The growing interest in deep-space exploration has sparked the need for
powerful long-lived rocket systems to drive spacecraft through the cosmos.
Scientists at the U.S. Department of Energy's (DOE) Princeton Plasma Physics
Laboratory (PPPL) have now developed a tiny modified version of a
plasma-based propulsion system called a Hall thruster that both increases
the lifetime of the rocket and produces high power.
The miniaturized system powered by plasma—the state of matter composed of
free-floating electrons and atomic nuclei, or ions—measures little more than
an inch in diameter and eliminates the walls around the plasma propellent to
create innovative thruster configurations. Among these innovations are the
cylindrical Hall thruster, first proposed and studied at PPPL, and a fully
wall-less Hall thruster. Both configurations reduce channel erosion caused
by plasma-wall interactions that limit the thruster lifetime—a key problem
for conventional annular, or ring-shaped, Hall thrusters and especially for
miniaturized low-power 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 on the
Laboratory's Hall Thruster Experiment (HTX) since then. The PPPL devices
have also been studied in countries including Korea, Japan, China,
Singapore, and the European Union, with Korea and Singapore considering
plans to fly them.
While wall-less Hall thrusters can minimize channel erosion, they face the
problem of extensive widening, or divergence, of the plasma thrust plume,
which degrades the system's performance. To reduce this problem, PPPL has
installed a key innovation on its new wall-less system in the form of a
segmented electrode, a concentrically joined carrier of current. This
innovation not only reduces the divergence and helps to intensify the rocket
thrust, Raitses said, but also, suppresses the hiccups of small-size Hall
thruster plasmas that interrupt the smooth delivery of power.
The new findings cap a series of papers that Jacob Simmonds, a graduate
student in the Princeton University 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 COVID-19 pandemic
and that could be resumed in the future.
The research was published in Applied Physics Letters.
Reference:
J. Simmonds et al, Mitigation of breathing oscillations and focusing of the
plume in a segmented electrode wall-less Hall thruster, Applied Physics
Letters (2021).
DOI: 10.1063/5.0070307
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