Dissociation and Ionization of Molecular Hall Thruster Propellants

Plasma propulsion devices such as hall thrusters (HTs) typically use Xe as the propellant due to its high mass and low ionization potential. However, due to the scarcity and cost of Xe, alternate fuels are being investigated which are also environmentally friendly and which are not potentially damaging to the spacecraft. Solutions containing, for example, TPA (tripropylamine, (CH₃CH₂CH₂)₃N) are attractive alternate fuels due to their ease of storage and low cost. However these complex molecules also result in complex plasma chemistry, producing a variety of ions that, when accelerated, produce thrust defined by the current and mass flow rate. For example, thermally decomposed TPA-like mixtures with air may produce H₂O, NO_x, CO_x, and CH_x. These decomposition products are further dissociated when exposed to the plasma environment. To provide insights into the variety of ions produced using alternate fuels for plasma propulsions systems, simulations were conducted with the Hybrid Plasma Equipment Model (HPEM) in an HT-like geometry. A feedstock mixture containing H₂O/N₂/CH_x/CO₂, representative of TPA-like decomposition product propellants were investigated. The resulting dissociation products, ion densities, fluxes, and velocities will be discussed.