Magnetic shielding of ion beam thruster walls

February 13, 2013

Electric rocket engines known as Hall thrusters, which use a super high-velocity stream of ions to propel a spacecraft in space, have been used successfully onboard many missions for half a century. Erosion of the discharge channels walls, however, has limited their application to the inner solar system. A research team at Caltech's Jet Propulsion Laboratory, in Pasadena, Calif., has found a way to effectively control this erosion 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. Thrust is then generated by the interaction of this current with an applied magnetic field that creates a strong electric field. The magnetic field is mostly perpendicular to the channel walls whereas the electric field is mostly parallel to the walls. This electric field then acts as the driving force on the ions, accelerating them to very high speeds (>45,000 mph) toward the exhaust opening. However, the presence of a plasma in the thruster's discharge chamber leads also to a small component of the electric field 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.

Guided by theory and numerical simulations, the research team designed a thruster configuration in which the effect of the plasma on the magnetic field lines along the walls is minimized, forcing the to be perpendicular to the lines. Based on the numerical predictions, the effect of this topology would be to accelerate away from walls while also significantly reducing their energy adjacent to the walls. Erosion then would be reduced without degrading propulsive performance. The method now known as magnetic shielding was verified by experiments in a vacuum facility using a modified . The combined results of the simulations and experiments demonstrated that there was 100 to 1,000 times less wall erosion when using . The results were published in the American Institute of Physics (AIP) journal Applied Physics Letters.

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not rated yet Feb 13, 2013
Could a similar thing be done to the outside of a ship? If so, it would significantly reduce the dangers of radiation.
1 / 5 (1) Feb 13, 2013
Not in that way the hav in mind.
In a Hall effect thruster you knwo where the positive and negative ions are. They are at spatially separate places, so the confined magnetic fields simulated can move them in definite directions depending on wher they are.

On the outside of a craft you don't know whether the next ion will be positive or negative - so employing that scheme would shield only from one type (while making the other type more likely to hit you - i.e. more abrasive)
not rated yet Feb 14, 2013
That's a tremendous reduction in wear. is that the primary wear point that limits the maximum thrust/pound design? Are there new application for this type of thruster due to the enhancement from better shielding? It sounds like the wear was already minimal enough it didn't cause problems unless the thruster was intended for a lifespan longer than needed to leave the solar system. So it might be the major benefit for this is for allowign the thruster non-destructive use on very long range space craft intended to go outside the solar system.
1 / 5 (2) Apr 05, 2013
This form of magnetic shielding is also proposed for space flight radiation protection. Its also surprisingly efficient: A magnetic field around the ship causes electrons to bend strongly-being much lighter than corresponding positive ions. This charge separation causes a LARGE electric field to form for little initial magnetic field drive. Hey presto an electromagnetic shield.

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