This tokamak produces magnetic bubbles called plasmoids that move at around 20 kilometers per second

Sunday, February 14th, 2021

A new type of rocket thruster proposed by a physicist at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) would apply magnetic fields to propel plasma:

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 devices known as tokamaks.

“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 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.”


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.

(Hat tip to Jon Jeckell.)


  1. CMOT says:

    This is a very, very low thrust, very, very high efficiency drive … if it works. But if we can get this working, we can also get a lot of other drive concepts to work as well, which would probably be as good or superior to this one for practical use.

    It is novel and that deserves celebrating, though …

  2. Wang Wei Lin says:

    Given there is no free lunch in physics, how does this differ than any other propulsion system that converts mass into thrust? Once out of the gravity well of earth the laws of physics remain the same so that the total energy needed to move a mass from point A to point B is fixed. Energy is energy. The magnetic connection-reconnection energy has to originate with its energy source residing in the fuel. I can imagine practical advantages of one system over another like simplicity, safety, cost, etc, but I don’t see a energy advantage.

  3. Gavin Longmuir says:

    The article was probably written by a Harvard graduate, so we should be pleased that the spell corrector worked and not expect too much in the way of understanding of physics. The process presumably makes more sense than the article, which left me with some questions:

    “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.”

    What is the mass of a plasmoid? If a magnetic bubble has no mass, then it is not going to have momentum, and hence would not add to the reaction force.

    “Third, … the magnetic fields in Ebrahimi’s concept allow the plasma inside the thruster 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,” Ebrahimi said. Scientists might prefer light gas in some cases because the smaller atoms can get moving more quickly.”

    A light particle moving quickly would have similar momentum to a heavy particle moving much more slowly. Where is the advantage in terms of the reaction thrust?

  4. Sindarian says:


    The more (a) acceleration you have, the less (m) mass you need to get the same (f) force. The less mass you need to get to point B from point A the more mass you have for cargo or life support.

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