Deep space exploration

Scientists demonstrate new rocket for deep space exploration

Growing interest in deep space exploration has spurred the need for powerful, long-lived rocket systems to propel spacecraft through the cosmos. Scientists at the US Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have now developed a small modified version of a plasma-based propulsion system called a Hall Thruster that both increases the lifespan of the rocket and produces high power.

The miniaturized system powered by plasma – the state of matter composed of floating electrons and atomic nuclei, or ions – measures just over an inch in diameter and eliminates walls around the plasma thruster to create thruster configurations innovative. Among these innovations are the cylindrical Hall thruster, first proposed and studied at PPPL, and a completely wallless Hall thruster. Both configurations reduce channel erosion caused by plasma-wall interactions that limit thruster life – a key issue for conventional annular or ring-shaped Hall thrusters and especially for low-power miniaturized thrusters for applications on small satellites.

Widely studied

Cylindrical Hall thrusters were invented by PPPL physicists Yevgeny Raitses and Nat Fisch in 1999 and have been studied with students in the Laboratory’s Hall Thruster Experiment (HTX) ever since. PPPL devices have also been studied in countries such as Korea, Japan, China, Singapore, and the European Union, with Korea and Singapore considering flying them.

While wallless Hall thrusters can minimize channel erosion, they face the problem of severe plasma thrust plume broadening or divergence, which degrades system performance. To reduce this problem, PPPL installed a key innovation on its new wallless system in the form of a segmented electrode, a concentrically joined current carrier. This innovation not only reduces divergence and helps intensify rocket thrust, Raitses said, but also removes hiccups from small Hall thruster plasmas that interrupt smooth power delivery.

The new findings cap a series of papers Jacob Simmonds, a graduate student in Princeton University’s Department of Mechanical and Aerospace Engineering, published with Raitses, his PhD co-supervisor; PPPL physicist Masaaki Yamada is the other co-advisor. “Over the past two years, we’ve published three papers on new plasma thruster physics that led to the dynamic thruster described in this one,” said Raitses, who leads PPPL low-frequency plasma physics research. temperature and the HTX. “It describes a new effect that promises new developments in this area.”

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

Plume concentration

The new device helps overcome the problem of wallless Hall thrusters that allow the plasma thruster to fire from the rocket at wide angles, contributing little to the rocket’s thrust. “In short, wallless Hall thrusters, while promising, have a fuzzy plume due to the lack of channel walls,” Simmonds said. “So we needed to find a way to focus the plume to increase thrust and efficiency and make it a better overall propellant for spacecraft.”

The segmented electrode diverts some of the electrical current from the thruster’s high voltage standard electrode to shape the plasma and shrink and improve plume focus. The electrode creates this effect by changing the directions of the forces inside the plasma, particularly those on the ionized xenon plasma that the system accelerates to propel the rocket. Ionization transformed the xenon gas used in the process into self-contained electrons and atomic nuclei, or ions.

These developments increased thrust density by shaping more of it into a reduced volume, a key goal for Hall thrusters. An added benefit of the segmented electrode was the reduction of plasma instabilities called breath-mode oscillations, “where the amount of plasma increases and decreases periodically as the rate of ionization changes over time,” Simmonds said. Surprisingly, he added, the segmented electrode made these oscillations disappear. “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, co-doctoral adviser to Simmonds who directs the Magnetic Reconnection Experiment (MRX) which studies the process behind solar flares, the aurora borealis and other space phenomena, proposed the use of a system of segmented electrodes without wall to power a CubeSat. Simmonds and his team of undergraduates working under Professor Daniel Marlow, Evans Crawford 1911 Professor of Physics at Princeton, took up this proposal to develop a CubeSat and such a rocket – a project that was halted nearly complete by the COVID-19 pandemic and which may resume in the future.

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