Mini-magnetospheric plasma propulsion

Mini-magnetospheric plasma propulsion is a form of spacecraft propulsion, a way to make a magnetic sail. A major problem with magnetic sails is that they would have to be superconductive. To circumvent this problem, NASA has attempted to develop a system using a conductive plasma constrained by the magnetic field, a sort of synthetic magnetosphere.

This mechanism creates an electromagnetic bubble or mini-magnetosphere around the spacecraft, using a cloud of plasma to conduct the current to create the magnetic field. The solar wind is deflected (being made of protons, it has mass) and the reaction accelerates the spacecraft.

The advantage is that no reaction mass is depleted or carried in the craft. It therefore can generate thrust for long periods of time without refueling.

The basic mechanism looks like a coffee can with both ends open. Gas is emitted from a valve in the coffee can. The coffee can confines the gas long enough for some electronics to strike an arc through the gas. Then a low voltage (low electrical pressure) high-amperage (lots of electrons) electric current is pushed through the arc. The magnetic field from this current "traps" the plasma of the arc. The conductive plasma finds it hard to move in the magnetic field because whenever it cuts the magnetic field, eddy currents form opposing magnetic fields and stop the motion. The magnetic field naturally wants to expand. This causes the arc to grow, and confines the ionized gas in the arc. This process is called "blowing the arc."

Blowing the arc is something like blowing a large soap bubble. The arc is fragile. If the electric current at some point becomes too low, the magnetic field decreases, gas deionizes and escapes, and the current and magnetic field decrease even more.

A tank of gas is required to replace the ions of the cloud that leak from the magnetic field. The prototype uses Argon for the plasma, but the researchers hope to develop Helium plasmas as the technology improves. An Argon plasma has more massive ions, which are easier to contain because they move more slowly for a given electric current and magnetic field. Helium would mass less, and therefore might yield more velocity per kilogram. However, more of it leaks from present designs.

Electric power is required to keep the cloud ionized, and keep the current flowing. The magnetic field and current requirements are small enough that they could be powered by solar cells on unmanned vehicles.

For mission capabilities and maneuvers see magnetic sail. Magnetic sails are not at all like rockets. They can maneuver using planetary magnetic fields and plasmas other than the solar wind.

A prototype is being developed at the University of Washington (link). It has been successfully tested on Earth, but not deployed in space.