New Rocket Thruster Concept Exploits the Mechanism Behind Solar Flares

A new type of rocket thruster that could take humankind to Mars and beyond has been proposed by a physicist at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL). The device would apply magnetic fields to cause particles of plasma(link is external), electrically charged gas also known as the fourth state of matter, to shoot out the back of a rocket and, because of the conservation of momentum, propel the craft forward. Current space-proven plasma thrusters use electric fields to propel the particles. The new concept would accelerate the particles using magnetic reconnection, a process found throughout the universe, including the surface of the sun, in which magnetic field lines converge, suddenly separate, and then join together again, producing lots of energy. Reconnection also occurs inside doughnut-shaped fusion(link is external) devices known as tokamaks(link is external). “I’ve been cooking this concept for a while,” said PPPL Principal Research Physicist Fatima Ebrahimi, the concept’s inventor and author of a paper(link is external) detailing the idea in the Journal of Plasma Physics. “I had the idea in 2017 while sitting on a deck and thinking about the similarities between a car’s exhaust and the high-velocity exhaust particles created by PPPL’s National Spherical Torus Experiment (NSTX),” the forerunner of the laboratory’s present flagship fusion facility. “During its operation, this tokamak produces magnetic bubbles called plasmoids that move at around 20 kilometers per second, which seemed to me a lot like thrust.”

Fusion, the power that drives the sun and stars, combines light elements in the form of plasma — the hot, charged state of matter composed of free electrons and atomic nuclei that represents 99% of the visible universe — to generate massive amounts of energy. Scientists are seeking to replicate fusion on Earth for a virtually inexhaustible supply of power to generate electricity.

Current plasma thrusters that use electric fields to propel the particles can only produce low specific impulse, or speed. But computer simulations performed on PPPL computers and the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility at Lawrence Berkeley National Laboratory in Berkeley, California, showed that the new plasma thruster concept can generate exhaust with velocities of hundreds of kilometers per second, 10 times faster than those of other thrusters.

That faster velocity at the beginning of a spacecraft’s journey could bring the outer planets within reach of astronauts, Ebrahimi said. “Long-distance travel takes months or years because the specific impulse of chemical rocket engines is very low, so the craft takes a while to get up to speed,” she said. “But if we make thrusters based on magnetic reconnection, then we could conceivably complete long-distance missions in a shorter period of time.”

There are three main differences between Ebrahimi’s thruster concept and other devices. The first is that changing the strength of the magnetic fields can increase or decrease the amount of thrust. “By using more electromagnets and more magnetic fields, you can in effect turn a knob to fine-tune the velocity,” Ebrahimi said.

Second, the new thruster produces movement by ejecting both plasma particles and magnetic bubbles known as plasmoids. The plasmoids add power to the propulsion and no other thruster concept incorporates them.

Third, unlike current thruster concepts that rely on electric fields, the magnetic fields in Ebrahimi’s concept allow the plasma inside the thruster to consist of either heavy or light atoms. This flexibility enables scientists to tailor the amount of thrust for a particular mission. “While other thrusters require heavy gas, made of atoms like xenon, in this concept you can use any type of gas you want,” Ebrahimi said. Scientists might prefer light gas in some cases because the smaller atoms can get moving more quickly.

This concept broadens PPPL’s portfolio of space propulsion research. Other projects include the Hall Thruster Experiment which was started in 1999 by PPPL physicists Yevgeny Raitses and Nathaniel Fisch to investigate the use of plasma particles for moving spacecraft. Raitses and students are also investigating the use of tiny Hall thrusters to give small satellites called CubeSats greater maneuverability as they orbit the Earth.

Ebrahimi stressed that her thruster concept stems directly from her research into fusion energy. “This work was inspired by past fusion work and this is the first time that plasmoids and reconnection have been proposed for space propulsion,” Ebrahimi said. “The next step is building a prototype!”

Scientists develop new tool to design better fusion devices

One way that scientists seek to bring to Earth the fusion process that powers the sun and stars is trapping hot, charged plasma gas within a twisting magnetic coil device shaped like a breakfast cruller. But the device, called a stellarator, must be precisely engineered to prevent heat from escaping the plasma core where it stokes the fusion reactions. Now, researchers at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) have demonstrated that an advanced computer code could help design stellarators that confine the essential heat more effectively. The code, called XGC-S, opens new doors in stellarator research. "The main result of our research is that we can use the code to simulate both the early, or linear, and turbulent plasma behavior in stellarators," said PPPL physicist Michael Cole, lead author of the paper reporting the results in Physics of Plasmas. "This means that we can start to determine which stellarator shape contains heat best and most efficiently maintains conditions for fusion." Fusion combines light elements in the form of plasma - the hot, charged state of matter composed of free electrons and atomic nuclei - and generates massive amounts of energy in the sun and stars. Scientists aim to replicate fusion in devices on Earth for a virtually inexhaustible supply of safe and clean power to generate electricity. The PPPL scientists simulated the behavior of plasma inside fusion machines that look like a donut but with pinches and deformations that make the device more efficient, a kind of shape known as quasi-axisymmetric.

The researchers used an updated version of XGC, a state-of-the-art code developed at PPPL for modeling turbulence in doughnut-shaped fusion facilities called tokamaks, which have a simpler geometry. The modifications by Cole and his colleagues allowed the new XGC-S code to also model plasmas in the geometrically more complicated stellarators.

The simulations showed that a type of disturbance limited to a small area can become complex and expand to fill a larger space within the plasma. The results showed that XGC-S could simulate this type of stellarator plasma more accurately than what was previously possible.

"I think this is the beginning of a really important development in the study of turbulence in stellarators," said David Gates, head of the Department of Advanced Projects at PPPL. "It opens up a big window for getting new results."

The findings demonstrate the successful modification of the XGC code to simulate turbulence in stellarators. The code can calculate the turbulence in stellarators all the way from the plasma core to the edge, providing a more complete picture of the plasma's behavior.

"Turbulence is one of the primary mechanisms causing heat to leak out of fusion plasmas," Cole said. "Because stellarators can be built in a greater variety of shapes than tokamaks, we might be able to find shapes that control turbulence better than tokamaks do. Searching for them by building lots of big experiments is too expensive, so we need big simulations to search for them virtually."

The researchers plan to modify XGC-S further to produce an even clearer view of how turbulence causes heat leakage. The more complete a picture, the closer scientists will be to simulating stellarator experiments in the virtual realm. "Once you have an accurate code and a powerful computer, changing the stellarator design you are simulating is easy," Cole said.