Abstract
A number of polymers have been proposed for use as propellants in space launch and thruster applications based on laser ablation, although few prior studies have either evaluated their performance at background pressures representative of the upper atmosphere or investigated interactions with ambient gases other than air. Here, we use spatially and temporally resolved optical emission spectroscopy to compare three polymers, poly(ethylene), poly(oxymethylene), and glycidyl azide polymer, ablated using a 532 nm, nanosecond pulsed laser under Ar and O2 at pressures below 1 Torr. Emission lines from neutrally and positively charged atoms are observed in each case, along with the recombination radiation at the interaction front between the plasma plume and the background gas. C2 radicals arise either as a direct fragmentation product or by a three-body recombination of C atoms, depending on the structure of the polymer backbone, and exhibit a rotational temperature of ≈5000 K. The Sedov–Taylor point blast model is used to infer the energy release relative to the incident laser energy, which for all polymers is greater in the presence of O2, as to be expected based on their negative oxygen balance. Under Ar, plume confinement is seen to enhance the self-reactivity of the ejecta from poly(oxymethylene) and glycidyl azide polymer, with maximum exothermicity close to 0.5 Torr. However, little advantage of the latter, widely considered one of the most promising energetic polymers, is apparent under the present conditions over the former, a common engineering plastic.
Highlights
Pulsed laser ablation (PLA) finds widespread use in diverse applications, including pulsed laser deposition,1 laserinduced breakdown spectroscopy,2 surface modification,3 and laser propulsion.4 Relative to PLA in vacuum, additional physical processes occur in the presence of background gas, including shock wave creation and propagation,5 plasma confinement,6 and charge exchange during plasma formation and expansion
A number of polymers have been proposed for use as propellants in space launch and thruster applications based on laser ablation, few prior studies have either evaluated their performance at background pressures representative of the upper atmosphere or investigated interactions with ambient gases other than air
And temporally resolved optical emission spectroscopy has been used to study plasmas formed by 532 nm pulsed laser ablation of poly(ethylene), poly(oxymethylene), and glycidyl azide polymer targets in low background pressures (10À2 p 1 Torr) of both argon and oxygen
Summary
Pulsed laser ablation (PLA) finds widespread use in diverse applications, including pulsed laser deposition, laserinduced breakdown spectroscopy, surface modification, and laser propulsion. Relative to PLA in vacuum, additional physical processes occur in the presence of background gas, including shock wave creation and propagation, plasma confinement, and charge exchange during plasma formation and expansion. Several previous investigations at (sea level) atmospheric pressure and in high vacuum have been reported, but propulsion-focused laser ablation studies of polymers in low-pressure ambient gas, as is relevant to upper-atmosphere operation, are much rarer.. Several previous investigations at (sea level) atmospheric pressure and in high vacuum have been reported, but propulsion-focused laser ablation studies of polymers in low-pressure ambient gas, as is relevant to upper-atmosphere operation, are much rarer.25 Such experiments pertain to outer-space propulsion, since the physics of both the ablation event and the resulting shock, normally used to infer momentum transfer, are qualitatively different for a confined plume. We explore the PLA of three different polymers (detailed in Sec. II) using spatially and temporally resolved optical emission spectroscopy (OES) and time-gated emission imaging, with particular attention to shock wave formation and propagation
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