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Mini-magnetospheric plasma propulsion

From Academic Kids

Mini-magnetospheric plasma propulsion (M2P2) is a form of spacecraft propulsion, a way to make a magnetic sail. Magnetic sails' efficiencies fall of as the square of their distance from the sun. 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 automatically expands as the solar wind becomes less dense.

This mechanism creates an electromagnetic loop or mini-magnetosphere extending out from 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 thrust can be in any direction at right angles to the solar wind, allowing efficient changes of orbit.

One advantage is that no reaction mass is depleted or carried in the craft. The "fuel" is the gas needed to maintain the cloud. It therefore can generate thrust for long periods of time without refueling. One can calculate a "specific impulse" for this system by computing the amount of "fuel" consumed per newton of thrust. This is a figure of merit usually used for rockets, where the fuel is actually reaction mass. The "specific impulse" of such a system is about 200 kN·s/kg (200 times better than the space shuttle main engine). The system requires on the order of a kilowatt of power per newton of thrust, easily supplied by a nuclear reactor.

A unique advantage, not shared by magnetic or solar sails, is that the sail spreads automatically as the solar wind becomes less dense. In simulations, the system generates the same thrust anywhere within the heliopause. The solar and magnetic sails have a thrust that falls off as the square of the distance from the sun.

Another unique advantage is that the system requires no large mechanical system to catch solar protons or maintain the magnetic field. This reduces its total mass.

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-current (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. Radon gas would be still more massive and would have at least a slight tendency to be self ionizing. 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 (http://www.ess.washington.edu/Space/M2P2)). It has been successfully tested on Earth, but not deployed in space. A beam-powered variant, MagBeam (link (http://www.ess.washington.edu/Space/magbeam/)), is also under development.

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