Thrust Characterization and Plasma Diagnostics for an Adamantane Thruster

The University of Southern California’s Advanced Spacecraft Propulsion and Energy (ASPEN) Lab is developing an adamantane (C10H16) solid fuel spacecraft plasma thruster. Adamantane offers advantages over legacy gaseous propellant due to the simplified storage requirements, including self-pressurizing capabilities and a high density, while offering comparable mass and ionization rates. In order to optimize fuel use, thrust, and specific impulse, a thorough investigation must be conducted. To characterize thruster operation, ASPEN has developed a suite of diagnostics to characterize thruster operation that includes a thrust stand and a double Langmuir probe, enabling thrust measurement and the analysis of plasma properties such as electron density and temperature. One method of characterization utilized is a null-balance torsional pendulum thrust stand. This stand was designed to improve on ASPEN’s previous hanging pendulum thrust measurements. The stand utilizes a 2-foot long arm balanced on two ⅜ inch pivot flexures. The thruster is mounted to one end of the arm, and when fired, it causes the arm to rotate around the flexures. The pivot flexures are designed to provide around 3 microns of deflection for the projected 29 micronewtons of thrust generated by the thruster. A campaign was conducted to test different thruster geometries in order to determine how changes in the thruster design correspond to thrust. A double Langmuir probe is used to measure the plume plasma properties. A previously constructed single Langmuir probe demonstrated the feasibility of adamantane as a solid fuel propellant source, with the plasma exhibiting a higher electron density and temperature than plasma generated from ambient air [1]. The current objective is to develop a more precise and advanced probe capable of providing more detailed measurements, enabling accurate comparisons across thruster iterations. A double Langmuir probe was chosen, as these probes often exhibit more consistent and reliable results compared to single Langmuir probes. The probe consists of two 0.51 mm diameter tungsten wires surrounded by alumina tubing spaced 1.0 ± 0.2 mm apart, protruding 5 mm. Both wire ends are placed within the plasma plume at variable distances from the thruster orifice inside a vacuum chamber. An oscilloscope measures the voltage signal that a waveform generator produces from a power supply. The data collected from the double Langmuir probe is sampled at a rate of 1 kHz, and then processed with oscilloscope data to create an I-V curve, which can be used to discern the plasma’s density, temperature, and potential. The double Langmuir probe is used to characterize the plasma from different thruster geometries in order to determine which thruster produces the most plasma with the greatest efficiency. Further experimentation will be conducted to refine the instruments used to characterize the thruster and optimize the thruster design and construction. [1] Coen, C., Zaretsky, A., “Characterization of an Adamantane Thruster by a Langmuir Probe,” AIAA, 2024-80550. 2024 Regional Student Conferences. January 2024. https://doi.org/10.2514/6.2024-80550