Plume Expansion and Ionization in a Micro Laser Plasma Thruster (Postprint)
AIR FORCE RESEARCH LAB EDWARDS AFB CA PROPULSION DIRECTORATE
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Ion density measurements have been performed on the plasma generated by an ablative diode laser thruster using a negatively biased flatplate probe. The biased probe data was coupled with measurement of the ablation crater through use of a scanning electron microscope SEM. The SEM was used to analyze the post-pulse ablation spots to determine the volume of fuel ablated. Themicro-laser plasma thruster uLPT discussed here ablates a target material through the back surface by focusing the laser through a transparent substrate in a process typically referred to as Transmissive mode T-mode ablation. The target materials investigated were polyvinyl chloride PVC and glycidyl azide polymer GAP, while the substrates used were cellulose acetate and KaptonR. Peak ion densities for a GAP targetacetate substrate were found to be 1.6x107cm-3, while for GAPkapton and PVCacetate the peak ion densities were 4.5x107cm-3 and7.9x109cm-3 respectively. Although these corresponded to low ionization fractions calculated from the observed mass loss, the results indicate there are ways to improve the ionization fraction and in turn increase the specific impulse.
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