Fatima Ebrahimi, a principal research physicist at Princeton Plasma Physics Laboratory (PPPL) in New Jersey, proposed the concept. The thruster would apply magnetic fields to cause particles of plasma to shoot out the back of the rocket, propelling it 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, generating lots of energy. Reconnection also occurs inside doughnut-shaped tokamak fusion devices.
“I've been cooking this concept for a while,” said Ebrahimi, the author of a paper 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). During its operation, this tokamak produces magnetic bubbles called plasmoids that move at around 20 kilometres per second, which seemed to me a lot like thrust.”
Current plasma thrusters that use electric fields to propel the particles can only produce low specific impulse, but computer simulations showed that the new plasma thruster concept could generate exhaust with velocities of hundreds of kilometres 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. Second, the new thruster produces movement by ejecting both plasma particles and plasmoids, adding power to the propulsion. The third difference is that unlike electric field thruster concepts, the magnetic fields in Ebrahimi's concept allow the plasma to consist of either heavy or light atoms.
“While other thrusters require heavy gas, made of atoms like xenon, in this concept you can use any type of gas you want,” she said. Scientists might prefer light gas in some cases because the smaller atoms can get moving more quickly.
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,” she said. “The next step is building a prototype.”
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