PROPULSION PERFORMANCE OF REPEATED, HIGHLY FOCUSED LASER ABLATION

Space debris is a growing concern for satellites. While most of the debris are small, though still extremely dangerous, nearly all of the mass currently on orbit is contained in relatively few large objects, waiting to be released by collisions with both large and small debris. To prevent an exponential growth in the number of debris, a tug spacecraft could be sent to deorbit all of the large objects. Unfortunately, the fuel required for any single tug to deorbit multiple debris grows exponentially with the number of objects deorbited. Laser ablation, which uses a laser to create a jet of high speed plasma from any solid material, provides an elegant solution – the necessary propellant is drawn from the mass of the debris object, rather than carried to it by the tug. To successfully deorbit large debris will require maximizing use of the debris mass. Other laser ablation propulsion schemes can address propellant efficiency issues by selecting both the ablated materials and their configuration, for example a strip of material the exact width of the laser and one laser pulse thick. By moving the ablation target, they are able to avoid questions about how damage done to the surface by one ablation event will affect the next ablation event. A laser ablation tug cannot afford to ablate its targets only once. This work used time-of-flight mass spectrometry to investigate how laser ablation propulsion performance changes with repeated ablation of the same location on an aluminum plate. The variation of performance metrics was considered as a function of the number of laser pulses applied to a given location, whether they were applied with a short or long delay between pulses, and whether the laser was slightly mis-aligned. It was found that, for up to 25 laser pulses, repeated ablation of the same location significantly improves the thrust-to-power ratio but makes only a small improvement to mass efficiency. After 25 pulses, a crater formed by repeated ablation deflects the plume towards the laser. The time delay between pulses and a slight mis-alignment of the laser had no significant effect