Plasma-Powered Rocket Designed for Deep Space Exploration

Graduate student Jacob Simmonds, center, with advisors Masaaki Yamada, left, and Yevgeny Raitses with figure of wall-less Hall thruster behind them. Credit: Yamada and Raitses photos by Elle Starkman/Office of Communications; Simmonds photo by Tyler Boothe. Collage by Kiran Sudarsanan.

The new findings cap a series of papers that Jacob Simmonds, a graduate student in the Department of Mechanical and Aerospace Engineering, has published with Raitses, his doctoral co-adviser; PPPL physicist Masaaki Yamada serves as the other co-advisor. “In the last two years we have published three papers on new physics of plasma thrusters that led to the dynamic thruster described in this one,” said Raitses, who leads PPPL research on low-temperature plasma physics and the HTX. “It describes a novel effect that promises new developments in this field.”

Application of segmented electrodes to Hall thrusters is not new. Raitses and Fisch had previously used such electrodes to control the plasma flow in conventional annular Hall thrusters. But the effect that Simmonds measured and described in the recent paper in Applied Physics Letters is much stronger and has greater impact on the overall thruster operation and performance.

Focusing the plume

The new device helps overcome the problem for wall-less Hall thrusters that allows the plasma propellant to shoot from the rocket at wide angles, contributing little to the rocket’s thrust. “In short, wall-less Hall thrusters while promising have an unfocused plume because of the lack of channel walls,” Simmonds said. “So we needed to figure out a way to focus the plume to increase the thrust and efficiency and make it a better overall thruster for spacecraft.”

The segmented electrode diverts some electric current away from the thruster’s high-voltage standard electrode to shape the plasma and narrow and improve the focus of the plume. The electrode creates this effect by changing the directions of the forces within the plasma, particularly those on the ionized xenon plasma that the system accelerates to propel the rocket. Ionization turned the xenon gas the process used into free-standing electrons and atomic nuclei, or ions.

These developments increased the density of the thrust by shaping more of it in a reduced volume, a key goal for Hall thrusters. An added benefit of the segmented electrode has been the reduction of plasma instabilities called breathing mode oscillations, “where the amount of plasma increases and decreases periodically as the ionization rate changes with time” Simmonds said. Surprisingly, he added, the segmented electrode caused these oscillations to go away. “Segmented electrodes are very useful for Hall thrusters for these reasons,” he said.

The new high-thrust-density rocket can be especially beneficial for tiny cubic satellites, or CubeSats. Masaaki Yamada, Simmonds’ co-doctoral adviser who heads the Magnetic Reconnection Experiment (MRX) that studies the process behind solar flares, Northern lights and other space phenomena, proposed the use of a wall-less segmented electrode system to power a CubeSat. Simmonds and his team of undergraduate students working under the guidance of Prof. Daniel Marlow, the Evans Crawford 1911 Professor of Physics at Princeton, took up that proposal to develop a CubeSat and such a rocket — a project that was halted near completion by the pandemic and that could be resumed in the future.

Reference: “Mitigation of breathing oscillations and focusing of the plume in a segmented electrode wall-less Hall thruster” by J. Simmonds and Y. Raitses, 22 November 2021, Applied Physics Letters.
DOI: 10.1063/5.0070307

Support for this work comes from the DOE Office of Science.

PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy.

See author's posts