Alfio E. Vinci, ThrustMe
Dmytro Rafalskyi, ThrustMe
Francesco M. Bianchi, ThrustMe
Heeral Bhatt, ThrustMe
Thibault Dubois, ThrustMe
Elena Zorzoli Rossi, ThrustMe
Alfio E. Vinci, Plasma physicist, ThrustMe
Several thousands of Earth satellites launched to date feature an onboard propulsion system based on Hall thruster technology. These devices are key characters for in-orbit maneuvering thanks to their remarkable thrust-to-power ratio and thrust density compared to other electric propulsion alternatives. Sub-kilowatt Hall thrusters can usually operate in the excess of 50 mN/kW on xenon propellant. Though, due to its limited production offer, krypton and argon are the most widely used propellants today for low earth orbit commercial missions that employ Hall thrusters. However, besides the significant cost benefits such a system choice brings, propulsive performance is severely impacted due to the propellant nature. This especially affects the low-power class of thrusters which typically feature lower thrust-to-power ratios.
In this context, the use of iodine as propellant brings unmatched advantages to electrostatic plasma thrusters, from fundamental plasma phenomena aspects to overall system performance and characteristics. This does particularly apply to propulsion systems designed for platforms below a few hundred kilograms. Iodine enables the same performance achievable with xenon at 1-2% of direct cost and about one third of storage volume thanks to its solid nature at room temperature. Systems designed to operate with iodine do not necessitate implementing valves and pipes rated for hundreds of bars – as typically required for noble gas fluidics – since the highest pressure achievable on iodine reads a few tens of mbar. This technology has already been demonstrated in space by ThrustMe, which currently boasts several tens of gridded ion thruster systems in orbit. As a result, combining iodine propellant and Hall thruster technology is a necessary step to ensure high performance in-space maneuvering with extremely reduced costs.
The paper will report on the progress results in the development of a low-power Hall thruster propulsion system operating on iodine propellant named JPT150. The system is designed to operate below 200-250 W of total power consumption and following autonomous decision-making to provide the desired performance metric during operation, namely thrust or specific impulse, which represent the input commands. It is conceived in a fully integrated architecture that comprises of a thruster head unit, a cathode neutralizer unit, a power processing unit, a flow control unit, and propellant storage. The development has been addressing in parallel both fundamental physics aspects and engineering challenges related to iodine plasma devices.
Experimental tests have been performed at ThrustMe premises, using an iodine-compatible vacuum facility with a diameter of 1 m and length of 2 m. Background pressure in the order of 10ā6 mbar is ensured by a system of turbo-molecular-primary pumps in combination with a cryogenic pump leading to an effective iodine pumping speed in the order of a few thousand L sā1. During thruster operation, facility pressure reads about 10ā5 mbar is maintained throughout iodine operation modes. Thrust is measured using a folded pendulum balance, which consists in a plate suspended by a simple pendulum on one end and by an inverted pendulum on the other, resulting in a dynamic response in the order of 1 Hz or less. The balance is absolutely calibrated in the thrust range of interest, and it features a detectable resolution smaller than 0.05 mN. Plasma plume properties are also mapped using several diagnostics mounted on a 2-axis movable arm. Characteristics such as ion current density and ion energy are recorded for several operating points.
We present a proof-of-concept for the entire propulsion system which shows full operational versatility in the range 50-250 W of thruster head power, with a delivered thrust spanning from 4 mN to 12 mN. Thrust-to-power ratios of the thruster head exceeding 60 mN/kW have been recorded, with specific impulse reaching 2000 s. The cathode neutralizer can reliably operate on iodine propellant providing the required electron current to sustain and neutralize the ion beam. Power processing unit and flow control unit also show successful functioning, completing the objective of developing a fully integrated system. The test results collected from more than 100 hours of cumulated firing time demonstrate the competitiveness of JPT150 system for its power class.