A big problem with the use of electric rocket engines known as Hall thrusters has been erosion of their discharge channel walls, limiting their use to the inner solar system.
Now, though, a Caltech team has found away around this by shaping the engine’s magnetic field in a way that shields the walls from ion bombardment.
Ions are produced in Hall thrusters when electrons from an electric current collide with the propellant atoms to form a plasma in the discharge chamber. The interaction of this current with an applied magnetic field creates a strong electric field, to generate thrust.
The magnetic field is mostly perpendicular to the channel walls, while the electric field is generally parallel to the walls. It’s this electric field that acts as the driving force on the ions, accelerating them to more than 45,000 mph toward the exhaust opening.
However, the presence of a plasma in the thruster’s discharge chamber means that a small part of the electric field becomes parallel to the magnetic field lines. This component then accelerates some ions toward the discharge chamber, rather than the exhaust opening – causing erosion by sputtering material from the walls.
Now, though, the Caltech team’s come up with a new thruster configuration in which the effect of the plasma on the magnetic field lines along the walls is minimized, forcing the electric field to be perpendicular to the lines.
Based on numerical predictions, the effect of this magnetic field topology would be to accelerate ions away from the walls, while also significantly reducing their energy near to the walls. This would decrease erosion without degrading performance.
The team’s tested its method, dubbed magnetic shielding, throughexperiments in a vacuum facility using a modified thruster. And, they say, the combined results of simulations and experiments show that there was 100 to 1,000 times less wall erosion when using magnetic shielding – giving Hall thrusters a longer life and therefore a bigger range.