm wavelength, 300 ns pulse length, and up to 20
J pulse energy was used to ablate several types of polymer targets with a range of observable chemical changes at the
surface following ablation. After 10 subsequent shots, the target samples were measured using Attenuated Total
Reflectance Fourier Transform Infrared (ATR FTIR) spectroscopy then compared to unablated samples of the same
polymer. An analysis of the results was made with an emphasis on laser propulsion applications, with a comparison of
the propulsion performance of the targets, specifically regarding the ablated mass per spot area (Δm<sub>a</sub>). Chemical
reaction pathways for combustion and vaporization are discussed on the basis of the differences observed in the FTIR
spectra, and the consequences for using such materials as laser propulsion propellants are explored.
Time-resolved force measurements and Intensified Charge-Coupled Device (ICCD) imaging techniques were applied to the study of force generation in the laser ablation of water and ice. A transversely excited atmospheric (TEA) CO<sub>2</sub> laser operated at 10.6 μm, 300 ns pulse width, and up to 20 J pulse energy was used to ablate water and ice held in various containers. Net imparted impulse and coupling coefficient were derived from force sensor data and relevant results will be presented for ice and water. ICCD imaging was used in conjunction with time-resolved force measurements in order to determine the dominating physical mechanism under which the thrust is produced. The effect of shock wave generation and propagation, as well as its contribution to the overall impulse imparted to the targets, was examined from the comparison of the timelines for the pertinent phenomena. The process of mass removal was investigated for each case, and specific impulse and efficiency were calculated from the data. Differences in the force-time curves for ice and water will be presented and discussed. Ballistic experiments were conducted in order to corroborate the force measurements.
A combination of techniques including launch ballistics, force sensing, and time-resolved ICCD imaging was applied to the study of the mechanisms of liquid ablation in the irradiance regimes from 10<sup>6</sup>-10<sup>8</sup> W/cm<sup>2</sup>. A TEA CO<sub>2</sub> laser (λ = 10.6 μm), 300 ns pulse width and 9 J pulse energy, was used for ablation of liquids contained in various quartz glass containers in order to examine dependencies on surface tension, absorption depth, <i>etc</i>. Dominant mechanisms of force generation were analyzed in order to determine their characteristics, and the evolution of the liquid surface was studied in depth. Net imparted impulse and coupling coefficient were derived from the force sensor data and ballistics experiments, and relevant results will be presented for various container designs and liquids used. The key differences between surface and volume absorbing liquids was observed. Various mechanisms including plasma formation, vaporization, bulk liquid flow, <i>etc</i>. will be critically examined and their relevance to force generation and propulsion will be determined.