Recombination of electrothermal plasma and decomposition of plasma-exposed propellants

Information about species involved in initiating ignition of propellents by electrothermal plasma is critical to determining any chemical effect on the enhancement of electro-thermal-chemical gun performance. An experimental study using a triple-quadrupole molecular beam mass spectrometer was conducted to measure species from plasma recombination and from plasma-induced decomposition of propellants (JA2 and M43). The mismatch between the plasma pulse and the sampling time requirement by the mass spectrometer makes direct probing plasma species difficult, requiring the use of a plasma-holding chamber to prolong the event for sampling. The secondary jet evolving from the holding chamber should represent the nature of the plasma jet as it propagates through the propellant bed in actual electro-thermal-chemical applications. Recombination species that can be consistently detected are C₂H₄, C₂H ₃, C₂H₂, CH₄, CH₃, H ₂, and H for a plasma produced within a polyethylene capillary. NO, CO and CO₂ are also detectable if sufficient air is present in the chamber or using a Lexan capillary. Species measurements of JA2 and M43 indicate the formation of NO and H₂CO during impingement by the jet, which are typical "dark-zone" species observed for these propellants.