The interaction of an electrothermal plasma with JA2 solid propellant

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Ryan, Michael David

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Electrothermal plasmas are being studied as an ignition mechanism for solid propellants in large caliber guns. Benefits of electrothermal plasma ignition over conventional primer charge ignition include a reduction of ignition delay and delay jitter (bootstrapping) and compensation for the variable burn rate of propellants at different initial temperatures. When JA2 solid propellant is exposed to plasma radiation alone, significant decomposition results. This radiative interaction is a possible mechanism that causes the bootstrapping and temperature compensation. In addition, the effects of plasma radiation exposure have the potential to increase the propellant burn rate. To characterize this radiation interaction, PLIF imaging of NO, a JA2 decomposition product, was conducted at the propellant surface. Also, simultaneous high speed video of the propellant surface and scattering of ejected particles has been performed. During the radiation interaction scattering particles and NO appeared between 100 and 150 µs after the beginning of the discharge and propagated away from the propellant surface. This ejected material appeared in identifiable structures that are irregular in shape and distribution. This suggests that the material was ejected at semi-discrete locations on the surface rather than diffused uniformly from the surface. During the plasma firing the propellant surface changed markedly by forming irregularly shaped decomposition structures that grew in size over the course of the discharge. No correlation was observed between the structure of the ejected material and the decomposition structures formed on the propellant surface during the discharge. After the plasma discharge, the propellant continued to react, with bubbles forming on the surface up to 9 ms after the discharge finished. These bubbles are probably the largest decomposition structures in images taken of the propellant surface minutes after radiation exposure. The delayed reaction of the propellant produced the majority of ejected particles after the plasma firing. This raises concerns that the potential burning rate increase by the effects of the radiation might not be completely realized in a plasma ignition event. Regions of the propellant exposed to plasma radiation could be consumed by the burning surface before all of the observed effects of the radiative interaction took place.