Water in a stream doesn't flow in a uniform sheet; it carves winding paths where the current runs faster. Electrons in a Hall thruster, a device that pushes a spacecraft with a jet of plasma, behave much the same way.
Inside the thruster, a magnetic field confines electrons, which ionize the gas and produce thrust. But some electrons leak across the field, reducing efficiency. For a long time, this leakage was thought to be smeared across the entire channel. New supercomputer simulations show that electrons actually gather into narrow near-wall streams that stretch toward the thruster exit.
The scientists compared three scenarios: conducting walls, ceramic walls with secondary emission, and an open plasma exit. The overall picture of near-wall channels remained unchanged; only their strength and connection to the exhaust region varied. This means the "pathways" are not a computational artifact but a fundamental property of the plasma.
This flow organization resembles how water in a stream picks its path. To improve the thruster, one shouldn't "smooth out" the flow but rather account for these natural channels — for instance, by tailoring the wall shape or the magnetic field.
Hall thrusters already operate on hundreds of satellites, accelerating ions to 20 km/s—60 times faster than the speed of sound in air.
In the series "The Expanse," ships use similar plasma thrusters, and their efficiency directly depends on managing such near-wall flows.