Experimental Investigation of Helicon Thruster Mode Transitions Using Molecular Propellants

Effective use of molecular propellants in electric propulsion (EP) devices has several benefits over the traditional monatomic gasses including, liquid fuel storage, in situ resource utilization for exploration missions, and multi-mode systems. These propellant advantages come at the cost of complications in both plasma-material interactions and plasma chemistry effects. The former complication can be mitigated by using an electrodeless thruster design however the latter is a relatively unexplored subject in the context of EP plasmas. For multi-mode applications the use of compounds such as Hydroxylammonium Nitrate (HAN), a principal component of the AF-M315E monopropellant, in EP devices is of interest. We present here performance metrics of our Helicon Plasma Thruster (HPT) operating on several products of HAN breakdown including N 2 , N 2 O, and CO 2 . We have also studied the performance of this thruster on Argon in order to compare with a more widely used monatomic gas. Measurements are made using a pendulum momentum flux sensor and an RF compensated Langmuir probe 1 which can sweep across the plume. Furthermore, we have studied how molecular propellants effect mode transitions in our HPT, an important aspect of RF thrusters. Chemical reactions such as dissociation change the chemical composition of the plasma along its axis 2 which effects the performance of the thruster.