Thousands of satellites take pictures, gather information and relay internet traffic from high above the Earth. Now, the challenge is making satellites that operate closer to home, in what is called a very low earth orbit (VLEO), where there is ample space for additional satellites, and the pictures taken would be clearer.
Working at an altitude with air would mean more force would be needed to propel the satellite forward, but many scientists believe there is also an advantage: the air could be used as the propellant. They say charged particles of air-breathing plasma – the fourth state of matter – could be used to propel the thrusters, potentially lightening the load and increasing the satellite’s lifespan. A satellite’s thrusters are essential to their ongoing operation, as they push the satellite around to keep it in the Earth’s orbit.
Researchers at the George Washington University (GWU) and the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) have teamed up to develop a proof of concept for such a propulsion system. Yevgeny Raitses, a managing principal research physicist at PPPL, is leading the Lab’s efforts on the project. PPPL’s expertise is needed to ensure the charged particles of plasma are neutralized as the plasma beam leaves the thruster. This is just one of many ways that PPPL’s plasma expertise supports critical U.S. infrastructure. The Defense Advanced Research Projects Agency (DARPA) has already provided $400,000 of an anticipated $1 million grant for the project.
“There is air available at VLEOs. So, instead of launching rockets with these propellants, such as xenon, krypton or argon, we can use what is naturally available: air,” Raitses said. “This should allow us to reduce the mass of satellites or allow them to dedicate the difference in mass to other aspects of the device. It might also extend the lifetime of the device.”
Air-breathing plasma propulsion
Current satellites have a lifespan limited by their power sources, propulsion systems and the propellant used to generate the plasma. Once the thrusters run out of the propellant, the satellite can no longer stay in orbit and needs to be replaced. The proposed thruster systems avoid this issue because the air that surrounds the satellite is used to generate the plasma. These air-breathing plasma thrusters would also eliminate the cost of the propellant.
PPPL has researched plasma thruster physics for decades, contributing multiple physics findings and engineering innovations. Several will be used to make the thruster prototypes. This includes the Lab’s ongoing work on plasma diagnostics, which will be used to study the thruster plasma. It is critical that positive and negative particles released from the thruster are leaving at the same rate so there is no net electric current in the plasma plume.
“It is important in order to avoid charging the satellites,” Raitses said, as that could cause charged particles released from the thruster to be attracted back to the satellite and cause a recoil. An electric current in the emissions could also cause electrical issues for the satellite.
“We will diagnose velocities of positive ions generated and accelerated from the thruster. From their trajectories and comparison with models by our partners at the George Washington University, we should be able to evaluate the neutralization process.”
Raitses will also use special probes developed at PPPL to measure key features of the plasma, such as its density and the temperature of its negatively charged particles.
The idea that the thruster itself can neutralize the particles is central to the DARPA project but raises additional questions that PPPL will help answer.
“The PPPL group led by Yevgeny has extensive experience in all these aspects,” says Michael Keidar, the A. James Clark Professor of Engineering at GWU. Keidar, a frequent PPPL collaborator via the Princeton Collaborative Low Temperature Plasma Research Facility (PCRF), is leading the GWU arm of the DARPA project. The GWU team includes doctoral student Anmol Taploo, who was a first author of the original work that led to this project. “The PCRF is well equipped with multiple relevant diagnostics tools, including laser-induced fluorescence that can help to evaluate dynamics of ion-charge neutralization,” Keidar said.
